I think the only combustion appliances allowed in houses should have
sealed combustrion chambers. There is no excuse for any possibility of
backdrafiting or spillage indoors. Combustion by-products simply aren't
good to breathe.
I think houses should be built as tight as possible. This is for energy
efficiency, comfort, to minimize mositure problems inside wall cavities,
to minimize pollutant entrey (e.g. radon, termiticides), and for
reliability. For all practical purposes, air only passes through a leak
if there is an air-pressure difference pushing or pulling it. I want my
ventilation to be reliable, not haphazard.
I think all houses should have mechanical ventilation. This doesn't
always mean an HRV. There are Geo ventilation systems and there are
Cadillac ventilation systems. An HRV happens to be fairly expensive and
it isn't always cost effective. Keep in mind that most houses need two
kinds of ventilation: local (usually in the bath and kitchen) and
general (because people don't confine themselves to one or two rooms).
I think it is always better to have air entering and leaving a house
through dedicated openings, rather than random openings. Anyone who has
seen the inside of a wall cavity in an old house knows how dirty it can
be. In the beginning, it acts as a filter (but, one that can't be
cleaned), then, later, it becomes a reservoir of crud that I personally
don't want my fresh air coming in contact with.
As Hal pointed out, the specific method of controlling a ventilation
system will depend on several factors. A CO2 controller costs several
hundred dollars, which means it is a Cadillac control. In a lot of
situations, a crank timer makes a great controller. Personally, I like
to oversize the system, then run it either intermittantly at full speed,
or continuously at a slow speed, then boost it manually for periods of
high occupancy. Occupant education is essential. An instruction plate
next to the control is a good idea.
I think the best way to insure that the air indoors is healthy it to
build with low-polluting materials (source control), then the
ventilation system only needs to supply fresh air for oxygen (1/2 cfm
per person is about right for this) and to dilute metabolic by-products
(about 10-15 cfm per person).
The bottom line: build tight with healthy materials, and you only need a
low-powered, inexpensive-to-run, inexpensive-to-purchase, quiet
ventilation system. Build with unhealthy materials, and you need a
higher-powered, more-costly-to-run, more-costly-to-purchase, noisier
ventilation system. Build a leaky house, and you simply lose control of
your indoor air supply. Fresh air shouldn't be left to chance.
-- John Bower The Healthy House Insttute Visit our web site: http://www.hhinst.com/ email: healthy@bloomington.in.us430 N. Sewell Road Bloomington, IN 47408 812-332-5073 fax 812-332-5073 __________________________________________________________________ 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. __________________________________________________________________