Outside air does not increase the efficiency of woodstoves. (A controversial post for my first one)
Googling around about outside air increases in efficiency didn't yield any quantitative results. So the engineer in me decided to calculate it for myself. I looked up the heat capacity of air, BTU of wood and all the parameters needed. Then I drew a diagram of the process and realized the net heat output of the stove was the same. You can prove this to yourself with a simple drawing:
Draw a house with a woodstove in it. Draw wood in the woodstove. Now get air to it. Whether it is ducted directly to the stove or if it seeps in the house through all the drafts, it always ultimately comes from outside, and must be heated to the same temperature before going up the flue(this is the perceived source of the loss). Doesn't matter what the outside air temp or inside air temp is.
Felt experience contradicts this; we feel the drafts running across the floor, it feels colder; Rooms furthest from the stove are the affected the worst. If you don't have a centrally located stove (long skinny house) then the ends will be colder since cold air is making it's way from the ends of the house to the stove. This can actually be alleviated by cracking a window at the location of the stove to decrease the pressure well inside the house pulling that cold air in. Counterintuitive that opening a window decreases draftiness and improves heat distribution! So this is what outside air kits achieve well, decreasing draftiness in the ends of the house and more evenly heating the house. An outside air kit can be considered a draft directed towards your stove! However, the same amount of heat is thrown off by the stove either way.
This of course neglects the small pieces, so I did the calculation anyway. The extra heat needed to warm up the "outside air" (ie any air that is going up your flue) corresponds to less than 2% of the stove's output. So any fringe arguments you can throw at this have a fraction of the 2% efficiency effect and usually have to do with heat distribution. For example: The basement the woodstove is in is heated to 90, so that hot air is being wasted. Even if those remote rooms feel cold, it's not a stove problem but a heat distribution problem. This is why you'll hear a lot of responses from people saying, "outside air isn't worth it, work on building envelope instead." And they are right, improving building envelope through weatherization will do more to offset heating costs and fuel use than anything else you can do for the same money.
A few notable exceptions. Sometimes outside air is demanded by code. In these cases you should probably put it in. New houses are tight enough in envelope that sometimes enough of a draft can't be generated without outside air; This is a good thing! But the solution is either to crack a window or install outside air. Outside air will allow indoor humidity to be slightly higher, since indoor sources of humidity will stick around longer.
I hope this clears up an answer I couldn't find on the internet! By the way I tried ChatGPT after I wrote this up and it got it wrong. It did tug at the threads of the right answer but made a few empirically incorrect statements like saying it would increase combustion efficiency, which is incorrect in any impactful way.
Cheers!! Looking forward to hearing all the pieces I neglected or your counter arguments
Googling around about outside air increases in efficiency didn't yield any quantitative results. So the engineer in me decided to calculate it for myself. I looked up the heat capacity of air, BTU of wood and all the parameters needed. Then I drew a diagram of the process and realized the net heat output of the stove was the same. You can prove this to yourself with a simple drawing:
Draw a house with a woodstove in it. Draw wood in the woodstove. Now get air to it. Whether it is ducted directly to the stove or if it seeps in the house through all the drafts, it always ultimately comes from outside, and must be heated to the same temperature before going up the flue(this is the perceived source of the loss). Doesn't matter what the outside air temp or inside air temp is.
Felt experience contradicts this; we feel the drafts running across the floor, it feels colder; Rooms furthest from the stove are the affected the worst. If you don't have a centrally located stove (long skinny house) then the ends will be colder since cold air is making it's way from the ends of the house to the stove. This can actually be alleviated by cracking a window at the location of the stove to decrease the pressure well inside the house pulling that cold air in. Counterintuitive that opening a window decreases draftiness and improves heat distribution! So this is what outside air kits achieve well, decreasing draftiness in the ends of the house and more evenly heating the house. An outside air kit can be considered a draft directed towards your stove! However, the same amount of heat is thrown off by the stove either way.
This of course neglects the small pieces, so I did the calculation anyway. The extra heat needed to warm up the "outside air" (ie any air that is going up your flue) corresponds to less than 2% of the stove's output. So any fringe arguments you can throw at this have a fraction of the 2% efficiency effect and usually have to do with heat distribution. For example: The basement the woodstove is in is heated to 90, so that hot air is being wasted. Even if those remote rooms feel cold, it's not a stove problem but a heat distribution problem. This is why you'll hear a lot of responses from people saying, "outside air isn't worth it, work on building envelope instead." And they are right, improving building envelope through weatherization will do more to offset heating costs and fuel use than anything else you can do for the same money.
A few notable exceptions. Sometimes outside air is demanded by code. In these cases you should probably put it in. New houses are tight enough in envelope that sometimes enough of a draft can't be generated without outside air; This is a good thing! But the solution is either to crack a window or install outside air. Outside air will allow indoor humidity to be slightly higher, since indoor sources of humidity will stick around longer.
I hope this clears up an answer I couldn't find on the internet! By the way I tried ChatGPT after I wrote this up and it got it wrong. It did tug at the threads of the right answer but made a few empirically incorrect statements like saying it would increase combustion efficiency, which is incorrect in any impactful way.
Cheers!! Looking forward to hearing all the pieces I neglected or your counter arguments
).
