Weird Furnace problem with Goodman GMP075-3 - Advice Welcome

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Weird Furnace problem with Goodman GMP075-3 - Advice Welcome

Post by donkeyjuks » Sat Jan 24, 2015 10:37 am

Hi there! I have a 19 year old Goodman GMP075-3 furnace. The problem I am finding is quite weird and not really sure what could be wrong.

The furnace stops working after reaching the set temperature. For example if I set the house to 22 celcius and it reaches 22 and after that the furnace will not turn back on and temperature just keeps dropping obviously without the furnace ever kicking in. So if i set the temperature to below existing temperature in the house (for example I bump it down to 18 wait 10 seconds and then bump it back up to 22) or if I turn off the power to the furnace and turn back on, the furnace starts heating as normal.

Any ideas?

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- Weird Furnace problem with Goodman GMP075-3 - Advice Wel

Post by Freon » Sun Jan 25, 2015 10:44 am

What make/model is the thermostat? How old?

When the house temperature drops below the thermostat set temperature, do you know if the furnace tries to light but fails in the ignition ?

I ask because you need to know if the thermostat is, for whatever reason, not sending the signal to the furnace to ignite. If the thermostat isn't sending the signal (24 volts AC on the W wire), then you may need a new thermostat.

But if the thermostat is sending the signal and the furnace tries to light but fails, then you need to know at what point in the ignition process the furnace stops.

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- Weird Furnace problem with Goodman GMP075-3 - Advice Wel

Post by LennoxHVAC » Mon Jan 26, 2015 9:59 am

I had similar issue with our Lennox G20 at the start of the season.

The thermostat setting at 70ºF would call, the cycle starts... and burner flame will come on. But the blower fan cycle will not, which will result in burner flame going off. This cycle will repeat umpteen times.

I suspected it to be Limit Switch issue which monitors temps inside the heat exchanger. If the air inside the heat exchanger has reached max temp it will cut-off flame for safety -- overheating. Hence no blower fan.

So a home temp may drop 10-20ºF, without blower fan cycle the trapped air inside the heat exchanger is overheated each time the burner flame starts. The air temp inside the heat exchanger is at max hence preventing the blower fan cycle -- the Limit Control Switch is doing its job.

In your case -- a heat exchanger must cool down to resume heating the home -- i.e. blower fan cycle.
Also check furnace return filter.

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