RZ,
It's normal for the fan to run for a short time after the fire goes out.
This is to distribute the remaining heat in the heat exchanger. In older
systems, the fan (blower) was controlled by a temperature sensor near the
heat exchanger. At the beginning of a heat cycle, when it sensed the fire
had heated the exchanger enough, it started. At the end of a cycle, after
fire, it remained on until exchanger had cooled a bit. Some newer furnaces
use electronics ( a timer) to determine how long it runs. Btw, you may want
to measure the temp. coming out of a heat register. Moving air feels cooler
than it actual is. You may find the apparent "cool" air is actually a few
degrees warmer than the room air, in which case you don't have a problem.
Newbie heat pump users experience this phenomenon because they expect
"warmer" air to be expelled, similar to a gas (or oiled) fired furnace.

I'll go out on a limb here and say the "bad pressure switch" idea is bogus.
Usually the pressure switch is used to verify the blower is blowing. If it
is not, the fuel is cut off to prevent the heat exchanger from overheating.

"RZ" wrote in message
...
I have a Lennox natural gas furnace, model G24M3/4-100A-2 with
electronic ignition. The furnace was installed new in 1994, and I see from
a
service log that the burners and inducer were serviced in 2002, and the
inducer was replaced one month later, after the servicing. We live in
Canada, where this furnace is heating for 5 months of the year.
I don't know if I have a problem or not,but the blower motor seems
to
run on longer than the burn period. So, you end up with unwanted cool air
blowing out of the heat registers, and a blower motor that is running an
electric bill up, unnecessarily.
Here is a run-down of a complete cycle:
1. thermostat sends signal to furnace to heat home
2. flue fan starts, and runs for 45 seconds
3. burners ignite, and burn continuously
4. blower motor starts 45 seconds after burners ignite
5. thermostat sends signal to furnace that ambient temperature is 1 deg
warmer than set desired temperature
6. flue fan stops and burners shut off simultaneously
7. blower motor continues to run for 3 minutes
8. blower motor shuts off
I was told by a furnace repairman, over the phone, that the problem
is
most likely the pressure-switch, and it should be replaced. This
pressure-switch is an easy DIY replacement, and would cost me $40, but
before I go ahead and get a non-returnable switch, and find out that I
still
have the blower running longer than the burn cycle, I want to know if this
is the normal operation for this furnace. And, is there anything else I
should be looking for, before I have to get a repairman to come have a
look
for $60/hr.

Does anyone have any experience with these furnaces, and what would you
suggest?

Thanks