"Bob" wrote in message
...
David Combs wrote:

What's an "A-coil"?

This discussion is about a combined forced air heating and cooling
system. In a typical installation the air first passes through a
squirrel cage fan, then the heat exchanger, then the cooling coil which
is often A-shaped. At the beginning of the heating season there is
frequently a burning smell from dust, etc. that have accumulated on the
heat exchanger or coil during the off-season.

When did this thread get re-ignited, and why is it about furnaces now? (-:
Anyway, Dave, Bob has given you the answer. I'll try to explain further,
and no doubt get something wrong, but hey, that's how we learn.

The "A coil" is a very much like the fin & tube constructions you see on the
back of refrigerators or inside freezers. Because space in furnaces is
tight, instead of a large rectangular grid, they fold it over to increase
the surface area. Warm air from the house is passed over these finned tubes
filled with cold refrigerant, warming the refrigerant and cooling the air.
In the process of cooling, the air gives up its moisture, which *should*
flow into a drain but in sick refrigerators and air conditioners, it often
shows up on the floor and sometimes gets blamed on the dog. (-:

This process causes a number of side effects. Moisture often forms on the
heat exchanger helping it corrode. The "A coil" also tends to trap, all
the dust, dirt and hair that make it through the filter and clot on the fine
fin work. The exchanger forms a skin of dust because it's usually moist
from condensation.

The first time the furnace runs in the winter that dust, hair, dirt and mite
laden "shell" on the exchanger "burns" off - not actually, but it's heated
to the "stinkpoint" pretty nicely though. If you've got a couple of dogs
and bad furnace filtration, as in no electronic air filter, the smell can
get pretty hideous. That's when people call the FD. We've already had one
round of such fire calls here in MD, but the calls will start when the
daytime temps fall and the houses don't retain enough warmth to pass the
night comfortably as they do now.

The warmed up refrigerant goes outside where it's compressed again and the
cycle begins again. My A/C was installed pre-internet days and I never
would have done it (retrofitted an old furnace) had I known then what I know
now (ducts were too small and in all the wrong places to support efficient
air-conditioning. It was a really expensive lesson, not only for the cost
of the unit, but for the increase in assessment and the cost of window air
conditioners, which turned out to be a much better idea in many dimensions,
at least for our teeny house.

Note: I'm just a Joe Homeowner, so don't believe anything I say unless you
also hear it from one of the many experts here.

--
Bobby G.