RZ wrote:
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

It is normal, in most modern furnaces, for the blower motor to continue
to run for some time. It would also be normal for the air coming from the
register to "feel" cool towards the end of this cycle. Just as a fan makes
the air "feel" cool it has not cooled it. Since your furnace should be
taking air from the room, running it through the furnace and then returning
it, it will not be cooling the air (ignoring the possibility of the ducts
running through a non-heated area). When working as intended, you will save
more in fuel cost by recovering more heat, than you will pay in electricity
to run the fan.

Now that said, there may be some adjustment possible to reduce the time
it runs. That time may be governed by a timer or by a ttemperature sensor
in the furnace. I have not checked to see what mine does, but as I recall
it is based on a timer and I have never looked to see if there is an
adjustment. It would be possible your thermostat could control it but I
don't think they do.

My advice is to enjoy the extra circulation and efficiency.

--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math