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Sweep's Library: Why burn outside combustion air?

Q: I read the following from your page, "... all woodstoves and fireplaces installed in Washington State must have the ability to draw their combustion air directly into the firebox from outside the house (this eliminates potential health problems deriving from poor indoor air quality 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

Sweepy A wood fireplace 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 than the atmosphere outside the house as long as the fire is burning. The tighter the house, the more pronounced the effect.

Controlled combustion heaters such as wood stoves and inserts don't consume nearly as much room air as open fireplaces, but even their small amount of air exfiltration can combine with that of other air evacuators such as radon pumps, kitchen and bathroom exhaust fans, attic ventilation fans, clothes dryers and other vented appliances to produce the same undesirable level of household depressurization. 

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 wood stove in that house, and the eventual occupants found that, unless they opened a window, the vacuum effect created by the wood stove 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 wood stove 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 combines with other evacuating factors to vacuum 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

Outside combustion air has been required for all wood stoves, gas stoves, oil stoves, furnaces and fireplaces installed in
mobile and modular homes (which were presumed to be of tighter construction than conventional housing) for decades. 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 wood stove, 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, which means extra fuel must be consumed to bring it up to room temperature. 


Visitor Comment:

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:

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 combustion. 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

Sweepy 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.

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:

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

Sweepy 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.

One way to introduce outside combustion air to a masonry fireplace is the Vestal outside air kit. 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 a historical perspective on outside combustion air:

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 Herbert's posting above, 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


Visitor Question:

I am having a problem with my Russo glass view stove. I cannot get the stove going without opening a couple of windows in the room. if the stove has been going all nite and I stick a couple of logs in the next morning the stove begins to smoke. I have extended my chimney and it still does the same thing. I also have a Russo insert on the other end of the house and that stove works fine. Any suggestions?

Aaron Inacio

Sweepy Sounds like both fires are trying to pull combustion air out of the house, and the insert is winning. If possible, connect one or both stoves to outside combustion air. If not, open a window in one room or the other whenever you're using both stoves.


Input from an Energy Star Home Builder:

Loved your Library articles on outside combustion air. I live in Western Wisconsin, where many of the fireplace installers still seem to be stuck on burning room air. They persist in singing the old song about the imagined potential ill effects of sealed combustion, such as backdrafting through the intake pipe during periods of wind-induced downdrafting. My business is pressure diagnostics, but even with my credentials, I can't manage to pound the science into their heads or change their protocol.

Our state has produced over 16,000 high performance homes. These homes are pressure tested (blower door) and surprisingly tight on average. We use ASHRAE 62.2 whole house ventilation standards (and test to verify). I tested a Habitat home today and it was 0.63ACH50 or 0.056CFM50/ft2 shell. That's friggen tight. Once the clothes dryer and kitchen ventilation are installed, the level of depressurization will, at times, easily drop below -30Pa. We allow only sealed combustion appliances and mechanical equipment with outside combustion air, so who cares? Even at that extreme level of room depressurization, we've had not one complaint state-wide about any backdrafting. (Except for ones inadvertently set up to burn room air).

I'd rather not live in an over-pressurized home: who needs warm moist air being pushed through the shell? Yet some of the same local yokels mentioned above are installing mechanical equipment to pressurize the house because they have trouble with stove and fireplace drafting! All they need to do is install all combustion appliances with outside combustion air and their problems, the homeowners problems, and my problems go away.

Eric Skinner, Partner
Home Performance with Energy Star
Focus New Homes Program
608 792 5018


Is there a scientific study supporting a conflicting opinion about outside air?

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