Outside Air Kit in non-airtight house. Does it make a difference?

[cougkid]

I have a 1914 farmhouse that is pretty tight (insulated, vinyl windows, etc.) but it is by no means state-of-the-art airtight. I know most people use outside air kits (OAKs) to prevent backdraft in airtight homes, but would it prevent drafts from coming in through small cracks, under doors, etc. in my home when burning a fire?

I thought this was the case, but I just spoke with my fireplace shop and they told me it won't make much difference. Wanted to get your thoughts before drilling holes and spending the money...

 

[webbie]

I agree with your local fireplace shop - especially given your climate and house. If you had an open fireplace or large central heater (more BTU = more air needed), then I might reconsider. But as far as stoves, I think most of them area PITA and not needed except for Mobile homes - and perhaps Pellet stoves. I'm not a Pellet Stove expert, but I think they do use a lot of excess air......more than a woodstove per BTU - my guess, anyway.

 

[Harley]

Just out of curiosity.....

Is it the PITA factor that they are not recomended more? I would think, in theory, in a non-airtight house - that reducing the amount of "inside" air going up the stack would reduce the amount of "outside" air drawn into the house.

and nope - don't have one on mine.

 

[TMonter]

Lets look at it from a scientific perspective, when you burn wood for every pound of wood consumed you need about 8 pounds of air for efficient combustion in most wood stoves.

Air has a density of about 0.076 lb/ft^3 which means you need 13.15 x 8 = 105.2 cubic feet of air for every pound of wood consumed.

Where does the air come from? From outside. This means that your wood stove is taking air from inside the house for combustion and it's being replaced by cold air from outside.

For every pound of air at 32 degrees F you have to heat up to say 70F it takes 0.2401 btu/lb*F to heat that air up. Each pound of air consumes ~9.21 Btu, much more if temperatures are colder.

So 8*9.21 = 73.68 Btu for each pound of wood burned.

Lets say the average stove takes 55 pounds of wood on a full load, that's ~4052 Btu lost because of air infiltration on a warmer day. At 0F you can more than double that number because of air density and the greater delta T.

OAK's save heat, no doubt about it.

 

[Highbeam]

Isn't it the law to have an OAK in Washington? I know it was for me in Pierce county. I put one in for my Heritage and supplied the outside air from my ventilated crawlspace. Installing OAK had nothing to do with preventing backdrafts.

The OAK is cheap to put in, probably will save you heat, can't hurt anything, and I honsetly believe that the OAK results in less drafts. My old non-OAK stove "felt" like it created more cold drafts across the floor towards the stove.

 

[bmstove.com]

If you can install an outside air kit you should. Think of it this way, combustion air is always coming from outside. Would you rather have cold air ducted directly to your woodstove or through your windows first?

Jack

 

[cougkid]

Thanks for all of your replies. I agree that it makes sense to do this, I was just a little turned off by the gal at the fireplace shop. I think I'll go for it. Nothing to lose but $60 and some time, and i think it will make a difference (at least in my mind

 

[Galroc]

Lets look at it from a scientific perspective, when you burn wood for every pound of wood consumed you need about 8 pounds of air for efficient combustion in most wood stoves.

Air has a density of about 0.076 lb/ft^3 which means you need 13.15 x 8 = 105.2 cubic feet of air for every pound of wood consumed.

Where does the air come from? From outside. This means that your wood stove is taking air from inside the house for combustion and it's being replaced by cold air from outside.

For every pound of air at 32 degrees F you have to heat up to say 70F it takes 0.2401 btu/lb*F to heat that air up. Each pound of air consumes ~9.21 Btu, much more if temperatures are colder.

So 105.2*9.21 = 968.892 Btu for each pound of wood burned.

Lets say the average stove takes 55 pounds of wood on a full load, that's ~5300 Btu lost because of air infiltration on a warmer day. At 0F you can more than double that number because of air density and the greater delta T.

OAK's save heat, no doubt about it.

Wow, this is what I was looking for...

My dealer said that pellet stoves didn't really use that much air and didn't recommend a OAK.

edit: checking the math...you were off a bit..

55 pounds * 968.892 BTUs/pound = 53000 BTUs

Which is much larger than 5300 BTUs

Or, you can put it another way, you lose more 12% of your BTUs due to outside air. (~8000BTUs/pound * 968.892 BTUs/pound)

 

[jgoodnow]

OAK’s save heat, no doubt about it.

I think it's not that simple. Double Clutch's post in another thread explains why.

There’s no such thing as a free lunch.

If you pull combustion air from outside, remember that you’re going to have to heat that cold air a lot more than the warm air pulled from inside your house (delta T) to maintain adequate stack temperatures to prevent creosote buildup. Also, if the heat from your wood is going into heating up the combustion air, that heat is not coming out of the stove into your house. It’s going into your exhaust gases. Finally, drawing cold outside air into your stove is going to have the effect of cooling your stove ... meaning that its temperature will drop, which means that there will be less of a delta T between your stove temperature and the room air temperature around it, meaning that you’re going to get less heat out of the stove through convection and radiation.

All of this is a long-winded way of saying there’s no free lunch. I just let the stove draw combustion air from the house. Adding a combustion air duct from ouside the house seems to me like more trouble and cost than it’s worth.

 

[DoubleClutch]

Another thing to consider is that on a finicky non-cat stove that's temperamental about running at full efficiency (read: VC Defiant), the colder air drawn in from outside is probably going to cool the firebox enough to make the stove really finicky.

Not my idea of fun. And even if using room air for combustion air costs me a little bit of heat (which I'm still not convinced of) it's worth it to keep the stove running more efficiently.

 

[stoveguy2esw]

i think with conventional woodburning stoves its a case by case basis unless code applies (such as with mobile homes or premanufactured homes) or unless dictated by the manufacturer. some structures and flues themselves are able to breathe well enough that negative pressure is not a factor, some are tight enough to restrict airflow from the house enough that an OAK will afford the proper airflow to allow the stove to burn at its optimal efficiency. this said , the different views stated above are both valid points. personally i would look at what i had for starters (especially if i had the means to easily install an OAK post stove install if i decided i would benefit) in my house when i was heating with an older (non OAK capable epa unit, the unit performed admirably however , i have found the pellet unit (with OAK) i have now is more easily able to heat the house even though it was is a smaller unit with a lower rated capacity for square footage. now ive insulated a lot , but the insulation was added post changeover, and even before i could tell that the house was less drafty with the oak than with the other system using house air. as to combustion efficiency , i do not think that the wood unit suffered at all, but if i were burning it now after tightening up the house considerably, with my flue's capability it would burn well, but i doubt that it would burn as efficiently as it did before(simply because of the higher restriction from the sealing up i have been doing with the house)

my book , its better to use an OAK but it may not be as cut and dried with a different structure

 

[Rockey]

OAK’s save heat, no doubt about it.

I think it's not that simple. Double Clutch's post in another thread explains why.

There’s no such thing as a free lunch.

If you pull combustion air from outside, remember that you’re going to have to heat that cold air a lot more than the warm air pulled from inside your house (delta T) to maintain adequate stack temperatures to prevent creosote buildup. Also, if the heat from your wood is going into heating up the combustion air, that heat is not coming out of the stove into your house. It’s going into your exhaust gases. Finally, drawing cold outside air into your stove is going to have the effect of cooling your stove ... meaning that its temperature will drop, which means that there will be less of a delta T between your stove temperature and the room air temperature around it, meaning that you’re going to get less heat out of the stove through convection and radiation.

All of this is a long-winded way of saying there’s no free lunch. I just let the stove draw combustion air from the house. Adding a combustion air duct from ouside the house seems to me like more trouble and cost than it’s worth.

I would bet that the amount of cooling effect the colder air would have would be offset by the fact that colder air is denser and contains more oxygen. Therefore you could choke the intake down a proportional amountand achieve the same burn as a larger volume of warmer air.

 

[IK_biker]

TMonter,

What you posted here is only partially correct.
While I am not going to dispute your numbers, I'd like to point out that air density and volume have nothing to do with your calculated results. In the formula you provided, we deal with degrees, pounds and BTUs.

...it takes 0.2401 btu/lb*F to heat that air up. Each pound of air consumes ~9.21 Btu, much more if temperatures are colder.
So 105.2*9.21 = 968.892 Btu for each pound of wood burned...

In the above you used 105.2 cu. ft, while you should have used 8 lbs instead.
Thus, to heat that 8 lbs of air from 30 to 70 °F it would take
(70 - 30) degrees * 8 lbs * 0.2401 btu/lb*F = ~76.832 BTU and _NOT_ 968.892 BTU as you posted.

There is a significant difference between 77 and 969 BTU to heat 8 lbs of air, right?

On the other hand, this is the heat to be spent on heating the _DRY_ air. Extra heat would be spent to evaporate the moisture from 8 lbs of air, but in no way would the total come close to 969 BTU.

OAK’s save heat, no doubt about it.

This would depend on lots of things, including prevailing winds (frequency, speed and direction) and their effects on the internal house pressure. This debate is ongoing, and I would spend more time researching this, but for now the heat savings from OAKs are highly questionable.
What is not questionable is that OAK does not rob Peter to pay Paul, namely it ensures the oxygen for the stove without having all the oxygen-consuming appliances in the house fighting each other.

Cheers!

 

[billb3]

There was a OAK 'study' done in Canada (I didn't save a link but the results and conclusion wre online) and , to make a long story short - the results basically claimed that an OAK on a fireplace was basically a wash. However, when and where a 'backdrafting' fireplace/burner was a concern or problem it wasn't the stove firebox that needed the OAK, but the whole house needed a make-up air system (the tighter the house the worse the backdraft problem) and the real concern is CO backdrafting invisibly down furnace and gas heater chimneys. In other wordswhen your wood stove puffed so is your other chimneys. The puff of smoke at your fire box isn't what should be worrying you if your furnace or gas heater is running.
Half way through their super airtight "test-house" with bath exhausts and kitchen stove exhausts running they realized they were getting a CO problem and it wasn't coming from the wood stove. But their wood stove draft problems were a damned good indicator of what was going on with other chimneys.

My house is over 100 years old. It could never be that tight unless I got a really huge seal-a-meal bag.
It's not surprising some of these new super tight buildings are having air quality issues.

 

[wallis54806]

In an older house like mine (build 1880s) I do not see any good reason for an OAK. This is my reasoning.

It is almost impossible to make the house tight enough to have back draft issues. The air that is used for combustion will originate outside the house regardless of whether it is first heated to room temperature or goes directly into the stove. In other words the air that the stove uses will require the same amount of heat (BTUs) to raise a given mass of air to the required temperature inside the stove.

In a tighter house you could reduce drafts around doors and windows with an OAK, but I question any efficiency improvements. I don't have numbers to back this up, but IMO the reasoning I have given is sound. You need oxygen and a certain temperature (don't remember how high) for wood to burn. Why cool it down with cold air if you don't have to?

 

[Rockey]

Why cool it down with cold air if you don't have to?

Once again colder air is denser and more oxygen rich so it will take a lower volume of cold air to support combustion as a larger volume of warmer air. That makes it a wash. Now, just like in an internal combustion engine, the colder the air you feed it the more power it puts out. The leaner you run it the hotter the combustion.

Either way, I think this would make a great mythbusters episode.

 

[Galroc]

There?s no such thing as a free lunch.

If you pull combustion air from outside, remember that you?re going to have to heat that cold air a lot more than the warm air pulled from inside your house heat out of the stove through convection and radiation..

You will have to heat it if it comes from the room, or directly into the stove via a OAK. If the air comes from the room, guess where that air came from? Answer: the outside.

Taking the air from the room means you are drawing air from the outside, which the stove will now have to heat.

I call that a wash....

20-30 degree cooler air via an OAK is a minor difference compared to a fire's temperature.

 

[wallis54806]

The colder, denser air theory only holds water if you need to run your damper wide open for most of your burn cycle. I run mine about half open, so I am not using all the air that is available anyway. Speeking of holding water, outside air will have less, so that is one advantage of OAK.

 

[Rockey]

You could achieve the same burn with the damper closed even more with an oak.

Since the oak won't lower the humidity indoors it would also be another added benefit.

 

[jgoodnow]

http://www.woodheat.org has a page which is highly negative on outdoor air:

The Outdoor Air Myth Exposed - Outdoor combustion air was a good idea . . . until it was studied

It is a relatively long article but worth reading. Here is one short quote:

Despite the fact that it is enshrined in some building codes and its adherents are often vocally forceful, there is no scientific evidence to suggest that outdoor air supplies, either direct to the combustion chamber or indirect supplies to the living space, are reliable and effective remedial measures for combustion spillage from the appliance for which the supply is intended.

...

In general, therefore, fireplaces that are vented by natural chimney draft should draw the air for combustion from the room in which they are located.

 

[builderbob]

http://www.woodheat.org has a page which is highly negative on outdoor air:

And the ChimneySweep in Washington State has a contrary opinion to Woodheat .org!

http://www.chimneysweeponline.com/hooa3.htm

You can go crazy going back and forth, yes or no, pro or con, do I or don't I.

But as mentioned in the Chimneyseep's post above, Washington State apparently mandates ALL wood/gas stoves, fireplaces, burning units be connected directly to an outside air source.

So, maybe that makes your decision for you?

 

[DoubleClutch]

Now, just like in an internal combustion engine, the colder the air you feed it the more power it puts out. The leaner you run it the hotter the combustion.

I think we're getting our theories mixed up here.

The only reason you can (not necessarily "will") get more power out of an IC engine fed cold air is because there's more oxygen in a given volume of air at a lower temperature; therefore you can burn more gasoline in that same volume of air. This requires you to increase the amount of gasoline being fed to the engine. You can get the same effect by aspirating an IC engine with nitrous oxide, which has more oxygen available for combustion in a given volume than air.

However, a gasoline engine doesn't run anywhere near as "close to the edge" as a woodstove regarding minimum efficient combustion temperatures, etc., so whereas cold air will allow a IC engine to burn more fuel in a given volume, a woodstove may not, due to the chilling effect of the cold air on the firebox. The colder combustion air may cause you to lose your secondary combustion, or even make the wood start to smolder, despite the fact that the colder air contains more oxygen for a given volume.

Finally, while running a 2-stroke IC engine leaner will make it run hotter (because the gas-oil mix is also used to cool the combustion chamber in a 2-stroke engine), it is by no means a universal principle that "the leaner you run it the hotter the combustion." In fact, in most cases, just the opposite is true: You need more fuel to make more heat, so the richer you run it, the hotter it gets. The faster you can burn fuel, the more BTUs per hour you can put out. And sometimes, burning more fuel per hour requires you to pre-heat the combustion air...which is exactly what happens when we draw combustion air from a heated room.

 

[Rockey]

Finally, while running a 2-stroke IC engine leaner will make it run hotter (because the gas-oil mix is also used to cool the combustion chamber in a 2-stroke engine), it is by no means a universal principle that "the leaner you run it the hotter the combustion." In fact, in most cases, just the opposite is true: You need more fuel to make more heat, so the richer you run it, the hotter it gets. The faster you can burn fuel, the more BTUs per hour you can put out. And sometimes, burning more fuel per hour requires you to pre-heat the combustion air...which is exactly what happens when we draw combustion air from a heated room.

We are getting off on a tangent a bit here but in fact at any ratio leaner than stoichiometry the spent gases will rise in temperature proportionally. That is definitely true in 4 stroke and probably 2 stroke but I wouldn't swear on it holding true with 2 stroke because all the data i have seen was associated with 4 stroke engines.


You make a good point about affecting secondary combustion by the cooling effect that the colder intake air may have. I however would like to see exactly how much the colder intake temps would affect the temps inside a stove compared to that of a stove with room air intake. I would really like to see 2 stoves side by side with each having the same number of bio bricks in them, the same stack, same everything else except the intake being an outside compared to an inside and see exactly what the delta T is for both. I would still believe, and i may be wrong, that the differnece is negligable because a smaller volume of cold air would be close in its cooling capability as a larger volume of warmer air.

 

[Highbeam]

Combustion is rolling on at 1500 or so degrees and you are concerned with the intake temperature differential of 50 degrees? BFD. You either heat 70 degree air to 1500 or you heat 20 degree air to 1500, the difference is so minimal that it is not a factor in real life. If you want to academically consider the temperature then you need to also consider that you need less 20 degree air to support the same combustion of a given amount of 70 degree air.

You won't snuff secondary combustion from this slightly colder intake air since the reaction of wood combustion will make plenty of heat. The coldness of outside air is of very small importance in reality.

Taking combustion air from outside the home means that the inside air remains the inside air, remains heated, and you don't need to worry about negative house pressures affecting your other chimneys or affecting the fire. Even on an old house like mine, I benefit from not leaving a window open.

 

[karl]

Would someone please explain the difference from taking cold air from an OAK and heating it up in the stove vs taking cold air from outside via a leaky window or whatever and heating with room air and then sending it through the stove? Either way you're burning wood to heat this air. With the OAK you're not diluting the the house, so I would think the OAK would help efficiency.

Is this not correct?