Goodman GMP-075-3 gas furnace circa 1994 (induced draft, hot surface ignition, ca. 80% efficient, horizontally mounted in unheated attic, no A/C). Honeywell Magistat thermostat.

The furnace worked fine for about 8 years, and still heats well and maintains the proper temperature. Starts with an exhaust blower for about 15 seconds to clear the flue of gases, then the burner ignites for about 15-30 seconds, then the air circulation blower starts. When the thermostat is satisfied, the burner turns off and the blower runs for another couple of minutes to extract the last of the heat from the system. The temperature and velocity of the air coming from the registers is normal.

The problem is that several times a day the furnace blower will come on and blow air for a couple of minutes while the thermostat is not calling for heat, and then turn off. Sometimes it even happens shortly after the thermostat has gone into overnight mode and the temperature setpoint is 10 degrees or more below the room temperature. In the past it did it with the thermostat off.

This started about 2 years ago at the end of the heating season when the temperature got warmer. But when I turned on the furnace last year in the fall the problem didn't show up... so we used the furnace all of last winter... until the problem came back last spring at the end of the heating season. This fall the problem showed up as soon as we started using the furnace and it has continued since then.

I install a new thermostat when the problem first showed up (the current Magistat) and that did not correct the situation. So I don't think that it's the thermostat.

Any suggestions on things I can check? The symptoms sound similar to the posting #91, but that one doesn't seem to have been resolved. If it's the circuit board, can you recommend a good source on the internet?

Thanks in advance for your help.

Try replacing the delayed fan timer located in the fau , this is probably where your problem lies

try replacing the delayed fan timer located in the fau , this is probably where your problem lies

Sorry for my inexperience, but I'll need more details. I can't find a "delayed fan timer" in the repair parts lists/diagrams or in the installation and operating instructions. And I'm not familiar with the term fau. I've been assuming that the fan timing happened on the circuit board (the circuit board is actually named the "Fan Control/Ignition Module). There is nothing that I can construe as a timer on the blower motor or fan housing drawings.

Where should I look for this device?

I myself am not familiar with some model numbers on goodman fau"s (forced air unit) but I know now what type you have and your problem is this fan control mudule , not diffucult to replace , its located in the blower unit compartment , it sounds like you can read your wiring schematic in your owners manual , get yourself the new module and replace wire for wire, remember shut all power off to the unit before toching anything

Replace the PC board it can be the only cause I can think of. Also when was the last time you had a safety inspection on that furnace, I'll bet you all the rivets are broken, your health is worth more than a new furnace.

Replace the PC board it can be the only cause I can think of. Also when was the last time you had a safety inspection on that furnace, I'll bet you all the rivets are broken, your health is worth more than a new furnace.

Thanks all for your input. I will replace the circuit board and see if that corrects the problem.

As for the safety inspection, I do my own every year when I start up the furnace. Two years ago that included removing the plenum and checking the exchanger from the top as well. There didn't seem to be anything broken.

I checked the records and the furnace is actually 12 yrs old, so it was 10 yrs at the time). I will take a look at it again when I'm replacing the circuit board.

By the way, I have a CO detector with a digital readout that gets nearly direct exposure to the air from one of the registers, and its reading is zero and stays zero when the furnace is heating. I can't vouch for the detectors calibration, but the readout does respond to an external source of CO when I've tested it.

Will let you know what happens with the circuit board.

Thanks all for your input. I will replace the circuit board and see if that corrects the problem.

As for the safety inspection, I do my own every year when I start up the furnace. Two years ago that included removing the plenum and checking the exchanger from the top as well. There didn't seem to be anything broken.

I checked the records and the furnace is actually 12 yrs old, so it was 10 yrs at the time). I will take a look at it again when I'm replacing the circuit board.

By the way, I have a CO detector with a digital readout that gets nearly direct exposure to the air from one of the registers, and its reading is zero and stays zero when the furnace is heating. I can't vouch for the detectors calibration, but the readout does respond to an external source of CO when I've tested it.

Will let you know what happens with the circuit board.

The heat exchanger is held together by what they call rivets they are the hole you see in the walls of the heat exchanger cells, when the break you will just fing dime sized rings laying around in there.