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 GBA 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 backdrafting, 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 effect 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 ventilation 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.