# Does Outside Air increase the Efficiency of woodstoves?

#### robllachance

##### New Member
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

I'm also happy to write a technical white paper on this if anyone would like to point to more than a blog post in the future on the OAK debate.

ctreitzell
Does it take an engineering degree to come to these same conclusions? No, but you did a great job explaining it.

If there is fire and therefore any draft, then there is air exchange, or else there’s a vacuum…and we can’t live in a vacuum devoid of fresh air.

I’ve said it here and elsewhere many times, just crack (unlock) the windows furthest from the stove and problem solved. Enjoy the convection loop it creates and actually warms the rooms furthest from the stove.

Last edited:
Not arguing, I've been a believer of the same, and it would likely take a real world lab test with drastically different results to change my mind.

I'm a big fan of using air from the house as combustion air, as it allows fresh air to be drawn into the house to replace the stale air. It's effectively free ventilation for the house.

I agree +/- OAK does not change the efficiency of the stove. I also agree with ABMax having ventilation can be a desirable feature.

It is not clear to me where the "2%" number came from. I am in 1200sqft/9600cuft. I burn 8 cords of spruce per year. I do not have an OAK. I do have a mold problem in the home, so I am with ABMax that ventilation via not having an OAK is a net positive for the total home environment.

However, if I do remediate all the mold and install an OAK, I find it difficult to believe I would still require 98% of 8 cords to maintain the home at the same temperature. I would still need 7.84 cords? That doesn't sound right to me. Imagine indoor air temp is +80dF and incoming make up air from outdoors is -20dF. As the temperature differential changes, the percentage of fuel savings should change also, but 2% sounds pretty low to me.

When I burn 35# of wood in my stove, how many pounds of air do I need and how many cubic feet of air is that?

When I burn 35# of wood in my stove, how many pounds of air do I need and how many cubic feet of air is that?

Depends on how much excess air your stove sends up the flue, but a general rule is 10-12 lbs of air per pound of wood (stoich is 6.4 lbs of air). 13.33 cuft per pound of air.

You're looking at 4665cuft to 5600cuft of air to burn down your wood. Basically your exchanging half the air in your house to accomplish this.

My house is quite tight, and I've tightened it up further in the last couple years, my (now defunct) radon detector proves that, it also proves that my wood stove is an effective mechanism for mitigation in winter months.

I agree it’s not an efficiency improvement but the OAK allows you to control where the air your stove needs comes from. Instead of increasing infiltration through already established leaks. These leaks can cool surfaces to below dew points and cause damage.

In terms of actual loss of heat I imagine (no numbers here) it could be off set by some simple air sealing of the highest floor of the house and adding insulation. So a good enough job air sealing and you might need the OAK.

My point, I don’t see a downside to the OAK.

in my opinion oak allows to reach higher temperatures inside the house, in same conditions and firewood consumption.
I tried, and the difference it's quite obvious.
Chimney sucks in large quantities of air, and replaced with cold air it's not the same thing.

Googling around about outside air increases in efficiency didn't yield any quantitative results.
Personally, I simply went to researchgate.net (or one can simple use the "site:researchgate.net" search keyword in your Google search to limit searching to a site that does publish quantitative content. Rather than simply being splattered by sites that do not, with a typical Google search.) and got this right off, as one example:

### Wood stove combustion air retrofits: A low cost way to increase energy savings in dwellings​

In Europe, wood-fired stoves remain as major renewable household heating and emission sources. This study focused on improving the performance of a wood stove (natural draft) traditionally used in Portugal by the adoption of alternative combustion air retrofits. Additionally, the performance of a new pellet stove (forced-air) was determined to investigate the highest achievable goal for solid-fuel stoves. In the wood stove, an outer chimney component was installed around the existing chimney to allow the vertical admission of outdoor air that was preheated before entering the combustion chamber. This measure increased the thermal efficiency of the wood stove from 62% to up to 79%. Another component was used to administrate secondary air to the wood stove reducing the carbon monoxide emissions by 39% to 2808 mg Nm−3. The two retrofits enhanced a more stable heat release from the wood stove, which reached a thermal efficiency 11% lower than that achieved by the pellet stove. This research suggests that retrofitting stoves with chimney components that allow the admission of combustion air can substantially increase energy savings in dwellings. Further efforts should focus on improving the interplay between the outdoor air and secondary air admission to achieve higher emission reductions at low-cost.

The above idea is in some ways actually commercially available in Europe for A-Rated houses:

Hope this helps.

Last edited:
ctreitzell
I totally agree it does not increase the efficiency of the stove. But it might increase the efficiency of the home by reducing the air turnover in the house.

While an OAK is required in some places, studies have shown they are not necessary for proper operation of a wood stove. I did not have one installed when we built our new home 6 years ago. My house is well insulated and relatively tight, and I have not had any issues. In fact, I've never had one in 40 years of burning so maybe I don't know what I'm missing.

While an OAK is required in some places, studies have shown they are not necessary for proper operation of a wood stove. I did not have one installed when we built our new home 6 years ago. My house is well insulated and relatively tight, and I have not had any issues. In fact, I've never had one in 40 years of burning so maybe I don't know what I'm missing.
Studies can say whatever they want but they absolutely are required in some cases

“Relatively tight”… you are missing a blower door test and number

I have air sealed many big gaps and now my stove no longer burns well with the air fully closed.
While an OAK is required in some places, studies have shown they are not necessary for proper operation of a wood stove. I did not have one installed when we built our new home 6 years ago. My house is well insulated and relatively tight, and I have not had any issues. In fact, I've never had one in 40 years of burning so maybe I don't know what I'm missing.

ctreitzell
I’ve said it here and elsewhere many times, just crack (unlock) the windows furthest from the stove and problem solved. Enjoy the convection loop it creates and actually warms the rooms furthest from the stove.
It would be better to open the window closest to the stove, not farthest from it, as the OP clearly explained. This has been discussed many times in other threads. One of the primary arguments for an OAK is that it reduces the amount of air being brought in through distant rooms, thus minimizing temperature differential throughout the house. Opening a window in some room far from the stove for the purpose of providing make-up air will usually completely destroy any possibility of weak passive convection loops heating the room containing the open window. I posted an explanation of this in another thread, just last night.

Imagine indoor air temp is +80dF and incoming make up air from outdoors is -20dF. As the temperature differential changes, the percentage of fuel savings should change also, but 2% sounds pretty low to me.
The OP was likely not using numbers as extreme as your case, but it would be very helpful if they could provide the spreadsheet, or Matlab or Maple worksheet they were using for these calculations, to test such cases against. Excel would be best, so even the non-engineers can play along.

Depends on how much excess air your stove sends up the flue, but a general rule is 10-12 lbs of air per pound of wood (stoich is 6.4 lbs of air). 13.33 cuft per pound of air.
^ This. You have roughly .08 lb/ft3 for air at room temperature and 1 atmosphere, so each 80 lb. load of wood through my stove is 70 ft3 of air up the pipe. 210 ft3 per day isn't so bad, for the ~1.5M BTU I'm putting into the house with three loads per day.

studies have shown they are not necessary for proper operation of a wood stove

I am not trying to be critical, but.... In my understanding, from the title, the thread is not about "proper operation", but "efficiency".

So do you have actual citations to peered reviewed journals** on such "studies" so we can judge what is exactly "proper operations" versus "efficiency", as they may be a bit different issues you are referring to. Ergo, you may be right on "proper operation", and maybe, or maybe not, correct on "efficiency".

** If not then a gray paper from a trusted source (example: university extension) may suffice.

ctreitzell
I'm surprised the OP didn't generate more likes. Aside from the slight confusion it may cause by initially citing the efficiency of only the stove, rather than the entire house as the system of interest, I think it was very well thought-out and he made an excellent argument. Good job.

Studies can say whatever they want but they absolutely are required in some cases

So true.

But... some things that are required may be regulated and required before all the facts are known. Causing their own set of issues till the facts are known. The facts may prove or disprove the prior regulations. Yet when an overwhelming amount of data and facts come in regulations that are proved are held up as visionary, while regulations that are disproved rather than getting changed, too often get entrenched into dogmatic denial rhetoric that "more science is needed". Go figure.

I remember the classic issue of requiring commercial kitchens to use plastic cutting boards as they were "declared" safer (without much data). It took awhile, till the actual science caught up. But when the science caught up, it started to indicate that wood was maybe no better or worse than plastic. So a lot of regulations, cost and waste for maybe no reason..... Science is slow but steady based on data and facts. Regulations (from politicians) are not uncommonly arbitrary or influenced by TPTB (-- with cutting boards maybe the plastic lobby? ).

### ​

Last edited:
I'm surprised the OP didn't generate more likes. Aside from the slight confusion it may cause by initially citing the efficiency of only the stove, rather than the entire house as the system of interest, I think it was very well thought-out and he made an excellent argument. Good job.

I for one find the post problematic on many levels.

- Starting with the title being a declarative statement as if it were fact. When the topic is really in the realm of "it depends", and one that "depends" on many factors. This is a bit of click bait. And not really even trying for a more science based objectiveness (e.g. outside air "may" not increase efficiency...)

- Then the use of Google as a singular source of research. The Journal of Science and Engineering Ethics (OP seems (?) to self declared to be an Engineer) even had an article about the limits and issues of this and basically said Google should not be one's only research source.

- I already provided a link that seems to contradict this flawed research. Again, helping to indicate that maybe do not rely on Google alone, and go to an actual research library and research some obscure journals for more information that is easily missed (sometimes you may need to speak a foreign language ... some of what I learned on this issue was in German for example).

- Given a failure of full and complete research and literature review, any scientist should proceed with caution with opinions based on limited or singular research, much less a white paper, no matter how "thought out" they may seem (because... the journal stated above was about "Ethics"). Even a flat earth argument may seem well "thought out", even if incorrect.

Side note: The OP self stated to be living in "Coloraod" Not sure where that is... Good grief... I make some horrendous typos myself. So not a critique. But he might want to adjust that if a typo.

Last edited:
...do not rely on Google alone, and go to an actual research library and research some obscure journals for more information that is easily missed...

I understand all of your criticisms, except the two related to his sources. I do research for a living, I've spent most of my career in R&D, and started it with several years as a Research Assistant and then later Research Fellow in my university days. I honestly don't come across many technical journals today whose transactions are not fully indexed and searchable by Google. There is no need to go to a library to research technical journals, good grief... this isn't 1995, or even 2005!

Especially when discussing a subject as basic as this, it's unlikely there are going to be sources so obscure that they have not been digitized and indexed by Google, which are going to make any wholesale change to the arguments made here.

The final content I am seeking is often behind a paywall, which is a business expense for me. But it does not change the fact that the entry point to this content in most cases are abstracts, bibliographies, and references which are all fully indexed by Google, nor the fact that pretty much all of this content (for at least my area of research) going back to at least WW2 is all fully digitized and available online, without driving to a physical library.

You have roughly .08 lb/ft3 for air at room temperature and 1 atmosphere, so each 80 lb. load of wood through my stove is 70 ft3 of air up the pipe. 210 ft3 per day isn't so bad, for the ~1.5M BTU I'm putting into the house with three loads per day.
there's a mistake here, or am I wrong 🤔
80 lbs. wood burned correspond to 6000 ft3 of air, at least.

Last edited:
Deleted by Hoytman

there's a mistake here, or am I wrong 🤔
80 lbs. wood burned correspond to 6000 ft3 of air, at least.
Actually, I think we both made a mistake. Not sure what I was doing there. 80 lb. / .08 lb/ft3 = 1000 ft3.

Actually, I think we both made a mistake. Not sure what I was doing there. 80 lb. / .08 lb/ft3 = 1000 ft3.

You're still wrong.

80lb * 6.4lbs air/lb wood * 13.33 cuft/lb = 6825cuft if you're burning at stoich, which you're not.

Your likely in the 10,000 to 12,000 cuft range.