Does Outside Air increase the Efficiency of woodstoves?

[fvhowler]

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".

Thanks in advance.

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

Hey there. Admittedly my internet research is limited but some reputable websites on wood stove operation contradict the need for an OAK. There is enough conflicting research and opinion based on real-life experiences out there that make the OAK at best an optional component to a wood stove installation and operation...unless it's required by code. The EPA does not require them, and manufacturers only make it an option. As someone said there probably is not a downside to an OAK but the benefits it seems are debatable and negligible.

 

[Ashful]

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.

Yes, you're right. Wasn't really taking the time to look at the problem. Sorry about that. Should probably refrain from doing ten second math between other things.

At the danger of another ten second math problem: 11k cu.ft. of air is basically a 25% change of the heated space, for my case, times three loads per day. I have no reference as to whether a 75% air change per day is "good" or "bad", but suspect others may be similar.

Speaking of which, time to go throw another 160 lb. of oak into two BK's.

 

[bholler]

Hey there. Admittedly my internet research is limited but some reputable websites on wood stove operation contradict the need for an OAK. There is enough conflicting research and opinion based on real-life experiences out there that make the OAK at best an optional component to a wood stove installation and operation...unless it's required by code. The EPA does not require them, and manufacturers only make it an option. As someone said there probably is not a downside to an OAK but the benefits it seems are debatable and negligible.

The benefits are not debatable or negligible if one is actually needed. I tried one on mine. It didn't make a difference. And in most cases I don't think they do much. But tge tighter a house gets the more important they are

 

[EbS-P]

Yes, you're right. Wasn't really taking the time to look at the problem. Sorry about that. Should probably refrain from doing ten second math between other things.

At the danger of another ten second math problem: 11k cu.ft. of air is basically a 25% change of the heated space, for my case, times three loads per day. I have no reference as to whether a 75% air change per day is "good" or "bad", but suspect others may be similar.

Speaking of which, time to go throw another 160 lb. of oak into two BK's.

Good indoor air quality shoot for a full exchange every 3-4 hours. Your house is not typically. Generally 100cfm is a nice round exchange rate for a bigger house, 50 for a smaller. Most powers ventilation is not continuous and your still have some natural ventilation to take into account.

 

[Ashful]

Good indoor air quality shoot for a full exchange every 3-4 hours. Your house is not typically. Generally 100cfm is a nice round exchange rate for a bigger house, 50 for a smaller. Most powers ventilation is not continuous and your still have some natural ventilation to take into account.

Yeah, figured this place wasn't typical, but was just using what I had to try to baseline what others might see without an OAK.

So, if seeing less than 1 exchange per 24 hours from the wood stove(s) alone, we'd be looking for a lot more help from the boiler, clothes dryer, bathroom exhaust, range hood, and convective attic losses, to hit your 3-4 hour target.

 

[robllachance]

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.

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?

you need an air fuel mass ratio of about 11:1

 

[robllachance]

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.

The point is that the temp differential between outside and inside doesn't matter. All the air comes from the outside no mater if it is ducted to the stove or not. 2% comes from the case where you'd consider that wasn't the case and a temperature differential of 50F (ie 20F avg to 70F average)

I may in the future post a google docs spread sheet

 

[robllachance]

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.

I agree, my post was just about efficiency

 

[robllachance]

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.

Sounds like you have a drafty house! The quality and distribution of the heat with an oak is definitely improved. This is what you experience. However the efficiency increase to how many BTUs the stove will put out with an oak is negligable.

 

[Prometeo]

Have you also considered the difference in air humidity between inside and outside?

 

[robllachance]

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:

Swift Air

Multifuel combustion air insulated Chimney system

www.schiedel.com

Hope this helps.

This is a great idea, to recover what engineers call the "waste heat" of the flue with a heat exchanger. This can definitely result in an efficiency increase. Care must be taken not to reduce the flue temps too much and cause condensation and fouling (creosote). My calc doesn't take this into account and simply looks at with an oak not preheated with waste heat vs without. BTW, I think you'll find the efficiency number generated is higher than any mass produced stove on the market. So they are doing something exceptional here.

Note that they mention the chmney heating the outside air, this can only happen when the outside air is introduced at a higher or same level as the exhaust, which I think is widely agreed is a bad idea. Could be achieved well with forced convection (fan) though

 

[robllachance]

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.

It would only change the "quality" and heat distribution, not the amount of heat introduced into the home. But draftiness can have a big effect on how warm a house feels

 

[robllachance]

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.

This is funadmentally incorrect by any appreciable margin. I concede that there is a large effect in the way a draft house feels with and without an oak

 

[robllachance]

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.


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.


^ 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.

I think I will clean up my calcs and post a google spreadsheet, but the point was a mass and energy balance. Anyone can see with the simple drawing that the two scenarios are identical wrt mass and energy flows.

 

[robllachance]

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.

The drawing i described does demonstrate the efficiency of the house

 

[robllachance]

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.

It is fact. A mass and energy balance proves it (that is the picture I described). Large concessions to heat distribution/draftiness resulting in a difference to how a house feels, particularly dependent on layout. However, there is no it depends that surpasses the single digit percentage mark outside of extraordinary circumstances.

As another poster pointed out, if you are preheating outside air with waste heat from the flue, you will do better with an oak in efficiency.

I'll keep the tpyo because nobody is prefect

 

[robllachance]

there's a mistake here, or am I wrong 🤔
80 lbs. wood burned correspond to 6000 ft3 of air, at least.

Mass ratios are approximately 11:1 air:fuel. Volume will depend on your elevation. I'm at 8600 feet

 

[Prometeo]

Sounds like you have a drafty house! The quality and distribution of the heat with an oak is definitely improved. This is what you experience. However the efficiency increase to how many BTUs the stove will put out with an oak is negligable.

drafts or not, the necessary air must always enter, otherwise create negative pressures inside the house and potentially your house sucks from the stove smoke/monoxide

 

[robllachance]

Have you also considered the difference in air humidity between inside and outside?

no. Mass and energy balance with a dry condition didn't break the single digit percent mark, so any humidity wouldnt move the needle much. Water vapor has about 1.9x the heat capacity of air as a reference.

 

[robllachance]

drafts or not, the necessary air must always enter, otherwise create negative pressures inside the house and potentially your house sucks from the stove smoke/monoxide

Yes, and this is exactly why an OAK doesn't improve stove efficiency. Same amount of air goes in and out of the house regardless

 

[bholler]

It would only change the "quality" and heat distribution, not the amount of heat introduced into the home. But draftiness can have a big effect on how warm a house feels

If the stove doesn't have enough air to run without one it certainly changes the quality and amount of heat it puts out.

But as long as there is enough makeup air your correct it won't change anything about the stove itself. But may decrease the air turnover in the home because the stove isn't pulling air out of the home

 

[Prometeo]

Yes, and this is exactly why an OAK doesn't improve stove efficiency. Same amount of air goes in and out of the house regardless

I think who uses OAK by efficiency he means, heating the house and not the efficiency of the stove without considering the heating. At your altitude, a lot of air will have to enter ( in the house ) to have the oxygen necessary to burn well. With OAK your home will be more efficient in maintaining heating. However, OAK born for the house without drafts and therefore it becomes necessary in those cases and not a choice!

 

[bholler]

Yes, and this is exactly why an OAK doesn't improve stove efficiency. Same amount of air goes in and out of the house regardless

Not nessecarily true.

 

[robllachance]

If the stove doesn't have enough air to run without one it certainly changes the quality and amount of heat it puts out.

But as long as there is enough makeup air your correct it won't change anything about the stove itself. But may decrease the air turnover in the home because the stove isn't pulling air out of the home

Agreed, as per my original post

 

[bholler]

Agreed, as per my original post

Ok so the original statement is inaccurate correct? Because clearly in some cases outside air does greatly effect a stoves operation right?