Does Outside Air increase the Efficiency of woodstoves?

[sneefy]

i just read this whole thread and i still have no idea what to do.

Install an OAK.

The OP of this thread started with incorrect assumptions and it went downhill from there.

 

[steveh2112]

thanks, i was planning to do that, the last thing i need is even more cold air rushing into my house.

 

[Dieselhead]

Combustion air/exhaust air needs to come from somewhere. This includes bathroom vents, clothes driers, etc. that somewhere is from outside. Anytime you exhaust air from your house the negative pressure sucks air in from outside sources. Oak just provides that controlled outside air to supply your stove, instead of uncontrolled air being brought in from unsealed recessed lights, poorly insulated windows and doors, OTHER exhaust vents not in use, etc etc...

 

[DoubleClutch]

I haven't read the thread, but I believe the net effect is a "wash." It's six of one, half a dozen of the other.

If you do use an OAK, the air coming in through the OAK will be colder than the air in the house, so you will get fewer BTUs/hour out of the stove because the outside-temperature air has a "chilling" effect on the stove, compared to using "inside the house" air for combustion. On the plus side, you won't be drawing as much (cold, outdoor-temperature) air into the house to feed the fire, but this will be offset by the fact that the stove will run cooler (compared to feeding it with "inside the house" air for combustion) due to the (Δt) chilling effect of the (colder) outdoor combustion air.

If you don't use an OAK, the stove will create a partial "vacuum" inside the house, so air will be drawn into the house through cracks, vents, gaps in windows and doors, etc. (the combustion air has to come from somewhere, after all) then be warmed to the "inside the house" temperature before being drawn into the stove for combustion. On the plus side, this (warmer) combustion air won't have as much of a "chilling" effect on the stove (compared to an OAK), so the woodstove will remain hotter and (thanks to Δt) thus put more heat into the room -- but this will be offset by the fact that you're drawing more cold, outdoor-temperature air into the house to feed the fire, chilling the house.

So long as the stove is burning efficiently (in terms of "combustion efficiency"), I suspect you get the exact same amount of heat in the house either way. But in some cases, using an OAK has the potential to chill the firebox enough to reduce the combustion efficiency of the fire, leading to more smoke, more wasted fuel and more creosote.

Even if you do have good combustion efficiency, you still have to consider your stack temperatures. No wood fire (at least in my experience) ever burns with 100% combustion efficiency. So you're always going to have vapors going up the stack that -- if they condense -- will create creosote. If you don't keep your stack temperature high enough (in other words, above the "dew point" of the creosote vapors), you'll get a lot more creosote building up faster. It will condense on the inside of your stack the same way that water vapor condenses into "sweat" running down the side of a cold can of beer. I used to run a fan next to my woodstove to get more heat out of it ... until I discovered that I got a lot faster creosote buildup when using the fan. Now I don't use the fan anymore. If your stack temperatures are "borderline" without an OAK, then using an OAK will likely reduce those stack temperatures even more, leading to accelerated creosote buildup, the need for more frequent chimney cleaning, and increased risk of chimney fires.

Unfortunately, with wood fires, there's no free lunch. If you try to pull too much heat out of the stove, you can reduce the firebox temperature enough to reduce your combustion efficiency -- leading to more unburned fuel going up the stack -- and you can reduce your stack temperature enough to greatly increase the deposition of that fuel as creosote inside the stack. Bad juju on both counts, at least for a woodstove or fireplace in a house that can burn down. If it's an OWB, where you have limitless wood and don't care that much if you get faster creosote buildup, or even a chimney fire, it's probably less of a concern (which is probably why many of them are run like smoke dragons for much of the time)...

 

[Dieselhead]

Drawing in exterior winter air is dry, low humidity air. Any air in your house that has moisture from cooking, showering, breathing, humidifiers etc is good air during the cold months. Opposite in summer when we try to remove it!

 

[bholler]

I haven't read the thread, but I believe the net effect is a "wash." It's six of one, half a dozen of the other.

If you do use an OAK, the air coming in through the OAK will be colder than the air in the house, so you will get fewer BTUs/hour out of the stove because the outside-temperature air has a "chilling" effect on the stove, compared to using "inside the house" air for combustion. On the plus side, you won't be drawing as much (cold, outdoor-temperature) air into the house to feed the fire, but this will be offset by the fact that the stove will run cooler (compared to feeding it with "inside the house" air for combustion) due to the (Δt) chilling effect of the (colder) outdoor combustion air.

If you don't use an OAK, the stove will create a partial "vacuum" inside the house, so air will be drawn into the house through cracks, vents, gaps in windows and doors, etc. (the combustion air has to come from somewhere, after all) then be warmed to the "inside the house" temperature before being drawn into the stove for combustion. On the plus side, this (warmer) combustion air won't have as much of a "chilling" effect on the stove (compared to an OAK), so the woodstove will remain hotter and (thanks to Δt) thus put more heat into the room -- but this will be offset by the fact that you're drawing more cold, outdoor-temperature air into the house to feed the fire, chilling the house.

So long as the stove is burning efficiently (in terms of "combustion efficiency"), I suspect you get the exact same amount of heat in the house either way. But in some cases, using an OAK has the potential to chill the firebox enough to reduce the combustion efficiency of the fire, leading to more smoke, more wasted fuel and more creosote.

Even if you do have good combustion efficiency, you still have to consider your stack temperatures. No wood fire (at least in my experience) ever burns with 100% combustion efficiency. So you're always going to have vapors going up the stack that -- if they condense -- will create creosote. If you don't keep your stack temperature high enough (in other words, above the "dew point" of the creosote vapors), you'll get a lot more creosote building up faster. It will condense on the inside of your stack the same way that water vapor condenses into "sweat" running down the side of a cold can of beer. I used to run a fan next to my woodstove to get more heat out of it ... until I discovered that I got a lot faster creosote buildup when using the fan. Now I don't use the fan anymore. If your stack temperatures are "borderline" without an OAK, then using an OAK will likely reduce those stack temperatures even more, leading to accelerated creosote buildup, the need for more frequent chimney cleaning, and increased risk of chimney fires.

Unfortunately, with wood fires, there's no free lunch. If you try to pull too much heat out of the stove, you can reduce the firebox temperature enough to reduce your combustion efficiency -- leading to more unburned fuel going up the stack -- and you can reduce your stack temperature enough to greatly increase the deposition of that fuel as creosote inside the stack. Bad juju on both counts, at least for a woodstove or fireplace in a house that can burn down. If it's an OWB, where you have limitless wood and don't care that much if you get faster creosote buildup, or even a chimney fire, it's probably less of a concern (which is probably why many of them are run like smoke dragons for much of the time)...

I understand what your saying but in practice the theory simply doesn't hold true

 

[begreen]

This is a really a poor test. Few thoughts, In the 2nd video he's going in and out like what 9-10 times? In the first video I only counted 3 outdoor entries. That's a lot of cold air dumping into the cabin. He also left the OAK wide open in the back of the stove instead of stuffing it. Meaning it's dumping in cold air into the room behind the stove. So no timing of the door opening. Also, no thermometer on the stove or weighing of the fuel. And he is running the stove wrong with the air wide open so there is a lot of heat wasted up the flue plus a lot of wood wasted. And I would have a more quickly radiant stove in there instead of a little soapstone one.
(And dude, wear some gloves when loading.)

 

[Ashful]

I haven't read the thread, but I believe the net effect is a "wash." It's six of one, half a dozen of the other.

You're only looking at the temperature of the stove itself, but I'm not heating a stove, I'm heating a house. Even if your "it's a wash" argument holds for the stove itself, which it might, you're ignoring the fact that you're bringing cold air into distant bedrooms, far from the heat source in your living room. This disturbs heat distribution through the house, making every place in your house other than right in front of the stove, anything but a "wash".

 

[danrclem]

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[vatmark]

I haven't read the thread, but I believe the net effect is a "wash." It's six of one, half a dozen of the other.

If you do use an OAK, the air coming in through the OAK will be colder than the air in the house, so you will get fewer BTUs/hour out of the stove because the outside-temperature air has a "chilling" effect on the stove, compared to using "inside the house" air for combustion. On the plus side, you won't be drawing as much (cold, outdoor-temperature) air into the house to feed the fire, but this will be offset by the fact that the stove will run cooler (compared to feeding it with "inside the house" air for combustion) due to the (Δt) chilling effect of the (colder) outdoor combustion air.

If you don't use an OAK, the stove will create a partial "vacuum" inside the house, so air will be drawn into the house through cracks, vents, gaps in windows and doors, etc. (the combustion air has to come from somewhere, after all) then be warmed to the "inside the house" temperature before being drawn into the stove for combustion. On the plus side, this (warmer) combustion air won't have as much of a "chilling" effect on the stove (compared to an OAK), so the woodstove will remain hotter and (thanks to Δt) thus put more heat into the room -- but this will be offset by the fact that you're drawing more cold, outdoor-temperature air into the house to feed the fire, chilling the house.

So long as the stove is burning efficiently (in terms of "combustion efficiency"), I suspect you get the exact same amount of heat in the house either way. But in some cases, using an OAK has the potential to chill the firebox enough to reduce the combustion efficiency of the fire, leading to more smoke, more wasted fuel and more creosote.

Even if you do have good combustion efficiency, you still have to consider your stack temperatures. No wood fire (at least in my experience) ever burns with 100% combustion efficiency. So you're always going to have vapors going up the stack that -- if they condense -- will create creosote. If you don't keep your stack temperature high enough (in other words, above the "dew point" of the creosote vapors), you'll get a lot more creosote building up faster. It will condense on the inside of your stack the same way that water vapor condenses into "sweat" running down the side of a cold can of beer. I used to run a fan next to my woodstove to get more heat out of it ... until I discovered that I got a lot faster creosote buildup when using the fan. Now I don't use the fan anymore. If your stack temperatures are "borderline" without an OAK, then using an OAK will likely reduce those stack temperatures even more, leading to accelerated creosote buildup, the need for more frequent chimney cleaning, and increased risk of chimney fires.

Unfortunately, with wood fires, there's no free lunch. If you try to pull too much heat out of the stove, you can reduce the firebox temperature enough to reduce your combustion efficiency -- leading to more unburned fuel going up the stack -- and you can reduce your stack temperature enough to greatly increase the deposition of that fuel as creosote inside the stack. Bad juju on both counts, at least for a woodstove or fireplace in a house that can burn down. If it's an OWB, where you have limitless wood and don't care that much if you get faster creosote buildup, or even a chimney fire, it's probably less of a concern (which is probably why many of them are run like smoke dragons for much of the time)...

Creating a partial vacuum in a house can create its own issues. As you stated (the combustion air has to come from somewhere, after all). Not only can that combustion air come from cracks, gaps, vents etc, it can also come from combustion appliances such as gas stoves, water heaters, furnaces etc causing combustion gases to be drawn into the living space. I never thought OAKs had anything to do with improving efficiency but more to do with safety.

 

[River]

You're only looking at the temperature of the stove itself, but I'm not heating a stove, I'm heating a house.

Notwithstanding, the title of this thread is "Does Outside Air increase the Efficiency of woodstoves?" (emphasis added).

...you're ignoring the fact that you're bringing cold air into distant bedrooms, far from the heat source in your living room.

Why is the source of infiltration restricted to "distant bedrooms, far from the heat source in your living room"? In fact, why is the heat source in the living room? What if it were in a distant bedroom?

For myself, I want those distant bedrooms cool (some might say, cold). So even if this scenario were more than just hypothetical, it does not seem (to me) to be a drawback. Further, when folks speak of infiltration of (in this case) cold air into a home, wind is never considered. Maybe (another hypothetical) those bedrooms would be cold even with an OAK.

Maybe hypotheticals are not overly useful, here.

 

[bholler]

Notwithstanding, the title of this thread is "Does Outside Air increase the Efficiency of woodstoves?" (emphasis added).


Why is the source of infiltration restricted to "distant bedrooms, far from the heat source in your living room"? In fact, why is the heat source in the living room? What if it were in a distant bedroom?

For myself, I want those distant bedrooms cool (some might say, cold). So even if this scenario were more than just hypothetical, it does not seem (to me) to be a drawback. Further, when folks speak of infiltration of (in this case) cold air into a home, wind is never considered. Maybe (another hypothetical) those bedrooms would be cold even with an OAK.

Maybe hypotheticals are not overly useful, here.

Yes of course wind has an effect on air infiltration into a home. But how does that change the discussion of outside air systems? It doesn't at all

 

[begreen]

Creating a partial vacuum in a house can create its own issues. As you stated (the combustion air has to come from somewhere, after all). Not only can that combustion air come from cracks, gaps, vents etc, it can also come from combustion appliances such as gas stoves, water heaters, furnaces etc causing combustion gases to be drawn into the living space. I never thought OAKs had anything to do with improving efficiency but more to do with safety.

This is why they are required for new, tight construction and for mobile homes.

 

[River]

Yes of course wind has an effect on air infiltration into a home. But how does that change the discussion of outside air systems? It doesn't at all

My comment in re: wind was in response to the statement:

... you're bringing cold air into distant bedrooms

which was used to support the author's conclusion:

This disturbs heat distribution through the house

So, if those statements are not germane to the the "discussion of outside air systems," then my mention of wind was also not. But if the quotes I used do not "change the discussion of outside air systems," why not reply to that author suggesting the same?

 

[Ashful]

Why is the source of infiltration restricted to "distant bedrooms, far from the heat source in your living room"? In fact, why is the heat source in the living room? What if it were in a distant bedroom?

I think you knew before you wrote this, that I never said it was restricted only to distant bedrooms. I'm saying that without an OAK, the draw created by the stove causes air infiltration to the home to increase everywhere, including rooms distant from the stove.

Let's not argue just for the sake or arguing. You made a good point, WRT to the temperature of the stove. And yes... that's (stupidly) the title of the thread. But no one is heating a stove, for the sake of heating a stove. We're heating our homes.

 

[Prometeo]

my oak includes a T that I open to draw indoor air if for some reason I have a puff of smoke, otherwise, I take air outside

 

[Das Jugghead]

Opinions, experiences, research, and set-ups vary. I found improvements with my OAK. I would agree that strictly speaking an OAK doesn't improve the efficiency of the stove per se. However it did, in my case, improve the ability of the stove to heat my house. I also pre-heated the incoming air prior to it entering the firebox and secondary tubes.

Pleasant Hearth WS 2417 Outside Air Intake Build

At the end of 2018 I purchased a Pleasant Hearth WS-2417 woodstove. That installation and subsequent learning process is described here: https://www.hearth.com/talk/threads/pleasant-hearth-ws-2417-install-challenges-questions-observations.173074/ Several issues have been on the list to...

www.hearth.com

 

[Wisco Shepherd]

Just went through this start to finish. I don't run an OAK on my small newer build for reasons unique to my setup. But the deep dive into the sciences was entertaining. At the end of the day we are burning dead plants in a metal box and that's going to have a lot of variability.

I'm pretty tired, I think ill go home now.