# Do stove fan's make a stove more efficient, or does it just help spread the heat around.



## SuperJ (Sep 25, 2017)

Does having a fan on a stove make it any more efficient?  Or does it just help even out the temperature in the room.  Or worded another way, will I burn less wood with a fan installed, than without (assuming the same room temperature)?  I'm assuming EPA stove ratings do not include a fan. 

My wood stove will likely be a bit over sized anyways, so it shouldn't have a problem keeping the room warm with or without a fan.  

I'm considering buying a stove without a fan, and then adding one if I have trouble moving the heat around.  The BK Ashford 30 is already more than I wanted to spend and adding a fan is another $300.


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## Michael Sean (Sep 25, 2017)

My personal opinion would be to run the stove with out the fan. As you progress through the burning season and get into some cold weather, you will be able to see how your stove heats and if it’s suitable for your needs. If it’s not, then go for the fan. If it heats just fine with out the fan then you save your $300.


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## WoodyIsGoody (Sep 25, 2017)

SuperJ said:


> Does having a fan on a stove make it any more efficient?  Or does it just help even out the temperature in the room.  Or worded another way, will I burn less wood with a fan installed, than without (assuming the same room temperature)?  I'm assuming EPA stove ratings do not include a fan.



Well, that's two different questions.

Yes, a fan does increase the burn efficiency but you will likely end up with a warmer house rather than burning less wood (since you said the stove was over-sized for the application). However, if you will be warming the house up from cold (like a 20 degree temperature rise) the fans will not only save you wood, they will heat the place up considerably faster. If you plan to burn 24/7 on a low setting, you don't need them.


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## lsucet (Sep 25, 2017)

Another thing to take into consideration is if the stove is convective or radiate the heat. We can say BK is a convective stove, what can make the fan a must to have. But everything depend if you can get away without them and keep/stay warm. I don't use them much on my princess but it makes a different those time i use them and just in low..


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## Highbeam (Sep 25, 2017)

On the thermostatic bk fans will drop efficiency. These cat stoves are most efficient at the lowest burn rates. Fans strip heat from the stove which makes the thermostat open and increases the burn rate. Any increase in burn rate results in a decrease in efficiency. 

On a noncat the fans help to capture some of the waste heat and can increase efficiency when used properly.


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## lsucet (Sep 25, 2017)

I don't understand how the fans are related to efficiency. I really will like to know more about it to understand better.  Then a furnace has no efficiency cause depends on fans? I understand that the fans can make the thermostat open more often but at the same time you can dial it lower if what you are looking is distribution of that heat vs more heat. I am not trying to argue or start anything. I just want to know and learn.


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## jetsam (Sep 25, 2017)

Imagine that you are burning your BK very low. The thermostat almost completely closes the air as the firebox warms up, and opens it a bit when the firebox cools down.

Now you turn the fan on. The firebox cools down, so the thermostat opens, so you burn wood faster and get more heat in the room.

I don't agree that this necessarily implies less efficiency.


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## yooper08 (Sep 25, 2017)

There's two efficiencies when it comes to wood stove operation. There's the burn efficiency, which is simply how completely does the stove burn the wood, which is related to the burn rate. The second is the heat transfer efficiency, which is how well the stove transfers heat to the living space.

There's ultimately an equilibrium.  If you're pulling more BTUs off the firebox than the burn rate can provide, burn efficiency will end up declining until you increase the burn rate to bring it back to equilibrium or in favor of the burn rate.

So, depending on how you'd run the fan, it could decrease your burn efficiency if you needed to maintain higher temps because of it.

Another option is an eco fan that runs about $100 and you place it on the stove and it uses no electricity. I can't imagine that would impact burn efficiency but it would help move warm air around. A manufacturer-funded study at a university showed the eco fan improves heat distribution while using less wood.


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## lsucet (Sep 25, 2017)

I think that is my point of view. It is true that wood burn faster but I know that different burn rates will give you different efficiency. It is confusing.


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## WoodyIsGoody (Sep 26, 2017)

Wow! I can see how the answers in this thread sound contradictory and confusing even though they are all somewhere between 70%-100% accurate when read correctly.

Let's focus on the claim that a fan could cause the thermostat to open from low-burn to a higher (and less efficient) burn rate. First off, this isn't necessarily true even on a thermostatically controlled stove (using a BK as an example). The BK's thermostat has a huge range that it can be turned past the point at which the air intake is closed as far as it's going to close. So if you turn the thermostat all the way low, turning the fans on is not going to cause the air intake to budge past fully closed as long as your fire isn't in it's last stages.

In a different scenario, say with the thermostat set such that the air intake is just barely fully closed with out the fan running, yes, turning the fans on could cause the air intake to open more and this will cause the wood to be consumed faster. However, it's not a given that the actual efficiency will be less because much more heat will be delivered to the room with the fans running. Even if the thermostat self-adjusts to a higher (and slightly less efficient) burn rate, the amount of heat reclaimed by the fans on even a medium setting would more than make up for the faster burn rate.

The bottom line is that the effect the fan has on efficiency is generally positive but to achieve the benefit of extra efficiency either:

1) You need the additional heat, or

2) you turn the stove to a lower burn rate using the thermostat while extracting more heat due to the fans.

Near the end of a burn the fans could stall the cat sooner which would be the least efficient scenario of all.

So, as your cat temp drops and is heading towards the "inactive" zone, it's a good idea to either re-load (if you need the heat) or to turn the fans off (if you don't need additional heat). Turning the fan off with a dwindling fire will increase efficiency by extending the amount of time the cat is active.  But, if you have a good fire going and you need additional heat, the fans will  increase efficiency but it may be necessary to turn the thermostat down to maintain the same burn rate (while extracting more heat).


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## lsucet (Sep 26, 2017)

WoodyIsGoody said:


> The BK's thermostat has a huge range that it can be turned past the point at which the air intake is closed as far as it's going to close. So if you turn the thermostat all the way low, turning the fans on is not going to cause the air intake to budge past fully closed as long as your fire isn't in it's last stages



This is my understanding. We have members here reporting they can dial the stove to low and the fans have no effect. I know that depends on setup, location, draft, etc.  Now, saying that, i think, including burning to faster/higher burn rates to those that are affected by the fans, the stove is supposed to burn into the efficiency rating that manufacture claims. The heat still going into the house, just at faster rates.
In my experience I can dial it lower if using the fans. As we all know, makes better distribution of the heat to us with weird floor plan and the wood consumption is not that much different, but is true that burns faster. At least that is my experience. I know it looks like we are off of the OP question but i think these statements are providing better information to him.


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## Highbeam (Sep 26, 2017)

This is all theoretical and really we're talking about minute differences in efficiency. The fans on all woodstoves move only a small amount of air and hardly have a significant effect on efficiency. Efficiency in this context is always total efficiency not combustion efficiency. Total as in heat delivered to the space vs. heat dumped up the chimney as a portion of fuel energy provided.


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## WoodyIsGoody (Sep 26, 2017)

Highbeam said:


> This is all theoretical and really we're talking about minute differences in efficiency. The fans on all woodstoves move only a small amount of air and hardly have a significant effect on efficiency. Efficiency in this context is always total efficiency not combustion efficiency. Total as in heat delivered to the space vs. heat dumped up the chimney as a portion of fuel energy provided.



At high burn rates the fans can improve efficiency substantially. They do this by reducing flue gas temperatures and the effect is not small. Last week I measured the air coming out of the blower outlet at around 550 degrees. The blowers blow across the flue collar and extract heat that would otherwise be wasted right up the chimney. The temperature of the flue collar and top of the upper chamber is reduced by the blowers and that lowered temperature causes those components to absorb more heat from the exhaust stream (due to the temperature differential being larger) before it exits out the stack.

I'm pretty sure the EPA efficiency figures are calculated using test data from stoves without blowers (because the blowers are optional equipment). Maybe if BKVP sees this he can correct me if I'm wrong about that.


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## cd36 (Sep 26, 2017)

I believe you are correct about the rating being based on having no blower. I remember when I got our zc fireplace the rsf manual stated that a fan would improve heat output slightly.


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## SuperJ (Sep 26, 2017)

One thing to keep in mind is that to calculate BTUs it involves a volumetric flow rate (CFM in this case) and a delta T.  You can get big BTUs moving around off a low flow rate if the delta is high enough.  So even though the stove fans don't move many CFMs they are still moving a lot of BTUs off the metal and into the room air.
I guess to simplify my original question it would be...

_Do you think you burn less wood to keep your house warm if you run a stove fan?_ 

I would think the answer would likely be yes; a fan should theoretically save a bit of wood (but the amount would be debatable), since you wouldn't have to unnecessarily run the stove as hot to get heat further from the stove.  If the immediate vicinity around the stove is overheated you will be losing more heat thru the walls, ceilings, and windows, the flue, than if you had a cooler but more even temperature in the room.


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## lsucet (Sep 26, 2017)

SuperJ said:


> _Do you think you burn less wood to keep your house warm if you run a stove fan?_



 It will be hard to tell you if it will consume less wood. everyone setup is different. like i mentioned before some members claim no changes in consumption with fan on or off. i think they have enough draft or better location, etc, allowing this. that is not my situation. I know that my burn times are shorter with the fan on but the heat is there and better distribution thru the house. I also can dial it lower to the point of not make the cat goes inactive. that helps to not see much differences in wood consumption between fans on and off. Another point to take into account in my situation, when I run the fans, I run then on low 90% of the time. I run the stove without fans 80% of the time.


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## WoodyIsGoody (Sep 26, 2017)

SuperJ said:


> _Do you think you burn less wood to keep your house warm if you run a stove fan?_



I'm going to have to agree with Isucet and say that will largely depend upon the particular install (and how the fans are used). In general and with typical usage, I would say the fans are likely to make the house warmer (and warm it up more quickly) with not a big change in fuel consumption. Probably the most utility offered by the fans in most situations is not that you will burn drastically less wood but in being able to increase the stoves output to warm the house sooner or deal with extra cold, extra windy winter conditions, especially in installs where the stove is not oversized for the situation (without burning more wood). The fans increase the maximum effective heating power of the stove. 

A scenario in which the fans could reduce wood consumption significantly is if the stove (without fans) is required to run wide open to maintain comfort but could be run at half-throttle with the fans going and maintain the same temperature. 

In reality, seasons change, weather varies and the fans are useful at least a small percentage of the time in any given install. But if they are only beneficial 10% of the time, and make the stove 15% more efficient, that is only an annual wood savings of 1.5% (less wood).

Honestly, I prefer silence to the sound of blowing air so I only run the fans on low and turn them to high only if the structure needs maximum heat fast, to bring it to room temperature from dead cold. Even on low they increase the efficiency of the stove (more heating power for less wood). If the room becomes too warm, I can turn the fans off and reduce heat output while maintaining about the same level of wood consumption. So, yeah, I'm burning less efficiently but the room is more comfortable (not as warm). And turning the fans off doesn't make the emissions dirtier (if anything, slightly cleaner or no change). Emissions do not necessarily rise and fall in direct proportion to how efficiently your stove is burning.

The fans are worth $300 but the stove will perform very well without them too, just less versatility. You can always add them at a later date.


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## yooper08 (Sep 26, 2017)

SuperJ said:


> One thing to keep in mind is that to calculate BTUs it involves a volumetric flow rate (CFM in this case) and a delta T.  You can get big BTUs moving around off a low flow rate if the delta is high enough.  So even though the stove fans don't move many CFMs they are still moving a lot of BTUs off the metal and into the room air.
> I guess to simplify my original question it would be...
> 
> _Do you think you burn less wood to keep your house warm if you run a stove fan?_
> ...



Bingo, you got there before I did.  It comes down to how many BTUs are needed to make the space comfy and where they end up vs where you wanted them.  Piling up near the stove and relying on natural convection can be inefficient use of BTUs, which is where fans come in and can provide the benefit of flattening the temperature gradient of the space by spreading them out.  In certain setups, there may not be a significant difference in wood used.  In others, it can be rather significant.  

Here's what I'd recommend to experiment with this before you make a fan purchase.  If you have a small floor fan, set it up so that it's blowing cold air towards the stove, just run it on low.  That alone will help you figure this out given your setup.  

If all you need is a little push to move some air around, the eco fans I mentioned earlier can do that, are silent, don't consume electricity (think power outage), and cost about $100.  It may be all you need.


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## yooper08 (Sep 26, 2017)

Unless someone can correct me, fans don't increase BTU output per se, they just redistribute what's already been put out.  Meaning, if my stove's heat transfer efficiency is 75%, 75% of the BTUs will get transferred regardless of fan status.  If I run the fan, I'm just distributing that 75% differently.  It's not like the flue is sucking a portion of that 75% back in and to the outside (except for combustion air but that happens anyway).  The only way to increase BTU output is a hotter firebox.


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## jetsam (Sep 26, 2017)

yooper08 said:


> Unless someone can correct me, fans don't increase BTU output per se, they just redistribute what's already been put out.  Meaning, if my stove's heat transfer efficiency is 75%, 75% of the BTUs will get transferred regardless of fan status.  If I run the fan, I'm just distributing that 75% differently.  It's not like the flue is sucking a portion of that 75% back in and to the outside (except for combustion air but that happens anyway).  The only way to increase BTU output is a hotter firebox.



Depends on the stove. On a BK, a fan blowing on the stove increases BTU output and wood consumption. This is because the stove's thermostat increases input air to make more fire as the stove cools off.


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## cd36 (Sep 26, 2017)

yooper08 said:


> Unless someone can correct me, fans don't increase BTU output per se, they just redistribute what's already been put out.  Meaning, if my stove's heat transfer efficiency is 75%, 75% of the BTUs will get transferred regardless of fan status.  If I run the fan, I'm just distributing that 75% differently.  It's not like the flue is sucking a portion of that 75% back in and to the outside (except for combustion air but that happens anyway).  The only way to increase BTU output is a hotter firebox.


It went increase how much heat your wood is capable of produce, that is set with the wood you're using.

It will increase how much of that heat your wood produces actually ends up in your house. Without a fan your stove will get to a certain temperature, heat the air around it and make itself a little insulating blanket that reduces how much heat can be conducted from the stove into the air. The heat that can't be conducted goes out the chimney. 

The fan blowing cool air onto the stove will disturb that insulating blanket of hot air around the stove, allowing the stove to give up more heat. The stove, now surrounded by cool air can conduct more heat out of the stove itself, instead of just sending it up the chimney.

So yes fans will increase how much heat ends up in your house from the wood you are burning. It's just simple thermodynamics. Think of windchill, do you feel colder on a still air day or on a windy day? The windy day disturbs your warm insulating blanket of air around your skin, allowing more heat to conduct out of your body into the air, making you feel colder. Really stoves are no different. 

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## Randy Acton (Sep 27, 2017)

Here is my 2 cents based on my heating situation.

A woodstove can/will exhibit all three types of heat transfer: conduction, convection and radiation.  Adding a fan, whether ceiling fans, a fan blowing across the stove or a fan on the stove, will interrupt what the stove does best all by itself (convect and radiate).  As soon as I turn on any kind of fan, any part of the house more than 20' away from the stove, starts to cool down.

I don't personally think a fan all by itself can improve efficiency, all it does is change heat transfer.  We all know that a fire needs 3 elements to burn and those elements need to have the proper ratio in order to get the most efficient burn.  Sure a fan can extract more heat but not without changing that ratio so somehow you have to get the heat back, so you increase air which in turn burns more wood and increases the firebox temp.  If all you do is extract more heat, efficiency goes down.


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## lsucet (Sep 27, 2017)

Randy Acton said:


> Here is my 2 cents based on my heating situation.
> 
> A woodstove can/will exhibit all three types of heat transfer: conduction, convection and radiation.  Adding a fan, whether ceiling fans, a fan blowing across the stove or a fan on the stove, will interrupt what the stove does best all by itself (convect and radiate).  As soon as I turn on any kind of fan, any part of the house more than 20' away from the stove, starts to cool down.
> 
> I don't personally think a fan all by itself can improve efficiency, all it does is change heat transfer.  We all know that a fire needs 3 elements to burn and those elements need to have the proper ratio in order to get the most efficient burn.  Sure a fan can extract more heat but not without changing that ratio so somehow you have to get the heat back, so you increase air which in turn burns more wood and increases the firebox temp.  If all you do is extract more heat, efficiency goes down.



 I understand that everyone's situation is different and i did mention that before. I don't know what type of stove you have, but I can tell you this. The way the fans on BK, blows over the top,( between top of stove and convection deck ) it really make a difference compare to my previous stoves. It is more effective and move the heat very good without the stove room feel overheated compared to the rest of the house. That was not my experience with previous stove fans, the was more noise than heat distribution. Like mentioned before, BK has a wide range of burn rate allowing you including sometimes depending outside temp, house insulation, etc, to use lower thermostat settings when using the fans. Now , I know depends on many factors. floor plans, home dimensions, heat demand needed, etc.

 Like also mentioned before here, depending on draft, setup, etc. Some don't see much different on wood consumption between fans on or off. Not my case, but still, not enough differences to take into consideration.


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## Highbeam (Sep 27, 2017)

WoodyIsGoody said:


> Probably the most utility offered by the fans in most situations is not that you will burn drastically less wood but in being able to increase the stoves output to warm the house sooner or deal with extra cold, extra windy winter conditions, especially in installs where the stove is not oversized for the situation (without burning more wood). The fans increase the maximum effective heating power of the stove.



It's magic. Drastically less wood consumption with increased output! Not true in my experience.

Those of us with experience know that fans will increase wood consumption on a thermostatic controlled stove. Give it a try this year! You'll see.

The fans do increase output, but they also increase wood consumed. No free lunch!


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## LondonNeil (Sep 27, 2017)

first think about inefficiency in a stove, then think if a fan can affect that.
A stove can be inefficient/wasting energy in 2 ways, #1 failing to burn clean and wasting energy as unburnt fuel(smoke), #2 wasting excess heat up the flue.  unclean burn has various causes but that smoke going up the flue is wasted energy.  one cause could be a cool stove, perhaps just lit or one dieing down, or just running very low and smoldering.  Here a fan will make things worse by sucking more heat from the stove / firebox.  So don't run a fan on a smoldering stove.

scenario 2, heat wasted up flue, occurs at the other end of the spectrum when the stove is running really hard.  At this point a fan will help efficiency by sucking a bit more heat off the stove and into the room.

So yes a fan can help efficiency if you a running your stove hard, or it could hinder efficiency if your stove is just lit or running very low.


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## Highbeam (Sep 27, 2017)

LondonNeil said:


> first think about inefficiency in a stove, then think if a fan can affect that.
> A stove can be inefficient/wasting energy in 2 ways, #1 failing to burn clean and wasting energy as unburnt fuel(smoke), #2 wasting excess heat up the flue.  unclean burn has various causes but that smoke going up the flue is wasted energy.  one cause could be a cool stove, perhaps just lit or one dieing down, or just running very low and smoldering.  Here a fan will make things worse by sucking more heat from the stove / firebox.  So don't run a fan on a smoldering stove.
> 
> scenario 2, heat wasted up flue, occurs at the other end of the spectrum when the stove is running really hard.  At this point a fan will help efficiency by sucking a bit more heat off the stove and into the room.
> ...



Unlike a wood furnace that blows across a flue gas heat exchanger in the flue gas path, the stove fans blow on the stove to cool the stove. The wood furnace situation where you can extract wasted flue gas heat has a real chance of increasing efficiency. It is the theory behind the magic heat device as well. Cool the flue gasses to the minimum required to increase the efficiency. Zero unburned fuel emissions and cold flue temps accomplish peak efficiency right?

The BK thermostatic stove will increase burn rate to maintain a set stove temperature so cooling the firebox will just increase burn rate. You just get more heat.


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## WoodyIsGoody (Sep 27, 2017)

Highbeam said:


> The BK thermostatic stove will increase burn rate to maintain a set stove temperature so cooling the firebox will just increase burn rate. You just get more heat.



True, if you want more heat you can just turn the fans on.

But if you want the same amount of heat at a slower burn rate, turn the thermostat down and the fans on. That will increase efficiency.


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## LondonNeil (Sep 27, 2017)

when a stove is running hard and wasting heat up the flue, no matter where you put a fan, stove top or flue, more heat is extracted to the room and less is left for wasting up the flue.  the amount of difference made may however be minimal.  an ecofan will not make much difference. measurable? probably, if youve sensitive equipment and can set up a control to test against (so a laboratory), noticeable in real life? errrm.....maybe not.  i run a 5kW stove that is a snug fit in its fireplace opening, i run it hard and have a cheap ecofan-alike.  i can feel the warm draught from the fan so i know i'm geting heat out to the room better, but i dont notice the flue thermometer jump if i move the fan, nor that i use less logs of an evening.


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## WoodyIsGoody (Sep 27, 2017)

Highbeam said:


> It's magic. Drastically less wood consumption with increased output! Not true in my experience.



Please read what I actually wrote. I said the fans WILL NOT drastically reduce wood consumption.



> Those of us with experience know that fans will increase wood consumption on a thermostaticly controlled stove. Give it a try this year! You'll see.



My stove has an adjustable thermostat. Yes, as the stove is cooled by the fans, the thermostat tends to open a bit more. But to isolate the effect of the fans increasing efficiency without increasing wood consumption it is necessary to have a little understanding of how to adjust your stove in different situations. Yes, I can turn it down! Unfortunately, you still have to know how to run a wood stove. A stove with a thermostat requires a little more in-depth understanding to get the most out of it. With more experience you might become more adept at controlling it.

Unless you want more heat, the operator must know that turning the fan on will cause the thermostat to open up a bit. So of course you can adjust it down a little lower to maintain the same burn rate. A thermostatically controlled woodstove is not rocket science but turning the fans on will cause the thermostat to let more air in and burn your wood faster if you don't know enough to turn the thermostat down a little.



> The fans do increase output, but they also increase wood consumed. No free lunch!



You keep repeating this fallacy, the amount of wood consumed is primarily regulated by the thermostat which can be adjusted lower to maintain the same burn rate. If you turn the thermostat all the way counter-clockwise the air intake will be as restricted as possible and turning the fans on/off will not affect the air intake at all. At higher settings you can ALWAYS turn the air down a bit manually.

I recommend you read the explanation of the thermodynamics involved a little more carefully. I posted this previously but I'll put it here for your benefit:



> The blowers blow across the flue collar and extract heat that would otherwise be wasted right up the chimney. The temperature of the flue collar and top of the upper chamber is reduced by the blowers and that lowered temperature causes those components to absorb more heat from the exhaust stream (due to the temperature differential being larger) before it exits out the stack.



The "free lunch" is simply reclaiming heat that would otherwise go up your chimney.


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## Highbeam (Sep 27, 2017)

WoodyIsGoody said:


> Please read what I actually wrote. I said the fans WILL NOT drastically reduce wood consumption.



By golly, you're right. I got tripped up by your very strange sentence structure. Which makes my comments inappropriate. I would edit but you've quoted and already set me straight.

I still think you've got some thinking to do about this. The thermostat controls burn rate but does so in response to firebox temperature. I have played the game of producing low/moderate heat with fans and low firebox temp vs. no fans and high firebox temperature and have found the stove is much happier making heat without the fans. Burns less wood, cleaner combustion, and quieter too.


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## WoodyIsGoody (Sep 27, 2017)

Highbeam said:


> By golly, you're right. I got tripped up by your very strange sentence structure. Which makes my comments inappropriate. I would edit but you've quoted and already set me straight.
> 
> I still think you've got some thinking to do about this. The thermostat controls burn rate but does so in response to firebox temperature. I have played the game of producing low/moderate heat with fans and low firebox temp vs. no fans and high firebox temperature and have found the stove is much happier making heat without the fans. Burns less wood, cleaner combustion, and quieter too.



I'm glad you're open-minded enough to think about this some more. I've been trying to keep it simple so there are a couple of factors I've omitted that I'll address below so you have even more to think about.  But to address your observation that the stove is "happier" making heat without the fans on, I'll point out that at low burn temperatures you need all that heat going up the chimney to maintain draft. There is a point, and each install is different, where the fans are counter-productive and will reduce efficiency (due to not enough draft). The fans are primarily beneficial at high firebox temperatures.

Two more complicating details (t-stat time delay and flue temperatures)

There is a time delay between when the fans are turned on and the thermostat self adjusts so, if your goal is to increase efficiency while maintaining about the same heat output, it's best to wait a few minutes to nudge the thermostat down a bit (after turning the fans on). But none of this is set in stone because it depends on what part of the burn cycle you're in (and the particular install). The above advice applies to a fire in the early stage.

Another factor that hasn't been discussed is the effect of lower flue temperature (from turning the fans on) on the strength of the draft. While it's true that turning the fans on can cause the thermostat to open more, the effect of this is partially tempered by lower flue temperatures that I'll explain more fully.

The air control valve is not the only thing that controls how much air is available to the fire because, for any given position of the air control valve, the amount of air drawn in is proportional to the amount of draft in your flue. And the amount of draft is dependent on the difference in mass of the hotter, lighter air in your chimney vs. the colder, heavier air outside. That is why the draft will be stronger on very cold days and weaker during mild weather.

By turning the fans on you will be reducing flue temperatures and thus the amount of draft. Of course the thermostat will eventually offset that lack of draft by opening more when it senses lower temperatures caused by turning the fans on and also by this reduction in draft. But the bottom line is that you will still need to turn the thermostat down a little when turning the fans on to maintain the same burn rate. The exact amount will vary depending upon your particular install, the indoor/outdoor temperatures, the size and type of fuel and the stage of burn cycle you are in.

Wood with a lower moisture content will give you more latitude to tune your heat output and efficiency by using the fans. Wood that is not well seasoned may very well not allow much fan use at all.


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## begreen (Sep 27, 2017)

Is the thermostat more affected by stove body temperature or room temperature? I thought it was supposed to be the latter. If so and the stove is being pushed for heat (hot stove), with the fan running the room should heat up a bit quicker, thus signalling the thermostat to close down a little.


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## lsucet (Sep 27, 2017)

begreen said:


> Is the thermostat more affected by stove body temperature or room temperature?



 I will say body temperature. The thermostat housing is way hotter than room temperature. It just give you a wide range of setting that you can find a setting that match the room temp that you want to see so so. Maybe i am wrong, But that is what i think.


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## begreen (Sep 27, 2017)

You could be right. Was not sure what BK engineers were shooting for.


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## Highbeam (Sep 27, 2017)

begreen said:


> Is the thermostat more affected by stove body temperature or room temperature? I thought it was supposed to be the latter. If so and the stove is being pushed for heat (hot stove), with the fan running the room should heat up a bit quicker, thus signalling the thermostat to close down a little.



The actual thermostat coil is mounted right up next to the firebox several layers of metal away from the room temperature. I think that the room temperature has some effect on the thermostat but not as much as the firebox. The thermostat does a lot of moving even when the house temperature is consistent. Another temperature consideration is intake air temperature as some folks use subfreezing outside air that runs through the cast metal body of thermostat.


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## Highbeam (Sep 27, 2017)

WoodyIsGoody said:


> I'm glad you're open-minded enough to think about this some more. I've been trying to keep it simple so there are a couple of factors I've omitted that I'll address below so you have even more to think about.  But to address your observation that the stove is "happier" making heat without the fans on, I'll point out that at low burn temperatures you need all that heat going up the chimney to maintain draft. There is a point, and each install is different, where the fans are counter-productive and will reduce efficiency (due to not enough draft). The fans are primarily beneficial at high firebox temperatures.
> 
> Two more complicating details (t-stat time delay and flue temperatures)
> 
> ...



Good to see you're thinking it through noobie. I think you're catching up nicely and that our ideas on this matter are nearly matching.

Efficiency is barely (if at all) the issue. It's about burn rate, thermostat, fan interaction.


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## jetsam (Sep 27, 2017)

I imagine that an external fan blowing into the air intake (and thermostat) would push up the burn rate a fair bit, but I don't know why you would want to do this, as you could get the same result by turning up the thermostat a little.

I've contemplated converting the stove to run off of room temperature in the next room (using a remote temperature probe and a servo to adjust the existing mechanical thermostat), but I already have great results with the unmodified stove, so motivation to mess with it is low.


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## WoodyIsGoody (Sep 27, 2017)

Highbeam said:


> Efficiency is barely (if at all) the issue. It's about burn rate, thermostat, fan interaction.



You would do well to study up on how efficiency is measured. By reducing flue temperature at the same velocity, higher efficiency is achieved. That's what the fans do when they blow across the flue collar and suck the heat right out of it. A cooler flue collar increases the temperature differential between the collar and the hot flue gasses causing more heat to continually be absorbed by the flue collar and blown into the room as hot air. 

I think you will eventually understand the thermodynamics involved if you keep studying and learning. This will help you increase the efficiency of your stove as you gain experience with operating the controls that are user adjustable. You will learn when the fans are counter-productive and when they can best be used to your advantage. I know it's not always intuitive but stick with it and you'll get the hang of it.


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## WoodyIsGoody (Sep 27, 2017)

lsucet said:


> I will say body temperature. The thermostat housing is way hotter than room temperature. It just give you a wide range of setting that you can find a setting that match the room temp that you want to see so so. Maybe i am wrong, But that is what i think.



Yes,  that's correct. Blaze King does not market their stoves as maintaining even temperatures in the house. It's all about controlling the rate of burn. You can set it on a high setting, a low setting or anywhere in between. In BK promotional materials they have a 30 hour burn graphed out and they mention how the thermostat self corrects at the point 2/3's of the way through the burn where the logs collapse in order to maintain a consistent burn.

The thermostat is designed to respond to the temperature of the firebox, not the room.


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## yooper08 (Sep 27, 2017)

Here's a fluid dynamics analysis that was done on a setup and what we're trying to all explain.  The left is modeled with just the stove running, the right is modeled with a thermoelectric fan.  In this study, they ran the stove to maintain the room at 72.5*F in pairs of burns so that the burns occurred under the (reasonably) same conditions.  In each pair they found improvement both in comfort and wood consumption, 14% less on average.

I've also found that SBI notes up to a 2% gain when using their fans on their stoves.


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## lsucet (Sep 27, 2017)

yooper08 said:


> they ran the stove to maintain the room at 72.5*F



That is interstate. The question is, how they control the burn rates in both cases to keep a steady 72.5 degrees fahrenheit?


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## WoodyIsGoody (Sep 27, 2017)

yooper08 said:


> In each pair they found improvement both in comfort and wood consumption, 14% less on average.



14% is a very nice wood savings! 

I've been thinking about this some more trying to figure out why Highbeam has had troubling getting the fans to work correctly and I think I recall him saying his flue is only 12'. That's going to cause marginal draft right there and explains why the fans could easily stall the cat. It could also explain his recent problems with the cat plugging up. I do think a better drafting flue could solve these issues. Probably worth the cost.


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## Highbeam (Sep 27, 2017)

yooper08 said:


> Here's a fluid dynamics analysis that was done on a setup and what we're trying to all explain.  The left is modeled with just the stove running, the right is modeled with a thermoelectric fan.  In this study, they ran the stove to maintain the room at 72.5*F in pairs of burns so that the burns occurred under the (reasonably) same conditions.  In each pair they found improvement both in comfort and wood consumption, 14% less on average.
> 
> I've also found that SBI notes up to a 2% gain when using their fans on their stoves.



The photo doesn't agree with the proposal. If the average temperature was the same in each scenario then I would expect the "fan" room to be a yellow color but it's all red.


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## Highbeam (Sep 27, 2017)

WoodyIsGoody said:


> 14% is a very nice wood savings!
> 
> I've been thinking about this some more trying to figure out why Highbeam has had troubling getting the fans to work correctly and I think I recall him saying his flue is only 12'. That's going to cause marginal draft right there and explains why the fans could easily stall the cat. It could also explain his recent problems with the cat plugging up. I do think a better drafting flue could solve these issues. Probably worth the cost.



You seem to be confused. My fans work great and I have no cat clogging problems. My 12' chimney meets the spec for my stove from BK though a little more height might make it work even better at very low burns.

It looks like 2% is the gain which is a lot. Much more than I would expect.


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## WoodyIsGoody (Sep 27, 2017)

Highbeam said:


> The photo doesn't agree with the proposal. If the average temperature was the same in each scenario then I would expect the "fan" room to be a yellow color but it's all red.



I  agree. It looks like not only did they burn 14 percent less wood while using the fans, but the fans also heated the room to a warmer average temperature! Nice!


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## WoodyIsGoody (Sep 27, 2017)

Highbeam said:


> You seem to be confused. My fans work great and I have no cat clogging problems. My 12' chimney meets the spec for my stove from BK though a little more height might make it work even better at very low burns.



I was basing your fan problems on a comment you made earlier in the thread where you said;

"I have played the game of producing low/moderate heat with fans and low firebox temp vs. no fans and high firebox temperature and have found the stove is much happier making heat without the fans. Burns less wood, cleaner combustion, and quieter too."

The fact you haven't been able to use the fans to increase efficiency (and actually use more wood) shows something is not quite right.



> It looks like 2% is the gain which is a lot. Much more than I would expect.



Actually, Drolet claims the blower increases efficiency 15% on their stoves which is in line with the 14% measured in the actual testing posted by Yooper8.

https://www.ruralking.com/drolet-wood-stove-fan.html


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## Highbeam (Sep 27, 2017)

WoodyIsGoody said:


> The fact you haven't been able to use the fans to increase efficiency (and actually use more wood) shows something is not quite right.



Oh I know exactly what isn't right.....

The BK is already 81+% efficient. If you think stove fans can get this over 95% then it is obvious that you are out of touch with reality.

You are exhausting.


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## WoodyIsGoody (Sep 27, 2017)

Highbeam said:


> The BK is already 81+% efficient. If you think stove fans can get this over 95% then it is obvious that you are out of touch with reality.




Nobody claimed 95% efficiency! The test you are referring to claimed the fans allowed them to burn 14% less wood while maintaining a more comfortable living space. Your statement also exposes that you don't understand what the efficiency ratings of a woodstove actually mean.



> You are exhausting.



It's always more difficult to advocate a position that is not supported by the facts than it is to point out the facts. No surprise you are becoming tired of it. It's well documented that removing more heat from the areas of a heater exposed to hot exhaust gasses increases efficiency. Look up "calculating efficiency using the stack loss method".

You will see your position that the blowers make your stove less efficient doesn't make sense unless there is something wrong with your install. The marginal flue length of 12' is probably only producing a marginal draft and fans cooling the exhaust stream can only make that worse.


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## lsucet (Sep 27, 2017)

I don't know about your stove guys, but mine is 100% efficiency right now cause is off and cold. 
No fire yet this year


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## WoodyIsGoody (Sep 27, 2017)

lsucet said:


> I don't know about your stove guys, but mine is 100% efficiency right now cause is off and cold.
> No fire yet this year



Wouldn't that be 0% efficient? 

I bet SuperJ had no inkling that his original question would cause so much controversy!


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## lsucet (Sep 27, 2017)

WoodyIsGoody said:


> Wouldn't that be 0% efficient?
> 
> I bet SuperJ had no inkling that his original question would cause so much controversy!



If 0% make you feel better then it is


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## Highbeam (Sep 27, 2017)

lsucet said:


> If 0% make you feel better then it is



Really.

I'm at the "whatever dude" point with this jackoff.


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## bholler (Sep 27, 2017)

WoodyIsGoody said:


> Wouldn't that be 0% efficient?
> 
> I bet SuperJ had no inkling that his original question would cause so much controversy!


Well no the input and output are equal so it is 100%


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## bholler (Sep 27, 2017)

Highbeam said:


> Really.
> 
> I'm at the "whatever dude" point with this jackoff.


He is right about the fan thing though.  By extracting more heat off of the stove and putting it in the room you are increasing one part of efficency.  But you are also right in some situations because it could potentially reduce the efficency of the birn by lowering firebox temps


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## WoodyIsGoody (Sep 27, 2017)

Highbeam said:


> Really.
> 
> I'm at the "whatever dude" point with this jackoff.



Can you try to remain civil here?

That's not appropriate here. It's a discussion about heating efficiency, not some kind of dong waving contest!


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## bholler (Sep 27, 2017)

But honestly the differences in efficency either way are typically not that great.  It all depends what works best in each situation.  People put way to much emphasis on the efficency numbers anyway


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## lsucet (Sep 27, 2017)

Highbeam said:


> Really.
> 
> I'm at the "whatever dude" point with this jackoff.



Just let it be. At the end we all want to help the OP but he should be more confused now.


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## WoodyIsGoody (Sep 27, 2017)

bholler said:


> By extracting more heat off of the stove and putting it in the room you are increasing one part of efficency.  But you are also right in some situations because it could potentially reduce the efficency of the birn by lowering firebox temps



Yes, and I think that's what's going on with Highbeam's setup when he says it makes heat better with the fans off. A stronger drafting flue could solve that. BK's have an uncanny ability to burn super low and slow and this is not the time to use fans to try to scavenge every last bit of it.

If you study the design of the BK stoves you will notice the fan kit mostly removes heat from the top of the stove and the stove collar. Of course it's unavoidable that this will slightly lower temperature in the lower firebox but the designers did a good job of minimizing that.


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## WoodyIsGoody (Sep 27, 2017)

bholler said:


> But honestly the differences in efficency either way are typically not that great.  It all depends what works best in each situation.  People put way to much emphasis on the efficency numbers anyway



Right, the numbers vary widely anyway in the real world, there is no one "efficiency" number when you're burning different species, different sized splits and have the stove attached to different flues with different drafts. 

What I care about is heating my home without burning all the wood up. And the fans help considerably with that, particularly noticeable when heating up a cold structure at higher burn levels. Last week I had 560 degree air blowing out of the blower vent! I was pretty impressed!


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## WoodyIsGoody (Sep 27, 2017)

bholler said:


> Well no the input and output are equal so it is 100%



Yes, but if no work is being done, there is no efficiency number...zero heat, no efficiency.


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## Highbeam (Sep 27, 2017)

WoodyIsGoody said:


> Yes, and I think that's what's going on with Highbeam's setup when he says it makes heat better with the fans off. A stronger drafting flue could solve that. BK's have an uncanny ability to burn super low and slow and this is not the time to use fans to try to scavenge every last bit of it.
> 
> If you study the design of the BK stoves you will notice the fan kit mostly removes heat from the top of the stove and the stove collar. Of course it's unavoidable that this will slightly lower temperature in the lower firebox but the designers did a good job of minimizing that.



My fans absolutely increase the stove output as well as the wood consumption at any particular thermostat setting. Real experience. I do not believe that they increase efficiency by any significant amount. Efficiency as total heat delivered per lb of fuel burned.

I get one vote.


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## yooper08 (Sep 27, 2017)

bholler said:


> Well no the input and output are equal so it is 100%



Add a fan, it'll lower the efficiency.

End of thread.


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## lsucet (Sep 27, 2017)

At the end, this is all good. It goes back to the point that same stove at different locations and installs perform different. What makes us not to agree at some points because our different own experience.

The only thing we have in common is that we all love our stoves regardless brand and technology used plus we all stay warm. What I don't know now is if I should use the fan or not this winter.


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## SuperJ (Sep 27, 2017)

WoodyIsGoody said:


> I bet SuperJ had no inkling that his original question would cause so much controversy!



I think I'm following the discussion alright...  I'm just sitting back, eating popcorn, watching the discussion. It's nice to see a vigorous discussion on something.

I've decided to hold off on the fans for now, mostly cause I blew the budget going for the BK Ashford 30.  Easy enough to add them later if necessary.


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## lsucet (Sep 27, 2017)

I am sure you will like it with fans or not. When you can or if need them don't hesitate. It will be a good addition to your setup. Congratulations


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## begreen (Sep 28, 2017)

SuperJ said:


> I think I'm following the discussion alright...  I'm just sitting back, eating popcorn, watching the discussion. It's nice to see a vigorous discussion on something.
> 
> I've decided to hold off on the fans for now, mostly cause I blew the budget going for the BK Ashford 30.  Easy enough to add them later if necessary.


Good plan.


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## SculptureOfSound (Sep 28, 2017)

WoodyIsGoody said:


> You would do well to study up on how efficiency is measured. By reducing flue temperature at the same velocity, higher efficiency is achieved. That's what the fans do when they blow across the flue collar and suck the heat right out of it. A cooler flue collar increases the temperature differential between the collar and the hot flue gasses causing more heat to continually be absorbed by the flue collar and blown into the room as hot air.
> 
> I think you will eventually understand the thermodynamics involved if you keep studying and learning. This will help you increase the efficiency of your stove as you gain experience with operating the controls that are user adjustable. You will learn when the fans are counter-productive and when they can best be used to your advantage. I know it's not always intuitive but stick with it and you'll get the hang of it.



So if considering an insert that is surrounded by insulation... can you over insulate it with regards to efficiency? Or would you just want your fan to be running faster to bleed off the extra heat trapped in what has effectively become another "shell layer" on the unit?

I'm considering a slightly smaller insert so that I can heavily insulate the sides, back (and potentially the top...while still leaving an inch or two clearance on top) because I don't want to heat my external masonry. What's the best approach here to maximize efficiency?


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## WoodyIsGoody (Sep 28, 2017)

SculptureOfSound said:


> So if considering an insert that is surrounded by insulation... can you over insulate it with regards to efficiency? Or would you just want your fan to be running faster to bleed off the extra heat trapped in what has effectively become another "shell layer" on the unit?



There are two components to efficiency.

1) How complete the combustion is. If all the volatile components are not combusting, efficiency will suffer. High firebox temperatures (or an active cat) are both good solutions to that. So insulation helps in this regard.

2) How much of that heat makes it into the room. If most of it just goes up the chimney, efficiency suffers even when the combustion is substantially complete. Insulation can make this worse. The trick is to scavenge the heat after combustion is complete while leaving just enough heat in the flue to maintain adequate draft and avoid condensation. And that's why fans are so beneficial. It provides a knowledgeable operator one more tool to increase efficiency through the burn cycle. Fans should not be left on continuously but used judiciously to increase efficiency and comfort.



> I'm considering a slightly smaller insert so that I can heavily insulate the sides, back (and potentially the top...while still leaving an inch or two clearance on top) because I don't want to heat my external masonry. What's the best approach here to maximize efficiency?



That's a good question for a new thread. Like most things related to wood-burning, it's dependent upon the particular stove and the particulars of your installation. In general, a highly insulated setup will be more dependent on fans, especially with an insert.

If you post some photos and a bit more detail in a new thread I bet you'll get some good feedback (and some not-so-good feedback). But such is the nature of a free/open forum where anyone can post their opinion whether it's well grounded or not. It's up to each individual to filter the wheat from the chaff.


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## SculptureOfSound (Sep 29, 2017)

WoodyIsGoody said:


> That's a good question for a new thread. Like most things related to wood-burning, it's dependent upon the particular stove and the particulars of your installation. In general, a highly insulated setup will be more dependent on fans, especially with an insert.
> 
> If you post some photos and a bit more detail in a new thread I bet you'll get some good feedback (and some not-so-good feedback). But such is the nature of a free/open forum where anyone can post their opinion whether it's well grounded or not. It's up to each individual to filter the wheat from the chaff.



I did just that (although the thread turned into a bit of a rambling monster as I wrack my brain - https://www.hearth.com/talk/threads/gotta-decide-tonight.163315/.

Would love to hear your thoughts Woody. One thing I really am curious about - never studied thermodynamics and am having a hard time making heads or tails of the info I'm finding - is approximately how many BTUs are being lost when people report their external masonry which must be 4-8" thick from the back of the firebox is 20 or 30 degrees higher than ambient temps. If I could get a rough approximation of what this is actually translating to in BTUs it would help me evaluate the actual gains of heavy insulation, and greatly help me with my choice.


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## WoodyIsGoody (Sep 30, 2017)

Another benefit of the fans is they vacuum up the cooler air near the floor. If you don't have an outside air intake, air intake of the stove achieves this function to a limited degree. But the stove fans typically move more air than the natural draft of the stove. I estimate on a med-low burn the normal sized BK's have a natural draft of about 5-7 cfm. The fans are capable of vacuuming a LOT more air than this. So, even on a very low fan setting, they can increase comfort by reducing the temperature stratification of the air in the room. If you have a ceiling fan in the room this won't matter as much but, if not, you can achieve more comfort, more quickly with a slightly lower overall burn rate.

For example, say your family took a weekend get-away and it's been a day or so without the stove active, the house will have cooled down (quite a bit if it's colder out). Now you build a fire and burn on med-high to bring the house back to 72F. After an hour or so the air might be 70F but the walls are still 64F and the windows are colder yet. So there are cold air convection currents flowing down the wall(s) and onto the floor. Rather than continuing to burn on med-high, lower the thermostat slightly and turn the fans on a med-low setting. After a while the thermostat will naturally open a bit due to convection cooling of the fans. Counter-act that by turning the thermostat down a bit more about 5 minutes after turning the fans on. You will find your house gets warmer and more evenly comfortable (without that cold layer by the floor) and heats up faster all while making your load of wood last longer. Turn the fans off when you have achieved the desired comfort level and the thermostat will automatically turn even lower now with the fans off. The "free lunch" you got was recovering heat that would have gone up the flue. Of course, it probably cost $0.0025 in electricity to run the fans.

If all that sounds like too much work you can just burn on high or medium-high without the fans on and lower the thermostat once the room becomes more comfortable but realize this method will consume your load sooner. Recognizing that cutting and transporting wood and bringing it inside is work too, I propose the former strategy of maximizing useable btu's is less labor intensive and offers more comfort and reduces cold air stratification more quickly and with less wood.


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## SculptureOfSound (Oct 1, 2017)

Very clearly said Woody,and I agree; definitely seems like less work when evaluating the big picture.


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## Highbeam (Oct 1, 2017)

WoodyIsGoody said:


> Another benefit of the fans is they vacuum up the cooler air near the floor. If you don't have an outside air intake, air intake of the stove achieves this function to a limited degree. But the stove fans typically move more air than the natural draft of the stove. I estimate on a med-low burn the normal sized BK's have a natural draft of about 5-7 cfm. The fans are capable of vacuuming a LOT more air than this. So, even on a very low fan setting, they can increase comfort by reducing the temperature stratification of the air in the room. If you have a ceiling fan in the room this won't matter as much but, if not, you can achieve more comfort, more quickly with a slightly lower overall burn rate.
> 
> For example, say your family took a weekend get-away and it's been a day or so without the stove active, the house will have cooled down (quite a bit if it's colder out). Now you build a fire and burn on med-high to bring the house back to 72F. After an hour or so the air might be 70F but the walls are still 64F and the windows are colder yet. So there are cold air convection currents flowing down the wall(s) and onto the floor. Rather than continuing to burn on med-high, lower the thermostat slightly and turn the fans on a med-low setting. After a while the thermostat will naturally open a bit due to convection cooling of the fans. Counter-act that by turning the thermostat down a bit more about 5 minutes after turning the fans on. You will find your house gets warmer and more evenly comfortable (without that cold layer by the floor) and heats up faster all while making your load of wood last longer. Turn the fans off when you have achieved the desired comfort level and the thermostat will automatically turn even lower now with the fans off. The "free lunch" you got was recovering heat that would have gone up the flue. Of course, it probably cost $0.0025 in electricity to run the fans.
> 
> If all that sounds like too much work you can just burn on high or medium-high without the fans on and lower the thermostat once the room becomes more comfortable but realize this method will consume your load sooner. Recognizing that cutting and transporting wood and bringing it inside is work too, I propose the former strategy of maximizing useable btu's is less labor intensive and offers more comfort and reduces cold air stratification more quickly and with less wood.



Even old highbeam can agree with that! Your description of the intense thermostat management required to use the fans for gaining efficiency is the important caveat. Even a caveman can use fans to increase comfort.


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## WoodyIsGoody (Oct 1, 2017)

Highbeam said:


> Even old highbeam can agree with that! Your description of the intense thermostat management required to use the fans for gaining efficiency is the important caveat. Even a caveman can use fans to increase comfort.



Comfort, yes, but it takes a little more knowledge to use the fans to reduce wood consumption. Because you need to know to turn the thermostat down a touch when applying the fans.


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## begreen (Oct 1, 2017)

WoodyIsGoody said:


> Comfort, yes, but it takes a little more knowledge to use the fans to reduce wood consumption. Because you need to know to turn the thermostat down a touch when applying the fans.


Though only applicable to the stoves that have a thermostat.


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## WoodyIsGoody (Oct 1, 2017)

begreen said:


> Though only applicable to the stoves that have a thermostat.



True. It's pretty hard to turn the thermostat down if it doesn't have one!


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## carlan (Nov 24, 2018)

cd36 said:


> It went increase how much heat your wood is capable of produce, that is set with the wood you're using.
> 
> It will increase how much of that heat your wood produces actually ends up in your house. Without a fan your stove will get to a certain temperature, heat the air around it and make itself a little insulating blanket that reduces how much heat can be conducted from the stove into the air. The heat that can't be conducted goes out the chimney.
> 
> ...




Very well said, all other comments were good but this one nailed it in my mind.


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## SuperJ (Nov 24, 2018)

I ended up with fans eventually.  Here are observations . 

 My experience matches @Ashful , I can get a long burn by riding the hole with the fans on and stay active.  You need to fill it up with good wood and char it properly.
The BTUs will be limited by the minimum air setting, but it really helps to even out the temperature in my house.  
I never run the fans much higher than low even with the stove cranked up.  
If it's above freezing I keep the fans off since I don't need the extra heat . 
I notice the surface temp of the wall behind the stove is much lower with the fans running. That means I'm losing less heat to outside from overheating my wall . 
I'm assuming the fan housing doesn't count towards my clearances.  The back of the fan is definitely closer than the minimum corner clearance, since I only had a inch extra before the fans.


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