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Sweep's Library:
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 this 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 such incident, 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 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
http://www.chimneysweeponline.com/hooa.
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?
Molly Murphy
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
dwellings."
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."
Fifteen years 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
its weakest.
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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:
****************
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.
Sincerely,
Tom Kara
Norwood MO
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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.
Terrel Miedaner
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
imagination.
This would seem to present a small marketing problem. Who would you sell this device to, and what would motivate them to buy it?
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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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