Outside Air Filter
October 7, 2009 6:07 PM   Subscribe

We want to add a wood stove to my wife's studio, currently under construction. Is outside combustion air a must or a maybe?

The studio is framed and roofed, and it's on a 2nd story above the garage. The place where we'd like the wood stove to go is above the garage, so we could do a vent through the floor and duct it to the outside.

I've read differing opinions as to whether outside combustion air is a good idea or not. I understand if the house is tight, it can draw exhaust down the water heater flue or in through bathroom fans, etc, but there won't be any other exhausts to back draft in the building. It's not super tightly-sealed but it is new construction; however there are french doors which are always going to let some air in, ventilated soffit, etc.

One thing I read said that outside combustion air is more efficient because your house becomes positively pressurized, which sucks the warm air towards the envelope of the house (and out?) which distributes the heat better in the space for greater efficiency.

But I also read this near-rant on the myth of outside air but it's very axe-grindy and seems like it comes from a guy who used to sell wood stoves that didn't offer outside air options... but it makes some good points.

My builder is a little worried about additional fire code requirements for the garage ceiling if the outside air vent comes through there, and it's always possible that the outside air could be added later, but I don't want the kit to become unavailable, or have to redo the ceiling as part of it, etc.

The stove is a Drolet Eldorado. It's Canadian, so you know it's good. The studio is about 600sf with a 200 sf office below in the back that I am calling "geekland".

So, do you have experience with outside air supplies for wood stoves? Do you think we should do this now, or maybe plan to do it later, or not worry about it?
posted by ulotrichous to Home & Garden (8 answers total)
Either you have the outside air or you have leakage sufficient to feed the fire. In modern tight construction you can have issues without outside air for a heavy combustion source like a wood burning stove.

At the very least, get a CO monitor.
posted by caddis at 6:22 PM on October 7, 2009

Every woodstove I've ever met (and I've met a lot) has only had a chimney. They all worked marvelously. And as fall falls, I'm progressively more envious of my woodstove-owning friends.

I wasn't even aware that there was an option for "outside air".
posted by Netzapper at 6:24 PM on October 7, 2009

The ventilated soffitt shouldn't allow outside air to come into the conditioned living space - if your building is properly insulated, the air from the soffitts should be completely separate from the inside of the building.

Did you read the owners manual for the stove that you indicated? Page 23 deals with outside combustion air requirements.

Basically, if you are creating a fairly well sealed space, you need some sort of outside combustion air. This can be as simple as cracking a window open, if that works for you.

Is there a reason you couldn't go directly through a side wall for the outside air intake?

I doubt that the kit will become unavailable any time soon, and at any rate, there is nothing particularly special about it that a HVAC guy couldn't whip up for you using off the shelf parts.
posted by davey_darling at 6:32 PM on October 7, 2009

Best answer: One of the big benefits of making a separate outside combustion air flue for your woodstove, is that you can generally insure that a very small amount of kindling being lit will be enough to start positive chimney air heating, creating a rising air column in the chimney, that gets the updraft in the stove going the right way quickly. That makes it much less likely your fires will smoke up the interior space in their early stages, particularly on windy days. Your effective chimney height is going to be greater than the actual chimney height, because your woodstove is on the second floor, and as such, windy days and your short real chimney are going to make for interesting lighting of fires many times. It will be nice, on those days, if you can at least close your fire door tightly, and keep combustion air going to the fire from the outside, as the fire builds heat in your stove, and creates chimney effect.

If the wood stove is intended as a functional source of heat, you may also find that a damper in the air supply flue, along with good chimney hardware, enables you to more effectively "bank" your fire, to make a single fuel load last as long as possible. This nod to fuel efficiency would be a big advantage for a second floor woodstove, where you have to carry all the wood up a flight of stairs, one way or another.
posted by paulsc at 8:27 PM on October 7, 2009

One reason to use external combustion air is for compatibility with modern
appliances, such as a vent hood over your stove, a bathroom exhaust vent, a
clothes dryer and traditional masonry fireplaces. All of these devices suck air
out of the house, and if you are not using external air for your fireplace, then
it has to compete with these other devices. When you are just starting a fire,
the fireplace loses the suction battle, and you fill your house with smoke.

There is a bureaucratic solution to this problem, which is to ensure that you are
not running the range hood, dryer, bathroom exhaust or another fireplace when
you are using your nice canadian fireplace. Or, you can just use the external air kit.

Another advantage of outside air for combustion is the fact that you are not
drawing heated air from the interior to feed the fire. That heated air
costs you some small number of cents per cubic foot, so you are essentially
throwing energy pennies up your flue.

This doesn't matter if this is a cosmetic fire, just there to satisfy your caveman
desire to have a fire; or if you are in a mild climate. If you are in a cold climate,
then that warm combustion air drawn from your living space is replaced by cold
air outside. It's a heat leak. It's energy inefficient.
posted by the Real Dan at 10:02 PM on October 7, 2009 [1 favorite]

Real Dan: but feeding the fire with cold air means that more combustion energy is used to get the fire to its appropriate temperature and keep it there. Either you lose energy in the form of warm air sucked out of your building and replaced with cold (which the fireplace must then heat conductively/radiantly), or your fire has to spend the same amount of energy heating its inlet air directly instead of your indoors air. The energy balance is the same, but extremely cold (like -40C) external-air may run a poorly-designed fireplace cooler and less efficiently, while internal air can give you annoying little cold drafts.

Much bigger efficiency difference is how much heat you can extract from the flue - if you want to maximize joules from your fuel, you want the flue gases leaving the chimney to be fairly cool but not so cool that you get nasty things condensingand causing corrosion; 150C-180C is a good temperature to aim for. Thermal power stations spend a lot of effort optimising exhaust temp to balance efficiency against condensation as the moisture content of the air and fuel (eg coal) varies.

OP: if at all possible, put the stove downstairs; upstairs will still get plenty warm because the flue (passing vertically through the upper floor, caged in steel mesh so no one burns themselves on it) will radiate so much heat, plus hot air rises anyway so upstairs will tend to be hotter if the spaces are connected. If your flue gases get too cool (eg from very long flue that radiates a lot of heat), you'll need to partially insulate the flue to keep exhaust above 150C with the fire banked.
posted by polyglot at 10:27 AM on October 8, 2009

Response by poster: Thanks very much, folks. The ease of getting the flue going faster is compelling, and although there isn't any other inlet now, we may add an exhaust hood in the future that would be in a very drafty spot... both good reasons to plan for outside air. It can't really go downstairs, although we're in Ann Arbor so it doesn't really get that cold here. Our canadian stove will be snickering at our weak-ass lower michigan winters.

It does seem like the question "is outside air more efficient?" may be the plane on a conveyor belt of woodstoves.
posted by ulotrichous at 8:43 PM on October 8, 2009

It does seem like the question "is outside air more efficient?" may be the plane on a conveyor belt of woodstoves.

Not really. The claim that the energy balance is the same ignores that without an outside combustion air system the room air will be cooler for the same fuel use so that you will need to burn more fuel to achieve the same level of comfort. Extracting heat from the flue was much more important with older designs. The newer stoves are fairly efficient; not that you couldn't improve that with fins on the flue etc.
posted by caddis at 11:57 PM on October 8, 2009

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