The Chimney Sweep Online Fireplace, Woodstove, Gas Stove and Barbecue Shop
Sweep's Library: Why burn outside combustion air?
Q: I read the following from your page, "... all woodstoves and fireplaces installed in Washington must have the ability to draw their combustion air
directly into the firebox from outside the house (this eliminates potential health problems deriving from living in an oxygen-poor environment in
Winter months when the woodstove is burning...)" I can understand how a vent free gas fireplace would reduce the oxygen level, as it would
consume the oxygen, then vent the exhaust into the house. I can't see how a wood fireplace could reduce the oxygen level, as it takes in air from the
house then vents it out of the house. The only way it could reduce the oxygen level would be if the house was so tightly sealed that it actually
reduced the air pressure in the house. I am not sure if this is even possible. Let me know your comments. Maybe I am missing something.
- Bryce
A: A wood fire consumes a VAST amount of oxygen, and the chimney updraft simultaneously vacuums a HUGE amount of both burned and
unburned air out of the house. This evacuation of air is known as the chimney flow rate, and it is measured in cubic feet per minute (cfm). A typical
open fireplace with a brisk fire burning will create a flow rate of somewhere around 500 cfm out the chimney, which is enough to totally evacuate the
air from a 1,000 sq.ft. house every fifteen minutes. Meanwhile, replacement air must squeeze in through tiny openings around doors, windows, etc..
If the house doesn't present enough openings to the outside atmosphere to allow static pressure stabilization, the atmosphere inside the house will
remain at a lower pressure (and a lower oxygen level) than the atmosphere outside the house as long as the fire is burning. The tighter the house,
the more pronounced the effect.
Several years ago, a progressive local contractor built one of the first "super energy efficient" house in our area. The house was of cinder block
construction, and incorporated plastic vapor barriers, gasketed windows and doors, etc. We put a woodstove in that house, and the eventual
occupants found that, unless they opened a window, the vacuum effect created by the woodstove chimney would actually overcome the updraft in
their gas furnace and water heater flues, pulling the gas exhaust backward into the house! The backdrafted exhaust blended with the room air, not
only lowering the oxygen content, but adding poisonous gases at the same time. Luckily, the woodstove was designed to accomodate an outside air
hookup, and when we installed the outside air kit, the problem disappeared. While it is true that most older homes aren't nearly as tightly
constructed as that one was, it is equally true that most newer homes aren't leaky enough to create neutral pressurization when a fire is going.
People with wood stoves and fireplaces often complain of smoke or a smokey smell or resperatory irritation or itchy eyes after the fire has been
burning for awhile. This problem occurs in even marginally tight homes whenever the exhaust rising up the chimney from a wood fire vacuums so
much air out of the house that the resulting negative pressurization eventually overcomes the natural tendency for the hot exhaust to rise up the
chimney, pulling wood smoke back down into the room. The solution is often as simple as providing outside combustion air to the fire so it doesn't
depressurize the house in the first place.
Another common problem that outside combustion air can solve is cross-drafting (where the vacuum affect created by a fire in one fireplace causes
the smoke from that fire to be drawn back into the house down an adjacent flue serving a second, unused fireplace). You can read about this
phenomenon here.
The outside air regulation has been in effect for several years for woodstoves, gas stoves, oil stoves, furnaces and fireplaces installed in mobile and
modular homes, which were presumed to be of tighter construction than conventional housing. The more recent Washington State legislation was
intended to extend this requirement to include all installations in newly constructed homes, which are of much tighter construction than site-built
homes used to be. The wording of the new legislation, however, has resulted in an interpretation by some local code authorities that mandates
outside combustion air in all new woodstove, gas stove, oil stove, furnace and fireplace installations, regardless of the age of the house or tightness
of construction. This undoubtedly results in some installations in leaky old homes where only a marginal indoor air quality advantage is gained.
Indoor air quality isn't the only issue, however: even people who live in older, leaky homes suffer from room depressurization. Remember Granny's
wing-back chair? The high, curved, solid wood back was designed to catch radiant energy from the fire, while shielding the back of her neck from
the cold drafts being drawn toward the fireplace by the chimney updraft. It is very common to hear complaints of drafts in older homes whenever
there's a fire in the wood, gas or oil stove, or that the furnace kicks on because of the cold air drawn into the house every time the fireplace is
burning. The drafts are the result of Winter-temperature outside air being drawn into the house due to the evacuation of room air by the rising
exhaust gases in the chimney. When the air evacuation is neutralized by the introduction of outside combustion air into the firebox, the drafts go
away.
Which brings up another advantage to burning outside combustion air; an increase in heating efficiency. While the fire is consuming room air in
Winter months, the replacement air being drawn in through cracks around doors, windows, etc. is COLD. If outside combustion air is provided for
the fire, the opposite happens: heated air expands, so the house becomes positively pressurized, which tends to carry heated air TOWARD all the
cracks, helping to distribute the heat throughout the house.
====================================================================
Visitor Comment: 12/16/05:
I just read your Q&A ... another positive aspect of supplying make up air directly would seem to be keeping humidity up in the house. Any cold
outside air needs water added, so reducing the amount drawn inside the house would help keep humidity levels up to an acceptable level.
B Herms
====================================================================
Visitor Question: 9/19/07:
I'm getting different explanations from different woodstove dealers regarding setting up a new woodstove. I'm presently using a stove built in 1980
and am probably one of the neighborhood's bigger polluters. So I'm trying to do it right. Many dealers sell "outside air kits" which allow one to
draw air from an outside wall directly into the stove, hence one does not have to use "room air" for oxidation. But other dealers are telling me that
there is no data to suggest that such kits make burning wood any more efficient...just crack a window (I don't live in a super-sealed house) and fresh
air (albeit cold) will replace the oxygen used up in the stove--you don't need outside air, they say, and that will save the expense of the kit and of
installation? Who's right? Is this debate over yet??
I want to do it right!!
Mike in Illinois
Hey Mike,
The outside combustion air debate rages on. Basically, our take is you're going to burn outside air no matter what you do, it's just a matter of
whether the air goes directly into the firebox or passes through your living space first. A direct connection to outside combustion air doesn't make
the stove work any better than an open window, but does eliminate the intrusion of cold air into the room. If, as you mention, you don't live in a
super-sealed house, you might not even need to crack a window.
Since practically every stove in today's markeplace is capable of direct outside air connection, you might consider trying the stove burning room air,
to see if you're comfortable. If cold air inflow or any other problems associated with negative indoor air pressure evidence themselves, the direct
connection can be your fall-back position.
====================================================================
Visitor Question: 1/4/08:
Have a good fireplace, good wood and nice warm/hot fires.... BUT I cannot keep the bedrooms (down the hallway) from becoming extra cold during
a nice fire in the LR. We have all this nice wood and have become scared of using it in a fire, because we are tired of cooling-down the other rooms.
What am I doing wrong?
Herbert Rogers
Hi Herbert,
Your wood fire is creating negative air pressure in the house, and it sounds like it is drawing cold replacement air in through tiny cracks around
windows, etc. in the bedrooms. If you don't want to crack a window in the living room whenever you have a fire, you might consider having an
outside combustion air kit installed in your fireplace. These provide combustion air directly to the fire, thus eliminating house depressurization.
The Vestal fireplace outside air kit can be viewed online here (see page BP-4), and ordered here. Note: this is a job for an experienced Brickmason,
and can't be done unless the fireplace is on an outside wall.
====================================================================
A Brickmason offers insight into the above question: 9/18/08:
Outside combustion air is not a new idea. Ben Franklin suggested and studied this by introducing OA through a form of heatilator in Colonial times.
It was his contention that at least the make-up air would be pre-heated. I started incorporating this technique in fireplaces I was building for
customers in the mid 70's. You can read more on this topic in a small book about Count Rumford, the originator of the Rumford style fireplace.
In regards to the 1/4/08 email posted, it may be that the thermostat for the central heat system is in the same room with the fireplace. I have seen
this many times: since the fireplace is making that room nice and warm, there is no call to the central heat system for heat to be delivered to the
rest of the house.
John Borlang
====================================================================
Is there a scientific study supporting a conflicting opinion about outside air? Click Here.
To return to the page you came from, use the < BACK button on your browser or click a button below:
The Chimney Sweep, Inc.
913 Harris Avenue
Bellingham, WA 98225-7032
1(888)-354-6722 toll-free in U.S. & Canada
1(360)-676-9080 FAX
http://www.chimneysweeponline.com
To send us E-Mail, Click Standard E-Mail Window or 
(our reply form)
Copyright © 1996 - 2008 The Chimney Sweep, Inc.
