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gotoman for ac
gotoman for ac, HVAC Technician
Category: HVAC
Satisfied Customers: 5158
Experience:  51 years of experience install design and repair of all makes and models of AC
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Lennox G20 furnace not firing up after temperature falls

Customer Question

I have a Lennox Whisper Heat G20 furnace and had been having problems with it not firing up every time the thermostat went below the desired temperature. I'd have to turn the furnace off/on to get it to fire up again. Then, after asking a tech here, they suggested I first try and clean the flame sensor with steel wool. Unfortunately, after doing so, the unit would fire up, but then turn off almost immediately, leaving no heat at all.
Today, I replaced the old RobertShaw ignition control module with a Honeywell 30W33 updated one. After I'd turn the gas back on and turn the power back on, it would lite the pilot, then fire up with no problem. The PROBLEM, however, is that it won't fire up again when the temperature falls below the desired temp at the thermostat. I have to turn off the furnace, and the gas as well if I'm going to start it again. Could this new module be faulty? The man at the parts store made it clear that they don't take back equipment unless it's faulty.
Submitted: 7 years ago.
Category: HVAC
Expert:  ComfortZone replied 7 years ago.

Perhaps it is the control board or the stat that is faulty.

Do you have a volt meter?

Customer: replied 7 years ago.

Actually, I do...but must admit, I'm not that comfortable in how to use it. It's a "50/60HZ Voltage tester GVT-82".

As for the thermostat, it's brand new since that was the very first thing I'd tried to change when the furnace started going cold (sorry I forgot to mention that).

Expert:  ComfortZone replied 7 years ago.

When the furnace does not start does the LED on the control board flash at all?

Customer: replied 7 years ago.
Yes, first the green light blinks six times and the yellow one, once...the second time the green blinks four times and the yellow, three...then it repeats.
Customer: replied 7 years ago.

I'm sorry, are you talking about the circuit board that is just above the blower motor (behind the bottom panel) or are you talking about the two LED lights on the Honeywell Ignition control module?

Expert:  ComfortZone replied 7 years ago.

The one above the motor where all the thermostat wires are attached.

Customer: replied 7 years ago.
just went down to check and didn't notice any lights at all on the circuit board in the blower compartment as I tried to start the furnace.
Expert:  ComfortZone replied 7 years ago.
I am not sure what your problem is, but I suspect it is the control board. Without voltage readings I would not condemn the board though. Since I am not a Lennox expert I will opt out and give another expert a shot at this.
Customer: replied 7 years ago.
thank you for trying.
Expert:  gotoman for ac replied 7 years ago.

I am an old Lennox Engineer. Most of the time on these Whisperheat units it is the igniter. Everytime I change one it is a diffferent manufacturer. Does the furnace lite now? Tell me what it doing now. Also do you need an install manual with all the trouble shooting procedures. So you can follow.

Customer: replied 7 years ago.

Thanks for your reply, Rus. As of right now, the only time the furnace will light is when I turn the gas and power off, then put the gas back on again (at the regulator) and turn the power back on. The furnace will then light the pilot, seconds later you hear some clicks, and seconds later the burner ignites...but then that's it. It does not come on again by itself when the temperature falls below the desired setting on my thermostat.

I guess the reason I'm wondering if it could be a faulty new ignitor module is that when I went to buy it at the appliance parts store, the box was already open when the man brought it to the counter. Just seemed "odd", especially now because it does work initially, but like I said, only the one time.

As for the install manual, the Honeywell 30W33 did come with a 4 page installation booklet, but there are no troubleshooting methods in it.

Expert:  gotoman for ac replied 7 years ago.

go to

for complete manual. This way we will be on same page. It is leaning toward igniter. Check trouble shooting chart in back of manual. Get back if you need help.

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