The run-on is normal in order to extract all heat from the heat exchanger.

In my furnaces the run-on is adjustable. It is a small switch box on the
outside of the furnace with a probe reaching into the hot air plenum. It has
a temperature dial (primitive) with two pointers. One pointer sets the temp
when the furnace blower will come on and the other one when the blower is
supposed to turn itself off because all hot air has been extracted from the
heat exchanger.. There is a small difference between the two settings. You
may be able to change the run-on at this dial, if your furnace has one.

If you set the run-on too short (shut-off temp too high), the blower will
turn off but will then turn itself on again briefly to pump any remaining
heat into the house. But, since the air in the ducts will have cooled off in
the meantime, you get a lot of cool air blowing into the house.

Just experiment until you get it perfect.

--

Walter
The Happy Iconoclast www.rationality.net
-

"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