Outside Combustion Air - Does It Make a Difference?

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Dipsea

New Member
Dec 29, 2010
4
NW Montana
I have a large river rock fireplace in a 2500 square foot home in Northwestern Montana. The home is a well made log structure with a 1500' main level and a 1000' loft open to below. I've been researching wood inserts (this website has been invaluable). I came into this with what I thought was the conventional wisdom that an outside air source supplied directly to a firebox sealed from room air was necessary to prevent the loss of heated (and humidified) air from the room, among other things.

To my surprise, only a few of these inserts make a provision for this. One is the Pacific Energy Summit insert - and I even found a posting here that asserted that provision was a farce. Some research produced this document http://www.woodheat.org/outdoorair/outdoorair.htm claiming that outside combustion air is unnecessary (although in reading this, it seems to me it may be somewhat subjective, I can't tell for sure). Still, given that manufacturers aren't putting a high priority on outside combustion air I'm leaning toward the idea that it may not be necessary.

Does anyone have a link to more objective research on this, or a concise explanation for either case?

Thanks.
 
Manufactured homes like mobiles and double wides built to HUD specs require it. Even gas fired water heaters must be of the sealed combustion type. It's a good thing to use if you already have one. Lots of air is pulled through any burner or fire, and that air has to come from somewhere. Depending on exhaust fans in a building, it becomes more necessary. A large commercial type range hood, electric or gas dryer, and bathroom exhaust fan can depressurize a building quick. If you have a burner or solid fuel appliance set up that requires barometric air pressure at it's intake, you have far less than the air required with mechanical fans on.
I have a wood stove with a 3 inch PVC pipe in the basement that feeds it air up through the pedestal. That pipe frosts up on cold nights to the point of needing gutter under it to catch the melting condensate. That's a lot of cold air that would be pulled through the house from every crack it can find. I'm sold on sealed combustion to the exterior air source.
 
Welcome Dipsea,

This is an active topic of frequent discussion. The answer is "Yes, but you have to decide if it's for you." Personally, think the reduction in drafts and heat and humidity loss is worth it, YMWillV.

Search the threads and articles for vigorous discussion and debate!

Happy burning!
 
I read "large river rock fireplace" and I am wondering how big the fireplace opening is. My small stove fits completely within the fireplace opening - a very safe place to put a stove - and it has a double jacketed firebox with a blower so it moves most of the heat into the room. Why not consider something like that if you have room to fit the whole woodstove into the fireplace opening?
 
I think that your fireplace flue will have a much different air flow charectoristic than a stove pipe will. If you are lining the chimney in order to facilitate the insert than your air flow will be quite different. Fireplace flues are very large and that causes the mass exodus of air from the house. It is quite uncontrolled. A stove or insert should have better air intake control, thus exhaust control making fresh air not a black and white issue. I believe the earlier reference to mobile homes and required fresh air is based on the limited cubic capacity combined with the air tight construction of such units. We have all read about people found dead in small cabins or campers because of asphyxiation. The fires ate all the oxygen in the space. It is not a completely bad thing to introduce fresh air into the house through minor air flow points in multiple locations.
 
mtcates said:
This link sums up the pros and cons of using outdoor air supply.

http://www.woodheat.org/outdoorair/outdoorairmyth.htm
Click to expand...
That is a popular link that naysayers like to use. His use of "Myth" in the title is the first clue that it is biased. I think it is a bit of half-truths bundled up as junk science and can say I've never had any issues with wind against my OAK. Many houses have significant pressure deficits from leaks and stack effect and the interior pressures could closely approximate the leeward outdoor pressure depicted in that article.

If the draft on the flue is so bad that wind would reverse it, you need to address it and OAK may not be for you. An OAK does not cure all that ails.
 
Thanks for all the thoughts and articles. After reading everything you guys suggested and anything else I can find, I would prefer to go with outside air to a firebox sealed from room air.

Next problem - Where do I find such a beast? I want a large insert - something designed to heat 2000 sq. ft. plus. The only one I've found so far engineered for outside air is the Pacific Energy Summit. Although, there seems to some question whether it's actually sealed from room air (https://www.hearth.com/econtent/index.php/forums/viewthread/61775/). There seem to be plenty of freestanding stoves that meet my criteria, but I can't find inserts that do. I do have outside air to the fireplace cavity (not the usual ash removal door, but two vents that empty in the center of the side walls - these were designed into this mid 1980's fireplace).

Anybody have any suggestions?

Thanks.
 
Dipsea said:
Thanks for all the thoughts and articles. After reading everything you guys suggested and anything else I can find, I would prefer to go with outside air to a firebox sealed from room air.

Next problem - Where do I find such a beast? I want a large insert - something designed to heat 2000 sq. ft. plus. The only one I've found so far engineered for outside air is the Pacific Energy Summit. Although, there seems to some question whether it's actually sealed from room air (https://www.hearth.com/econtent/index.php/forums/viewthread/61775/). There seem to be plenty of freestanding stoves that meet my criteria, but I can't find inserts that do. I do have outside air to the fireplace cavity (not the usual ash removal door, but two vents that empty in the center of the side walls - these were designed into this mid 1980's fireplace).

Anybody have any suggestions?

Thanks.
Click to expand...

I'll give you my thoughts, and wiser souls can interject as they wish.

I'm wondering if you have a few options here.

The first one would be to find an insert with an easy connection for outside air, as you asked.

Barring that, I wonder if, compared to most, you aren't already ahead of the outside-air game, by having outside air inlets in your cavity. I'm guessing most inserts would happily use that air, with your main problem being outside air leaking into your house when the stove is off, and possibly the stove still sucking house air when burning.

One way you might be able to fix that is by making an airtight seal around your insert surround. Perhaps others can say whether that is reasonable.

Another solution might be to rig a cable-operated shutoff on those vents, so you could close them when the stove is off, just like with a flue damper on a fireplace.

Just some thoughts. Good luck and happy burning!
 
Oh, I like that idea. If the insert draws some or all of its combustion air through the front, then at least some of the outside vent air will find it's way into the firebox and reduce the amount being drawn out of the room. If the insert draws all of it's combustion air from inside the fireplace cavity, then I can seal the surrounds to the masonry and so much the better. The existing outside vents have convenient sliding closures on them, so your cable idea should be simple to hook up if necessary.

I can't really tell where these things draw from the info I've found on the web - but, really, either situation sounds workable to me.

Thanks again.
 
I would think that by sealing the front or trim of the insert fairly well,and then not creating an absolute seal on the chimney around the flue or liner,that most inserts would tend to draw outside air down the chimney.Obviously this is not the case with most freestanding stoves,but it would apply to most inserts.Of course if I am incorrect someone will enlighten me.
 
mtcates said:
This link sums up the pros and cons of using outdoor air supply.

http://www.woodheat.org/outdoorair/outdoorairmyth.htm
Click to expand...

Also note that the link incorrectly assumes the OAK has to have only one intake. If your house is situated as such for high winds you can easily design OAK intakes for multiple intake points as to eliminate the low pressure zone that would cause the downdraft effect through the stove.
 
tommystunes said:
I would think that by sealing the front or trim of the insert fairly well,and then not creating an absolute seal on the chimney around the flue or liner,that most inserts would tend to draw outside air down the chimney.Obviously this is not the case with most freestanding stoves,but it would apply to most inserts.Of course if I am incorrect someone will enlighten me.
Click to expand...

Hey TT,

Your idea is creative, but may not be the best way to get outside air. I'm thinking that a problem is that it in effect creates a second chimney, well heated by the flue/liner. This will tend to create suction rather than pressure at the stove inlet, fighting the real chimney's draft. But good on you for being creative and putting your ideas out there!

Happy burning!
 
Dipsea said:
I can't really tell where these things draw from the info I've found on the web.

Thanks again.
Click to expand...

You're very welcome, Dipsea, I hope the idea actually is a good one. :)

Can experienced folks clarify where inserts generally draw from? From the front, from inside the recess, or random?
 
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