MLD wrote:
"Robert Gammon" wrote in message
...

Al Moran wrote:

On Sat, 16 Sep 2006 21:39:14 GMT, "MLD" wrote:




You are not taking into consideration the fact that the 80% furnace

uses

inside air for combustion and the 90% furnace brings in outside air.

The

inside air has already been heated, that has cost you $$$ and you're
throwing it up your chimney . This air has to be replaced and the new

air

is coming into your house from the outside--cold air that has to be

heated

to room temperature--more $$$$. It gets into your house through

cracks,

windows, under doors--so your house gets drafty. All this newly heated

air

has to be humidified because it is going to be very dry. My point

being

that the $$$ difference between the 80% and the 90% furnaces is more

than

the 10-12% in furnace efficiency.
MLD



That's the dumbest thing I've heard in awhile. That air your talking
about that is already heated and gets put up the chimney, did you ever
stop to think that it must pass the burners to get there in the first
place? Now if by chimney you mean the real chimney, any idiot knows
to close the damper when the fireplace is not being used. As for a 90%
furnace getting air from outside, bull****. They can get air from the
living space also. Stick with something you know cause hvac ain't it!


Most, but not all new 90% furnace installations DO bring in outside air,
in a PVC pipe to supply the burners, particularly in new custom homes as
extra attention is paid to proper sealing.

Manufacturers have recommended this practice for a LONG time, same as
fireplaces that bring in combustion air from the outside so that you
keep the doors closed most all the time.


Once again--the air used for combustion on an 80% furnace is supplied by air
that is already in the house. A 90% furnace brings in air from the outside.
In my search to install a new furnace I didn't see one manufacturer that did
it otherwise. The exhaust air from a 90% furnace has just about all the heat
squeezed out of it-- that's why it can safely be vented outdoors. You can
put your hand in the air stream and it feels luke warm. If you vented the
90% exhaust gases into the chimney you'd have a problem sooner or later.
Why? First of all there would be problems creating the proper draft because
of the relatively cool air moving up the chimney--not too good to have
poison gas reentering the house. Second, because of the condensation that
would form in the chimney. I don't know who Al Moran is but I do know that
he isn't too bright.
MLD



Well, this is contrary to what the installers told me here when my 18
year old 65% furnace died and I was forced to replace it with an 80%
(the cost delta between 65% and 80% will NEVER, NEVER be recovered in
the lifetime of this furnace even if gas prices TRIPLE!!!)

I had stuffed fiberglass insulation around the plenum of this up flow
furnace. They removed it and said that combustion air comes from the
attic. Very little air comes thru the crack at the bottom of the door
to the closet where the furnace is installed.

90% furnaces are condensing and the flue must be PVC and routed
HORIZONTALLY outside. The water that condenses and drips outside is
caustic, and limestone or marble chips need to be placed under the drip
zone to protect neighboring plants. 90% furnaces should NEVER be vented
out a chimney.