The Chimney Sweep Online Fireplace, Woodstove, Gas Stove and Barbecue Shop
A conflicting opinion about outside combustion air?
Q: Hello Chimney Sweep, just finished reading your article on using outside air for your wood burning stove at
http://www.chimneysweeponline.com/hooa. I was thoroughly convinced. Then I went to http://www.woodheat.org. Their opinion is almost exactly
opposite of yours. Now I'm really confused. Will you please give me your thoughts on what woodheat.org has to say.
Thanks, Jerry Pestel
Hi Jerry, thanks for the inquiry! We too have spent quite some time on Woodheat.org's excellent site, and we tend to agree with their
opinion about passive outside air intakes that allow "makeup" air to flow into the room to prevent depressurization (you might as well open a
window as install one of those things). What mystifies us is Woodheat's stand against providing outside combustion air directly into the firebox. We
can certainly find no hard data on their website that would support that opinion.
If we understand Woodheat's argument correctly, they don't like direct-to-firebox outside air supplies for two reasons:
Their point #1: A 1989 study showed that direct connection to outside combustion air did not stop smoke spillage during periods of extreme room
depressurization. Simply stated, the study quoted on Woodheat.org's website found that, when air is being pumped out of the room to the extent that
the vacuum overcomes the chimney updraft, an outside combustion air intake would not prevent wood exhaust from being drawn into the room.
Our response: We agree that if you live in a house that develops depressurization to the tune of -10 Pascals (the level used in the study), you will
quite likely experience, among lots of other household problems, a tendency for your fireplace to spill smoke into the room, even if the fireplace is
connected to a source of outside combustion air. However, since the same study found that spillage would occur at -10 Pascals whether or not
outside combustion air was being supplied, we can't see where any kind of case was made against outside combustion air.
Their point #2: If, as shown in the accompanying illustration, the outside air intake were to be installed horizontally through a wall instead of straight
down into a protected crawlspace, and a strong enough wind were to blow at a downward angle onto the side of the house opposite the outside air
intake termination, the combination of wind-induced chimney pressurization and the depressurization that would occur on the leeward side of the
house could cause hot wood exhaust to be simultaneously forced back down the chimney and sucked backward through the intake duct, causing a
potential fire hazard where the uninsulated intake ducting is in proximity to combustibles.
Our response: We reluctantly allow that there might be at least a possibility that this could happen, although to our knowledge there has never been
a single reported incident of fire damage due to reverse airflow through an outside air intake, and here in Washington State all wood, gas and oil
stove and fireplace installations must be directly connected to outside combustion air. Whenever the Northeaster kicks up here in Whatcom
County, we get sustained high winds and 60+ mph gusting for hours at a time, and even this extreme wind condition has never resulted in a single
reported case of fire damage due to air intake backdrafting. In our experience, this "potential hazard" is more theoretical than real.
Even if we were to accept the theory that exhaust backdrafting through the outside air intake is remotely possible in certain extreme wind
conditions, we must realize that, in the absence of an outside air connection to the firebox, this same extreme downdrafting would fill the house with
poisonous wood exhaust. Again, the folks at Woodheat.org haven't made a case showing that burning room air is the better option. And anyway,
wind-induced downdrafting isn't the insurmountable problem they seem to present it to be. We have found that the fix is simple: install a Vacu Stack
draft cap on the chimney, and voila! The harder the wind blows, the greater the chimney updraft.
The bottom line as we see it:
If your house develops depressurization at a rate even approaching -10 Pascals, find out what's causing it and fix it: fireplace back-puffing will be
just one of your problems.
If your fireplace back-puffs when the wind blows, install a Vacu-Stack draft cap.
If you're installing a wood-burning stove or fireplace, consider hooking it up to outside combustion air for all the good reasons stated in the article at
Hang on, gentle reader, we're not through with this topic quite yet:
Q: Love your Sweep's Library, and have spent many pleasurable hours steeping myself in hearth-related lore. I read your comments about
Woodheat.org's anti-outside combustion air stance, and you had me convinced that their arguments didn't hold water. Then I visited the Woodheat
site, and found an article about an actual laboratory test that seems to support their opinion and refute yours! You say you've spent a lot of time on
their site: did you miss this article, or am I missing something?
A: Hi Molly, thanks for the kind words about our Library! If you'll read the article on Woodheat.org's site again carefully, you'll find that there is
nothing whatsoever in the test laboratory's findings that recommends against outside combustion air.
The fact is, the study wasn't designed to compare outside air to room air at all: in fact, it only addressed one issue: whether the outside air supply
systems in the manufactured fireplaces of the day were adequate to prevent smoke from being drawn into the house (a phenomenon known as
spillage) during periods of combined weak updraft and extreme room depressurization.
The study was inspired by a then-new trend in housing design : super energy efficient homes were just coming into vogue, and house plans were
beginning to incorporate forced-air heat exchangers and attic ventilation systems which could combine with kitchen and bathroom exhaust fans,
clothes dryers, etc., and potentially cause brief periods of extreme negative pressurization inside the home. The typical outside air intakes
incorporated in manufactured fireplaces of the day were relatively small, and many introduced the outside air into the circulation air plenum, not
directly into the firebox. The purpose of the testing was to determine if these intakes were supplying adequate combustion air to prevent smoke
spillage into the house during periods of room depressurization of 10 Pascals, which is an almost unheard-of extreme in the real world: to quote the
study, 10 Pascals is "...a rare level of depressurization in most existing houses, although it is attainable, especially in mechanically exhausted
The study determined that the air intakes in the manufactured wood-burning fireplaces back in 1989 didn't supply sufficient combustion air to
prevent smoke from being drawn backward into the house during all phases of the fire if the room was depressurized to this uncommonly extreme
level. To quote the article: "All fireplaces would spill, during fire diedown, if a room depressurization of roughly 10 Pascals was maintained."
The final recommendation of the study was that, in houses that experience depressurization at this extreme level, "combustion air inlets have to be
roughly 2-3 times as large in order to match the fireplace exhaust rate at low burn." In other words, the study called for larger (100 mm was the
recommended size) outside air intakes for the manufactured wood-burning fireplaces of the day. We think this was a valuable study, and have no
quarrel with its findings and recommendations, summed up in this final quote from the article:
"The 100 mm combustion air duct connected directly to the fire chamber can supply the total air requirements for a low burn fire... if the firebox can
be sealed tightly from the room. Once operating, this type of fireplace is relatively insensitive to house pressures, and would work well in houses
where intermittent high depressurization occurs."
Today, nearly a quarter-century after the conclusion of the study, we can easily see the impact its findings have had on the hearth product industry. The airtight
woodstoves and high-efficiency manufactured fireplaces sold today feature tighter-sealing doors and relatively larger outside air intakes that
deliver the air directly into the firebox to combat the effects of room depressurization, even during low fire periods when the chimney updraft is at
Q: Just wanted to say that I appreciated reading your rebuttal to the anti-outside air intake article at woodheat.org. I almost didn't install one in the
tightly sealed greenhouse I built, but fear of asphyxiation led to me to do it anyway. While my own experience has been total satisfaction, it was
good to see a reasoned rebuttal against this argument for the benefit of others who may be misled. FYI, here's the email I sent to woodheat.org:
Before installing my greenhouse woodstove I read your article against outside air intakes. While I was impressed with the apparent scientific rigor
of the article, I continued to be concerned about using a wood stove inside a very tightly sealed building (the 26x20 ft greenhouse is mainly solar
heated, but backup heat is required during cold, cloudy periods). As a result, I installed a 4" round flexible metal pipe through the wall and directly
into the firebox using a fabricated adaptor. The air intake is located on the east side of the building about 2 feet above the ground and is otherwise
not protected. The stove is located on the floor of the greenhouse which mostly has 10 foot interior walls and a 15 foot peak at the ceiling.
I would just like to say that after repeated uses during this unusually cold and cloudy winter, the outside air intake has performed beautifully, both
during heavy winds and calmer periods, with no backdrafts. In fact, the updraft in this stove is superior to that in the house and at times it's like
having a fan assisted draft. I've been amazed at the rapid heat output relative to my house stove. I read a rebuttal of your article at
chimneysweeponline.com which you're probaby familiar with. They link their article to yours in the interest of providing both points of view. Perhaps
you should consider doing the same.
You write more cogently and concisely than the writer on woodheat.org. This usually comes from being smarter, more open minded, and therefore
better informed. Providing a link to contradictory sites is smart. So, to the point.
After studying woodheat.org's outside air vent issue, it is clear that you can solve the problem very simply and make a few bucks in the process.
Simply build an outside vent with a barometric damper (an air diode). This need only consist of a sheet metal flap, balanced so as to be responsive
to a very low pressure gradient, such as that induced by a woodstove fire. The valve will automatically close whenever negative outside air pressure
is stronger than chimney updraft, thereby eliminating the objections presented in the woodheat articles.
You can't get a patent on such a device, since I recall seeing one on the basement ductwork in a house I lived in as a child. But you can contract out
the design and building of various models for sale and make a few bucks. If you get rich enough on the idea to send royalties my way, great! If not,
perhaps you'd send me some free prototypes, or offer a discount on the device when it becomes available.
Thanks for the kind words, and for the input! Visions of vast piles of greenbacks resulting from the successful marketing of the Terrel
Barometric Outside Air Termination Kit were swimming in our heads for a minute there.
But only for a minute.
You see, we've long maintained, and still believe, that wind-induced backflow of burning material through the outside air terminus is not possible.
We can certainly find no evidence of any kind that this phenomenon has ever occurred, outside of the author of the woodheat.org website's
This would seem to present a small marketing problem. Who would you sell this device to, and what would motivate them to buy it?
I want to thank you for your knowledge and very informative website. I have decided to do the outside-air connection near the tail end of a 6 month
long living room remodel project (I guess it takes awhile when you only have one day a week to dedicate to it!). As an engineer, the outside air
connection has been on my mind since day one... but after reading through your site the other night, it helped me to finalize the decision.
I clicked on the link and read the woodheat.org website info. as well, but I don't buy their argument for outside air connections being over-rated. I
am not so concerned about downdrafting as I am pulling cold air into the house. They mention that a house should have all of its air displaced (i.e.,
refreshed) every 3 hours by design anyways... the way I see it, it is one thing to have natural air circulation and currents blow through the cracks
and freshen up the air in the house, but it is a completely different scenario to create a vacuum within the structure and pull in 10-25 CFM of
30-degree outside air while you are trying to heat your house!
Los Osos, CA
Hi, Having done quite a bit of reading on the pro's and con's of hooking up outside air to a sealed combustion system, you stated you never have
heard of back drafting thru the outside air connection. We have heard of two such incidences. One was an outside air connection out the north wall.
Worked fine until a mild spring day and the winds came around the home from the south. The exhaust was drawn down the flue and out the intake
air on the side of the home. Fortunately the home owner was doing some spring yard cleanup and saw the smoke coming out of the intake. He shut
the stove down immediately and we came and disconnected the solid connection from outside to the stove. I have other stove installers in
Wisconsin, Baraboo, that have experienced the same thing. While direct connection may work the majority of times, can you be comfortable that
the 1% that it doesn't work, will not burn the house down? I don't think you or anyone else has a 100% answer for outside air in tight sealed homes.
I don't think direct connect is the answer. I am willing to listen to any data you may have for me and my customers. Thanks.
American Home Fireplace & Patio
"22 years serving your Fireplace and Jacuzzi needs"
Thanks for the input! Actually, our website doesn't say we've never heard of the CONCEPT of outside air intake backdrafting: what we say
is we have never seen a video, a photo, a single shred of after-event evidence, or even direct testimony from anyone who has personally witnessed
this phenomenon. And, more to the point, we state that we have never been able to find a single case where a fire department, Fire Marshall or
insurance adjuster has ever cited outside air intake backdrafting as the cause of a fire.
You ask, "While direct connection may work the majority of times can you be comfortable that the 1% that it doesn't work, will not burn the house
down?" Lynn, I wonder where you got that 1% figure? All woodstove installations in mobile and manufactured homes in all states have required
direct connection to outside combustion air for over 26 years now. All woodstove installations in ANY type of home here in Washington State have
required direct connection to outside combustion air for over 12 years. In order to qualify for Oregon State's Wood Stove Energy Rebate, the
woodstove must be direct connected to outside air. Wood stove manufacturers who offer outside air adapters report that between 50% and 66% of
their customers order them with the stove. This is hundreds of thousands of outside air installations. If, as you seem to be implying, 1% of these
installations result in a house fire, we would be reading about thousands and thousands of OA backflow fires every year.
But that is not the case. As stated above and elsewhere on our website, we've never seen even one such fire report. And neither has any hearth
product shop owner, installer, Chimney Sweep, Fire Marshall, fireman, or insurance adjuster we've ever asked. Therefore, we conclude that the
potential fire risk associated with direct connection to outside combustion air to be exactly.................... 0.
Let me turn your question around for a minute. According to your signature, you've been in the business for 22 years. In that period of time you've
been told by two people that they've witnessed OA backdrafting. Neither of those reported incidents resulted in a fire or other damage to the home
or the people within. Why would you conclude that direct connection to outside combustion air is dangerous?
Don't get me wrong, I love your website, and have looked to it many times for reasonable answers to my wood-burning questions. But your answer
to Lynn Meyer's concerns about outside air kit backdrafting comes across as unreasonably harsh in my opinion. It seems Lynn is simply willing to
err on the side of caution to protect her customers from a potentially dangerous situation she believes could happen.
Duane, I just re-read my response to Lynn, and you have a point. The reason I might have been a trifle short with her is that, as a 22-year
hearth product industry veteran, she knows there are many very real and documented benefits outside air kits can provide, including eliminating
life-threatening situations such as poisonous exhaust fumes being drawn into the breathing space. My unspoken (unwritten?) question to Lynn was,
how can you take away the health benefits OA kits can provide without any hard evidence whatsoever that OA kits can be hazardous?
To read our article about the value of outside combustion air, click here.
To read Woodheat.org's opinion about outside combustion air, click here.
To read the article about the laboratory study of outdoor air supplies for manufactured fireplaces, click here.
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