This is a complex subject, and I hesitate somewhat to wade in, since it
is in my field of interest and you might not be able to stop me once I
get going. However....
CMHC did some amazing, ground-breaking research in this field in the
70's and 80's. Before their work, no one really understood how venting
worked in houses. "The house as a system" concept originated from this
very comprehensive body of work.
Having said that, and having studied the literature on outside air, and
having drawn some conclusions on my own on the subject, I'd like to add
the following two cents:
- there are no studies to support the notion that direct combustion air
has ANY benefits. There may indeed be some, but there are no research
results to support this assertion, that I know of. I'd love to hear
otherwise from somebody.
- outside combustion air vented directly into the firebox, although this
is somewhat counterintuitive at first, does NOT reduce "susceptibility"
to spillage. What it does do is reduce spillage quantity because your
combustion air intake is not located in the house when it backdrafts and
becomes a chimney. It has no effect on spillage through leaky gaskets on
loading doors, for example. Even if your doors are airtight, then the
connections in your factory-built chimney will leak unless they are
siliconed in place, which they won't be if they are installed according
to manufacturers instructions. "Aerodynamic decoupling" is a myth.
- there are some reported downsides to outside combustion air, such as a
potential fire hazard when backdrafting does occur, and the combustion
air supply becomes a chimney.
We did a study in this area as it relates to masonry heaters, and there
is a paper located at
http://mha-net.org/msb/html/papers-n/airreq/cmhc-rep.htm
The real culprit here is house depressuriztion, and the guy wearing the
black hat is the Jennair downdrafting kitchen range, although there is a
long cast of supporting actors. One of the more common ones that I see
all the time is the tall owner-built house with a mickey mouse vapor
barrier at the cathedral ceiling - ie., house stack effect on drugs
(sometimes with rotting ceiling rafters as a side effect).
The proper way to address this is to follow the recommendations in
"R-2000 Makeup Air Guidelines", published by the Canadian Homebuilders'
Association (http://www.chba.ca/)
Here's a list of some do's and don'ts:
DO have your stove and venting system installed by a certified
professional, and checked annually
DO take your old stove to the dump and buy an EPA-certified stove -
you'll be doing the environment a big favor.
DON'T run your chimney up the outside of your house.
DON'T put your stove in the basement without first doing a negative
pressure test as outlined in the CHBA booklet, above.
DON'T load your stove full of wood and turn the combustion air supply
down before you go to bed at night.
DO burn wood at 20% moisture content or below. If you can't tell, then
you shouldn't be burning wood in the first place.
--------------------------------------------------------
Norbert Senf email: mheat@mha-net.org
Masonry Stove Builders mheat@hookup.net
RR 5, Shawville website: http://mha-net.org/msb
Quebec J0X 2Y0 fax: 819.647.6082
voice: 819.647.5092
__________________________________________________________________
This greenbuilding dialogue is sponsored by Oikos (www.oikos.com)
and Environmental Building News (www.ebuild.com). For instructions
send e-mail to greenbuilding-request@crest.org.
__________________________________________________________________