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Green Building Discussion
Topic: Outside Make-up Air for Fireplaces
Discussion Participants:
- Don Compton asks- How much outside air?
- Richard Kadulski on Canadian code
- Bion Howard has reservations
- Norbert Senf does some calculations
All postings are the opinion of the writer, and BuildingGreen can take no responsibility
for their accuracy or appropriateness.
Don Compton asks- How much outside air?
From: Don Compton
I have a need to know the amount of air or the size of source air opening needed for a 24 X 36 fireplace with an 8" flue.
Is there a source air vent with a thermostat that controls the amount of air coming in to control the fire size?
Don Compton - ENERGYPRO
Top of fileRichard Kadulski on Canadian code
From: Richard Kadulski
Sent: Thursday, September 12, 1996 6:16 PM
The latest edition of the National BUilding Code of Canada has removed requirements for specific combustion air requirements (into the fireplace). This is based on recent research that has shown that combustion air ducts at times work as flues, exhausting hot gases (and thus being a fire hazard), and not providing enough air when the fire is in full fire (the air requirements are so strong and the fire is such a powerfull exhaust device, that combustion air ducts will not draw enough air, and the fireplace will will get all the air it needs from the house in any case). However, where combustion air is provided directly into the fireplace, the duct has to meet requirements for fire safety (in other words, it has to be able to perform as a chimney).
So in other words, it does not appear that there is a magic number. You may want to look at what prefabricated wood burning units are doing in their designs.
Bion Howard has reservations
From: bhoward2@sprynet.com
Sent: Monday, September 16, 1996 6:51 PM
You might want to check local codes. I have read reports from Canada that makeup air ducts can back-draft under certain conditions, leading to fire hazards and failures.
Some people will recommend you use a zero-clearance or prefab stove or insert, but I think a full masonry heater "Russian Fireplace" type of arrangement is much better. They slow down the exit of exhaust gases, store heat in thermal mass (Alex Wilson of EBN is one of the world's experts on thermal mass...BTW) can provide structural support for a building (masonry after all) and are aesthetically pleasing.
There is information on the topic available from the Brick Institute of America (BIA) and the National Concrete Masonry Association (NCMA). I believe Fine Homebuilding magazine did a special issue on masonry recently with a couple of really nice fireplaces / masonry heaters.
Also, look into the TuliKivi system from Finland, which is a highmass stone clad heater system for homes. -- Bion Howard ----------------------------------- Mr. Bion D. Howard bhoward2@sprynet.com (business) Building Environmental ZowWatt@aol.com (fall-back) Science & Technology Voice:(301)627-2780 FAX:(301)627-4735
Norbert Senf does some calculations
From: Norbert Senf
Sent: Monday, September 16, 1996 7:40 PM
The Masonry Heater Association has a website at www.mha-net.org General information on masonry heaters can be found by clicking on "library".
There is a technical paper on outside air supplies for masonry heaters located on our own site at http://mha-net.org/msb/html/papers-n/airreq/cmhc-rep.htm
Using the "Woodsim" software program developed by CMHC, we simulated the tightest house we could find, with an equivalent leakage area (ELA) of 25 sq.in., and using a typical air consumption for a masonry heater at full burn of 15 litres/sec., it resulted in a house depressurization of 2 pascals. Anything under 5 pascals is considered not worth worrying about. In other words, you don't need outside air for woodstoves or masonry heaters (although it is required by the Canadian building code). A couple of studies done to date also found that outside air supplies directly to the firebox had no effect on whether a fireplace (or stove) would spill combustion products into the house or not. You get spillage when negative pressure in the house (from turning on a Jennair, for example, or opening an upstairs window) is greater than the negative pressure (draft) that the chimney provides to the firebox. Both of these pressures are negative in relation to a reference, namely the ambient pressure outside the house. Bringing the firebox to outside pressure with makeup air doesn't help.
The worst case scenario is when your outside air inlet is located on the leeward side of the house. Whenever the wind blows, there is negative pressure on that side of the house, and there have been reported cases of flow reversals from the firebox into the outside air duct.
Best...Norbert Senf
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