"Jonathan Kamens" wrote in message
...
Are you sure it's the blower that's coming on and not the
chimney exhaust fan? I suspect that it may be the latter.

There are a number of possibilities (assuming that it may be
the exhaust fan that's coming on):

1) Your igniter is bad.

2) Your exhaust fan is bad and isn't producing enough of a
draft.

3) Your chimney is blocked, so the exhaust fan can't produce
enough of a draft.

4) Your pressure switch is broken so the furnace thinks that
the exhaust fan isn't producing a draft when in fact it is.

Your first step in figuring this out is to make sure you
understand the difference in sound between the exhaust fan
and the blower :-). My apologies if you already understand
the distinction.... some people don't.

If your exhaust fan comes on when the furnace decides that
heat is needed, and then a little while later you hear clicks
from the furnace, and then it starts the cycle over, then
it's probably the igniter -- the clicks are the furnace
turning on the gas and trying to ignite it.

If your exhaust fan comes on but then there are no clicks, or
there's just one click as the fan shuts off, then it's one of
the other three possibilities. If the exhaust fan sounds like
it's working just fine, it's probably one of the last two.

The executive summary of all this is that you should have
a trained professional come out and look at your furnace.
You don't want an amateur messing around when there's gas,
fire and carbon monoxide involved.

This is Turtle

In 1984 Vent fans type furnacers were rare.

TURTLE