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How to Provide Makeup Air for a Wood Stove

Should your wood stove get ducted outdoor combustion air direct to the firebox, or to a location near the stove — or perhaps no ducted outdoor air at all?

Posted on Dec 19 2011 by Scott Gibson

Wood stoves used to be pretty uncomplicated devices. Even though they weren’t airtight and they weren’t especially efficient, these cast-iron stoves warmed plenty of New England farmhouses in the dead of winter.

Our forebears never considered the source of makeup air to replace all the heated combustion gases that were going up the flue. They didn’t need to, because back then, houses were leaky. As the stove burned its load of oak or maple, makeup air had no trouble finding its way into the house.

In the era of airtight construction, however, a wood stove is a different animal altogether. For one thing, stoves are more efficient. For another, the current emphasis on air sealing has reduced the number of cracks and leaks that were traditional sources of makeup air.

Writing in a Q&A post at GreenBuildingAdvisor, David Meiland delves into the problem by describing two common methods of solving the problem.

“I've been wood stove shopping lately and noticed that ‘outside air’ for wood stoves is done in a couple of different ways,” Meiland writes. “The European stoves in general do not seem to have a direct connection for the outside air duct — it's what the salespeople are calling ‘proximity’ air, meaning a 3-inch duct from outside terminates very close to the stove, but does not connect.

“The American stoves are much more likely to have a direct connection. To me, a direct connection makes perfect sense, whereas the ‘proximity’ air looks like an air leak.”

Anyone care to venture an opinion?

Adding a trap to control air intake

John Klingel is among those who wonders whether a trap could be installed in the incoming air duct in those cases where the stove is set up to use proximity air.

Much like a P-trap in a plumbing drain prevents sewer gases from backing up drain lines and getting into the house, an air trap might block unwanted cold air from getting into the house when the stove wasn’t operating.

Klingel passes along a link to a trap design from the City of Fairbanks, Alaska. He adds that a trap he and his son installed in a boiler room wall “seems to be working fine; no cold air in, no icing outside, indicating moist, warm air leaving.”

Michael Maines had heard much the same thing from a well-respected home inspector who once suggested a trap in a line for makeup air for the boiler. “He said that cold air would settle in the bottom of the trap and minimize air leakage,” Maines writes. “The concept makes logical sense to me but we didn’t do it on that house, and I’ve never seen an actual example. Is his logic flawed?”

Indeed it is, says GBAGreenBuildingAdvisor.com senior editor Martin Holladay. “’The ‘trap in the line’ concept works with plumbing drains, but it doesn't work with air intake ducts,” Holladay writes. “If air is leaking out from the top of the house, air will enter a leak at the bottom of the house — whether or not the leak at the bottom of the house has a trap in the line. A hole is a hole.”

To verify his understanding of the P-trap myth, Holladay contacted Gary Nelson of the Energy Conservatory for a second opinion.

“You're right,” Nelson tells Holladay. “The P-fitting maybe increases the resistance slightly, but not much. That ‘trap’ should not be called a trap; it should be called a siphon. It doesn't act like a trap at all. The reason some people like it is that when they install a so-called trap, they usually terminate the pipe so it aims at the ceiling.

“A pipe without a trap is usually aimed at the floor, causing the incoming cold air to pool at the floor. The pipe with the trap — the one aimed at the ceiling — allows all that air rushing into the house to mix with the air in the room before it falls to the floor, so the temperature of the air doesn't feel as cold by the time it reaches the floor compared to a pipe without a trap aimed downward. But just as much air is coming in, in either case.”

One argument in favor of proximity air

There is one reason that a duct dumping outside air near the stove might be better than a direct connection, Holladay says, and that’s the danger of fire.

“A few house fires have occurred in homes with outside combustion air intakes that connect directly to the wood stove,” he writes. “Here's the scenario for trouble: a gust of wind causes backdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney., and hot coals blow into the air-intake duct. Some homeowners have used PVC for these ducts — a bad idea. Obviously, metal ducting is safer, but hot coals in the air intake duct are always scary.”

Even so, Lucas Durand is planning on installing an airtight stove and in anticipation ran a 3-inch ABS line under the footing and slab that he will hook up to the stove with a kit provided by the stove manufacturer.

“Martin is right though that there have been fires caused by backdrafting into the air supply...” Durand adds. “I haven't been too worried about it, though. Chimney draft in my case will be pretty strong... Whatever route you choose, just don't run the stovepipe out through and then up the outside of an exterior wall.”

Dick Russell also has a direct connection for his wood stove, a deciding feature when we bought it. “Of course a direct connection brings in only the air that the stove needs, and not additional cold air according to how leaky the house is and the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. of its configuration,” Russell writes. “But the other benefit is that you don't have all that cold air dumping into the house when the stove burns out overnight, and any other time when the stove is not burning and the outside air damper is not closed (if there is one).”

The European perspective

Is it all much ado about nothing? That’s the impression Jesse Thompson got after speaking with representatives of Jotul, a Scandinavian stove manufacturer.

“They say very clearly that in Scandinavia, houses with HRVs [heat-recovery ventilators] and balanced ventilationMechanical ventilation system in which separate, balanced fans exhaust stale indoor air and bring in fresh outdoor air in equal amounts; often includes heat recovery or heat and moisture recovery (see heat-recovery ventilator and energy-recovery ventilator). don't use outside air intakes, that it's a ‘strange Canadian thing’ that causes more problems than it fixes,” Thompson says. “They mentioned the same issue as Martin: backdrafting into your fresh-air intake is disastrous and a real fire hazard.

“The idea is that if you have a balanced ventilation system, your house will be at the same internal pressure as the outside, and you won't get the strong pressure differentials that can create back-drafting. As well, they quoted very low cfm needs for the combustion in modern EPA stoves, I remember 15 cfm? In any case, it's much lower than a big tube through your wall would be providing and doesn't need additional supply beyond normal house leaks.”

Our expert’s opinion

GBA technical director Peter Yost added this:

I thought this was going to be easy: just go to the EPA website on their labeled wood stoves and download their guidance on make-up air for high-efficiency wood stoves. It turns out I could not find even a mention of this issue there! So I emailed and called the designated contacts listed on the EPA website and got this response: “Sorry but we don't have any regulation or guidance on outside air for wood stoves.” That’s it. No, “we are working on this issue,” or “no worries, make-up air is not really an issue.” Now, while I appreciate the heartfelt apology for the lack of information on the subject, I was expecting something a bit more substantive and proactive.

But there are those who seem to be very forthright in handling the issue. According to John Gulland of the Wood Heat Association:

“The supposed benefits of outdoor air are not supported by research. Laboratory and field reports have revealed that providing outdoor air is not a simple or effective cure for spillage, and that some designs could create a fire hazard.”

Unfortunately, Mr. Gulland does not provide any citations for the research studies and laboratory reports supporting his unequivocal position. And placing fireplaces and wood stoves in the same conversation about combustion make-up air is far too vague and even disingenuous; we need a focus on combustion air for high-efficiency wood stoves.

And then there is a very interesting direct rebuttal to Mr. Gulland’s article on the Chimney Sweep website. “The Chimney Sweep” also deals with both fireplaces and woodstoves (the former having MUCH higher combustion air requirements) but feels that dedicated make-up air is both important and safe, when designed correctly to deal with any windy-day downdrafts in the flue.

So, I see two related questions:

First, how much combustion air do high-efficiency wood stoves really draw?

I could not find a study for this, but found several websites that state the “typical” draw for high-efficiency wood stoves as low as 10 cfm with a focus on about 15 cfm. That is mighty low, but sure seems as though they would have a range of draw extending well above 15 cfm.

Second, given that they draw so little, do high efficiency wood stoves need dedicated combustion air or can we just treat this draw as incidental and deal with it through inadvertent background air leakage/exchange?

Of the three ways to try and provide make-up combustion air, passive air intakes just don’t work — they are not really dedicated to the wood stove and Martin Holladay has adequately addressed the general performance issues with passive air inlets.

I think the same issues can apply to “proximity” combustion air ducts for high efficiency wood stoves — they are not really dedicated to the pressure issues of the wood stove system and therefore are more subject to a wider range of pressure issues the whole house is experiencing.

Moreover, a dedicated metal duct for combustion air can be laid out and outfitted with a back-pressure damper to properly supply the wood stove, prevent house pressure variances from interfering with wood stove draw, and eliminate the opportunity for embers from the stove making their way into the combustion air duct. I am partial to the Tamarack Cape backdraft damper. It operates with pressures as low as 4 pascals and when located at the building enclosure boundary, it is located away from the firebox.

Finally, I frankly don’t know if draws as low as 10 cfm really need dedicated combustion air, but my gut tells me that over the range of interior and exterior performance conditions a high-efficiency wood stove will face, a dedicated combustion air duct is a very good idea.


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Image Credits:

  1. Justin Fink/Fine Homebuilding magazine
1.
Mon, 12/19/2011 - 17:41

Stoichiometry
by K Willets

Helpful? 0

You can calculate the draw from the stoichiometric ratio. Dry wood is 6.1:1 (http://wiki.gekgasifier.com/w/page/6123822/Stoichiometric%20Combustion%2...). Dry air at 70F is 0.075 lbs/cf, so one pound of wood burned over an hour will take in 81.3 cf of air, or 1.36 cfm.

Humidity adds to that of course, and the stove will likely admit a higher ratio of air.


2.
Tue, 12/20/2011 - 08:28

Edited Tue, 12/20/2011 - 08:30.

Very Interesting
by christian corson

Helpful? 0

I received this from a friend (quoted in this article) a short time ago.
http://woodheat.org/outdoor-combustion-air-in-the-canadian-national-buil...
This is a very interesting conversation. So, There is NO data relative to combustion air in super air tight construction? I am talking Passive House and beyond...... .2-.6ACH/50. Now, I have to thank Kendall here..I get that if you only need 1.36 ish CFM to combust (completely), one pound of wood that you may not need make up air if the ambient mechanically ventilated airspace can with a supply rate of 80CFM can support burning...............10 lbs,20 lbs of wood at a time?
But what I dont understand is how when you close the door of the wood stove and that said door is truly airtight, where does the combustion air come from? .............IT CANT COME FROM THE HOUSE.
My gut tells me as well, that I want a well designed thoughtfully dampened make up inlet. I also want it connected to the stove. At the very least in new construction, I would want it available for the possible future use; so when that "oh shit" moment happens; when the fire goes out while someone decides they want to dry their clothes in the clothes dryer sucking 110cfm. i can then send someone over to connect it.
Thoughts?
This is a very interesting question.


3.
Tue, 12/20/2011 - 09:06

Response to Christian Corson
by Martin Holladay, GBA Advisor

Helpful? 0

Christian,
I think that even in a Passivhaus, when you close the airtight door on your woodstove, the fire gets its combustion air through the (dampered) air inlet that every stove has. This air inlet is usually part of the door itself. And yes, there should be enough air indoors to supply the fire with oxygen -- just as there should be enough air indoors to supply you with oxygen.

Moreover, when a powerful exhaust fan is turned on, reversing the flow of combustion gases up the chimney flue -- something that can easily happen with a range hood fan -- the fire won't go out. Rather, the combustion gases will be sucked down the chimney, and the increased draft will probably cause the fire to burn hotter, not go out. You'll probably see a puff of flame coming out the door of your wood stove.


4.
Tue, 12/20/2011 - 23:27

Edited Tue, 12/20/2011 - 23:29.

CO2
by James Sarcia

Helpful? 1

I insulated my attic over the weekend. Also, I added a new boiler in the basement with a power vent last summer. Sunday, our first fire of the year in my Jotul 500, was a struggle all day. I got continuous smoke rolls in the house even with the vent open all the way and opening the door slowly. I finally got it going good by opening a window. I closed the window that night to go to sleep. The next morning my wife put clothes in the dryer and jumped in the shower with the bathroom vent on. Then the heating system kicked on. A minute after that all the CO2 detectors went off. We evacuated and opened all the doors to air out. I determined the smoldering fire and the negative pressure in the house sucked the gas down the flue into the house. I definitely need makeup air. I would love to see a plan or get instructions for the wood stove and my boiler. Any suggestions?


5.
Wed, 12/21/2011 - 00:49

Outside Air Kit
by K Willets

Helpful? 0

James, the Jotul 500 has an outside air kit, which direct-connects to the stove.

http://www.jotul.com/en-US/wwwjotulus/Main-menu/Products/Wood/Wood-stove...


6.
Wed, 12/21/2011 - 01:15

Airtightness
by K Willets

Helpful? 0

As I understand it, an EPA stove can't be completely closed off. The various joints and seals in the stove are airtight, but the air supply has to be sufficient to keep the stove burning cleanly. Since the stove doesn't know if it's burning or not, there's no "off" setting.

I'm not sure if that's an explicit regulation, or a side-effect of the clean burning requirement, but it definitely interferes with the ability to shut down the stove, or close the air leak when it's not operating.


7.
Wed, 12/21/2011 - 06:11

More on outside air kits
by Martin Holladay, GBA Advisor

Helpful? 0

There is an interesting document containing information on outside air kits at the Hearth.com website:
http://www.hearth.com/econtent/index.php/wiki/Outside_Air_Kit_-_Why
"The Outside Air Kit (OAK) enables a stove to pull air from outside of the house rather than from the room the stove is sitting in. Unless you have a draft problem (difficult starts and backpuffing), an OAK probably will be of no value to you. Some argue that OAKs increase overall heating efficiency because the stove is no longer pulling cold outside air indirectly though the entire house but instead is pulling directly from the outside. However others would argue that home ventilation is actually very important, especially in modern tight houses, and that the effects of an applicance slowly replacing indoor air is actually quite beneficial."

More information on Jotul's outside air kits can be found here:
http://www.jotul.com/FileArchive/Technical%20Documentation/Wood%20Stoves...

Jotul outside air kit.jpg


8.
Wed, 12/21/2011 - 08:35

Martin
by christian corson

Helpful? 0

Thank you for your response. First I realize that all wood stove have dampers. Those dampers allow air into the stove in a regulated fashion to allow for effective combustion. Ok, granted. If you need an additional 5-25 cfm for combustion which to me in reality seems low( I think these numbers are based on truly seasoned hardwood 2+years seasoned, ) Segue, to occupancy habits for a sec. Most people do not burn seasoned hardwood. they are sold "seasoned" wood that is one season old...three months. This greatly affects the amount combustion air necessary to make the stove operate, thus increasing the amount of make up air necessary.
Second. Passive House. Having designed and built a few,what I consider to be pretty airtight homes, I have noticed a couple of things. Besides the obvious and primary benefit of air tight construction; increasing the durability of the structure(i am a wood guy, we build with wood)by mitigating air infiltration and thus controlling moisture infiltration as well. There is the comfort factor. Now i am talking homes < .8ACH/50. There are NO drafts. The natural comfort of the house is increased considerably by adding high quality triple glazed windows thus eliminating even naturally occurring convective currents. Fine. Now we need to ventilate, so we do that in a balanced way while adhering to the ASHRAE 62.2 standard. Now that being said, we have established a ventilation rate of say 84 CFM for a two bedroom two bedroom home of 1600sf. 24 CFM from each bathroom 35CFM from the Kitchen and then match the supply, voila. Now 25CFM which to me is a LOW number for combustion is one third of the entire ventilation rate. That air is designed to be breathed by the occupants. Occupants trump wood stove. Another note.
"even the best rated airtight firebox will still have minimal leakage and secondly, the ventilation system could potentially significantly depressurize the house, if for example one of the two fans breaks. If the failure goes unnoticed until the house has been depressurized smoke can be drawn into the living space from the firebox or stove, no matter how airtight it is."

Lastly ......."Moreover, when a powerful exhaust fan is turned on, reversing the flow of combustion gases up the chimney flue -- something that can easily happen with a range hood fan -- the fire won't go out. Rather, the combustion gases will be sucked down the chimney, and the increased draft will probably cause the fire to burn hotter, not go out. You'll probably see a puff of flame coming out the door of your wood stove."

This is a backdraft, and it is very dangerous. I dont want gases coming out of the chimney into the house.
I am in a hurry, and trying to quickly have a thoughtful discussion about this, so forgive me if I left out something obvious.
cheers


9.
Wed, 12/21/2011 - 08:47

Response to Christian Corson
by Martin Holladay, GBA Advisor

Helpful? 0

Christian,
You wrote, "This is a backdraft, and it is very dangerous." No kidding! That's why I advise people that powerful range hood fans are incompatible with wood-burning fireplaces, wood stoves, or atmospherically vented water heaters or furnaces.

More information here: Makeup Air for Range Hoods.


10.
Thu, 12/22/2011 - 00:08

Edited Thu, 12/22/2011 - 00:08.

Saskatoon Loop
by Greg Labbe

Helpful? 0

Around here (Toronto), the combustion air inlet 'P trap' was known as a "Saskatoon loop" and though the idea was promising, it didn't take alot of pressure to overcome the dense air "trapped" at the bottom of the loop.. maybe if the P was really deep.. Not used much anymore.

Regarding Passive houses, do wood stoves belong in PH?


11.
Thu, 12/22/2011 - 07:02

Outside air and HRVs
by Ben Hemberger

Helpful? 1

I've been round and round with this issue on the Jotul stove we're planning to install soon in a little 1550 sf house that is built tight, but vented with an HRV. Our first instinct, going on advice from the HVAC folks, was to install makeup air.

We encountered a little trouble as to exactly how much air the little stove would need, and in calling a Jotul representative directly, were told we'd actually need a damper on the metal chimney flue to keep the stove from overheating! His thinking was that the straight shot of 25' metal chimney that is all interior(first floor through second floor and cathedral ceiling to the peak) would become a strong enough updraft to suck more air through the stove than was intended.

My fear is that when she turns on her kitchen hood, she could backdraft the stove, but it sounds like the HRV is essentially setup to allow more air through already, so why poke another hole in a house we're trying to keep tight!

Based on numerous discussion with everyone we could talk to that had an official or informed opinion is that the HRV will allow enough makeup outside air. No one can give a definitive answer as to how this exact setup will work in this particular house, so before cutting another hole in the side of the house, we'll install the stove and monitor it's behavior closely, adding a duct for makeup air only if necessary.


12.
Thu, 12/22/2011 - 08:58

Edited Thu, 12/22/2011 - 09:13.

Martin- Thanks.That was
by christian corson

Helpful? 0

Martin- Thanks.That was funny. I actually laughed out loud as I was waiting for a sandwich yesterday. I understand what you are saying. My trepidation is relative to PH specifically. Can a dryer vent cause a backdraft in truly airtight construction? I dont know a bout that, probably not. Could it prevent the stove from completely combusting..........Maybe? I think with the combination of unseasoned wood ,low volume air supply, occupancy habits, and a dryer thrown in there, raises some valid questions.
Greg
In the first PH that we built I really pushed for a condensing dryer and NO wood stove. We have double sized our heat load with a small ASHP in a 7345 HDD environment. So...that being said, a wood stove is totally unnecessary. However, I am one of those guys who is ALWAYS looking for a different solution. Even when the solutions we have are adequate. To me that is how we evolve.
There are a few things that appeal greatly to me in regards to wood stoves in PH.

1st -By returning to biomass we can further reduce our primary energy demands. What if one day in my lifetime I can design and build a house that has a SPED(specific primary energy demand)of 0. That is probably not possible, but it is a cool idea and worth working towards.

2- If the power goes out there is not only heat but the capacity for boiling water and baking bread.

3- increases the potential for off grid living.

4- Client driven requests. I dont want to put a fireplace in any home. So.... when I have a prospective client say " I am a leo, and I have to have fire" I think Rais Rondo w/ the turntable. hence these other concerns

final note- as far as I know PHIUS strongly urges to stay away from wood. If wood is used they are adamant about make-up air. I am on the fence myself, I can definitely see both sides. And I feel there are VERY valid points but NO actual "real life"data. Having seen how "air tight " construction performs, I tend to think make up air MIGHT be needed in certain situation.

We built a house 7-8 years ago with a masonry heater (with no MU air) and the clients still tell me we love the house everything is great (we never had a single call back) but we cant get a fire lit unless we open the dining room window. It still bothers me. Even though it is in a predominately cooling climate and that heater gets used probably twice a year. Christmas and New years.


13.
Thu, 12/22/2011 - 09:53

Edited Thu, 12/22/2011 - 09:58.

More on Passivhaus and wood stoves
by Martin Holladay, GBA Advisor

Helpful? 0

Christian,
1. As you know, most Passivhaus advocates advise against the use of a wood stove in a Passivhaus building. For those who live in rural, forested areas of the North, that raises many questions -- including, "Why bother with Passivhaus? A small wood stove meets my needs, and I know it works. So I don't need to aim for 0.6 ach50."

2. Every HRV and ERV manufacturer is in agreement on one point: they all explain that an HRV is not intended to provide combustion makeup air because it is a balanced ventilation appliance. Air in = air out.

3. Nevertheless, Passivhaus advocates (and others) often overstress this point. I call it the "smart air fallacy": namely, the false belief that air always follows the "smart arrows" you see in the ventilation diagrams. In fact, even in a house with an air leakage rate of 0.6 ach50, makeup air usually finds its way in to the house when a bathroom exhaust fan is turned on or when wood smoke goes up the chimney. This air comes from all the usual cracks around windows, doors, and framing members, as well as through the ventilation intake ports. If the house needs more makeup air, it finds its way in.

4. Nevertheless, if an exhaust fan is big enough -- the classic example being a huge range hood fan -- atmospherically vented appliances will backdraft. That's why we should be very careful to keep exhaust fans as small as possible, and to whittle down (or even eliminate) the number of atmospherically vented appliances in our homes.


14.
Sun, 12/25/2011 - 09:16

Modulating air loads
by shane claflin

Helpful? 0

With all of these variables, a tight home construction needs comprehensive air management. An HRV at one setting will suffice for a woodstove alone, however, with other de-pressurizing appliances operating, more air is needed. Hence, an HRV that can modulate with the air-load requirements of the home. Do these exist? Or, alternatively, an OAK for the stove.


15.
Sun, 12/25/2011 - 15:24

Makeup air via HRV
by Dick Russell

Helpful? 0

At least with some HRVs, if it is on when some other exhausting device is operated, the in/out air flow via the HRV will become unbalanced and tends to allow more air in than is sent out. I know this is the case with the Lifebreath 195ECM that I have. When the blower door test was done, we had to turn off the HRV, of course. With it on, I could feel the difference in the air temperature coming in while the blower door fan was running. With the HRV off, an internal damper closes the outside air flow so as to prevent unwanted air intrusion.

A woodstove is a mildly exhausting device, in that there is a strong stack effect due to the large difference in flue gas density vs that of the combustion air entering. Still, I would expect that a strong exhausting device (the proverbial huge range hood fan) could backdraft a woodstove without a directly connected OAK, despite any tendency of an HRV to makeup air by running out of balance.


16.
Mon, 12/26/2011 - 23:29

Have had outside combustion air source for several years....
by William Ridley

Helpful? 0

I have a Jotul 500 and used it for a couple years without an outside combustion air source, and it worked well. Then I installed a metal duct from the outside to supply combustion air to the stove using the connection kit from Jotul. It works even better now than before. I have a pretty tall interior chimney (30') with a strong draft and have never noticed any backdraft issues mentioned above with hot embers being puffed back into the intake duct.
I only have anecdotal evidence but I am convinced that when we use a humidifier now, it seems to have to run less often because of the combustion air supply for the stove being in place now.
Another added benefit in my mind is that when the fire goes out there is not a supply of warm house air slowly drifting up the chimney being pulled by the natural draft of the chimney. All it can do now is suck some cold air from the outdoor air supply! I am sold on using a dedicated and connected combustion air supply for a woodstove.


17.
Mon, 02/20/2012 - 00:26

Clarifying
by Charlie Weiss

Helpful? 0

My understanding of the proximate air duct is that outdoor air, usually cold, when added to the hopefully hot firebox, reduces the fire temperature and so the efficiency of the burn, increasing pollutants. After reading everything out there, I will soon be adding a proximate air port to the exterior wall behind my woodstove. Yes, in a <1 ACH@50 house, without the port we absolutely have to open a window (4' tall by at least 2" open) to start a draft. Once started, less outdoor air is required. This discussion clarified for me that an HRV can most effectively heat the same amount of air it is exhaling -- and if exhale is happening faster due to wood stove or range hood, the incoming air will just get colder. The proximate stove air, with the outlet fastened at the back baffle of the stove (about 1" from the back of the firebox) will allow the stove to pre-heat the air as it enters the space, without compromising the fire. That seems good -- but will require someone (me) to remember to close the port when the stove is cold. Imperfect but this seems the best option when you want a fire in a tight house.


18.
Sun, 11/18/2012 - 12:39

Flow rate based on stoichiometric ratio?
by Peter Mullen

Helpful? 0

It appears as if the low flow rates tossed around are based on the stoichiometric combustion ratio mentioned at the beginning of the thread. Won't a fire draw a much larger volume of air than the simple ratio? In addition to the chemical reactions between the fuel and the air, there is expansion due to heat which pushes gases up the exhaust and draws in a correspondingly larger volume of air.

In a sealed fireplace with combustion air supply, this isn't an issue. However, my particular problem is with a wood-fired oven. I have a tight house and am hoping to minimize the volume of interior air exhausted due to burning in the wood oven. To that end, I have constructed a makeup air apparatus consisting of a 4" duct vented to the outside and terminating in close proximity to the mouth of the wood burning oven. Spent some time trying to figure the fire draw, but in the end just went with a 5" duct on a guess.


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