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

[Highbeam]

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.

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.

 

[WoodyIsGoody]

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.

 

[LondonNeil]

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.

 

[WoodyIsGoody]

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.

 

[Highbeam]

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.

 

[WoodyIsGoody]

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.

 

[begreen]

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.

 

[lsucet]

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.

 

[begreen]

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

 

[Highbeam]

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.

 

[Highbeam]

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.

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.

 

[jetsam]

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.

 

[WoodyIsGoody]

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.

 

[WoodyIsGoody]

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.

 

[yooper08]

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.

 

[lsucet]

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?

 

[WoodyIsGoody]

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.

 

[Highbeam]

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.

 

[Highbeam]

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.

 

[WoodyIsGoody]

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!

 

[WoodyIsGoody]

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

 

[Highbeam]

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.

 

[WoodyIsGoody]

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.

 

[lsucet]

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

 

[WoodyIsGoody]

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!