On Sat, 26 May 2007 10:01:44 -0000, Art Greenberg
wrote:

On Sat, 26 May 2007 02:57:43 GMT, aemeijers wrote:
Probably the exhaust for the high-efficency furnace. But there is
usually a second pipe to suck combustion air in.

4" is too big for a furnace. Maybe a water heater - mine is like that.

That's interesting. I woudl have thought a water heater fire is much
smaller than a furnace fire, and would use a smaller exhaust pipe.

BTW, I've seen new construction with high efficiency heating that do
exhaust to the outside, but take combustion air from inside - just a
short stub on the furnace intake with a plastic grille on it. I thought
that was wierd.

That is wierd, but otoh, don't all the previous prior to
high=efficiency furnaces, suck air from the inside and exhaust it to
the outside.

I guess colder air has more air in the same volume, and that's good,
but indoor air is pre-heated and that's also good, right? Or maybe
those are the exact opposites, and only one can be good.