I’ve been fighting a furnace problem for the last two months and could really use some help.
I own a Goodman GMN120-5 that is twenty years old. The unit is down flow and uses natural gas. All vents in the house are 100% open
At the end of April, I noticed the furnace was cycling on-off. I pulled and inspected the high temperature limit switch. The plastic portion of the switch was crumbling so I ordered a replacement switch on-line and scheduled an inspection by a local heating/cooling technician. The technician noted the on-off cycling of the furnace (new switch had no arrived yet), told me my heater capacitor was weak, said the heat exchanger had NO cracks or holes, said the AC was ok, but strongly recommended a new heater/ac due to the age of the heater. He was acting more like a “sales rep” than a “technician”. I declined his “generous offer” and assumed that there would be a cheaper fix. A few weeks later the weather got hot and I needed the AC. It ran for a few hours and then it stopped… Dead capacitor. I replaced the capacitor and the AC is fine but I’m still fighting the heater cycling. Needless to say, I have no faith in this vendor to solve my heating problem; he just wants to sell me a new heater and AC!
I’ve completed the following tasks (not exactly in this order) and I still have the on-off cycling:
I replaced the weak AC capacitor and the cooling system works fine.
I replaced the high temp switch and the furnace still tripped on high temp. Cut out switch is designed to trip at 170 degrees and reset at 140 degrees. I confirmed that the MAIN high temp switch is the problem and not the switch in the vent stack blower by checking voltages across each switch when the furnace tripped.
I drilled a 1/8 inch hole in the furnace panel (right above the temp switch) so I could get an independent temp reading from a digital meat thermometer. I got many high temperature trips (170 degrees) when the meat thermometer read 120-130 degrees. For the record, I’m 100% positive that I did not drill into the heat exchanger
Due to the above, I suspected that the replacement temp switch was defective so I bought another one from a different vendor. Furnace still tripping after installation of second temp switch.
My heater also tripped twice on high roll-out temperature during my testing so I replaced both roll-out switches. Furnace still tripping on high temp but no more roll-out trips.
I inspected the inlet ducting. Looks good. No blockages or collapsed ducting. Duct is 20-25 ft in length and oval shaped at both ends. Approximately 20” by 15” at one end and 22” by 12” at other end
I test ran the furnace with the air filter removed. Got high temp trip.
I tested the furnace with the air filter removed and with the top panel of the furnace removed. In this case I was pulling air from the house and the garage. No high temp trips. A new clue!!!
I removed the main blower and cleaned the blades on the chance that the small amount of dirt on the blades was restricting air flow. No broken or missing blades. Spins freely with no vibration. Furnace still tripping after blower cleaning.
An inspection of the secondary heat exchanger and the evaporator coil showed no obvious blockages. I’m the original owner of the house and regularly cleaned and/or replaced the air filter.
I’ve removed and inspected the vent stack blower. Blower appears to be fine; turns freely, no rubbing sound, no broken or missing blades.
I ran a plumbers snake up the vent stack line and found no blockages. I can’t say with 100% certainty that there are no restrictions but it’s not severely blocked.
I checked the gas pressure. I got 3.4 “ WC. Design is 3.5” so the unit is NOT over-firing.
The only way I can get the furnace to run without tripping on high temp is to remove both the furnace filter AND to open the panel that covers the blower.
Typical temperatures readings when the furnace trips on high temp (170 degrees) are as follows:
Inlet air temp = 71 degrees, outlet air temp = 121, temp rise = 50. Nameplate temperature rise is 35 to 65 degrees so I’m within spec.
During these tests I installed a digital meat thermometer within 1 inch of the high temp switch. The meat thermometer reads 130-135 degrees when the furnace cuts out on high temp at 170. I have also used an infrared temp gun to measure the skin temperature of the high temp switch. I get a reading close to 170 when the heater trips. I don’t understand the variation in temps between the different gauges.
Pressure drop readings for three cases as follows: Suction side + discharge side = total static pressure.
(My pressure readings may be slightly off because I’m using a cheapie manometer and plastic tubing without a “static pressure tip”.)
Case 1: Filter installed and furnace panel installed: 1.1” WC vacuum + 0.5” WC pressure = 1.6” WC static pressure
Case 2: Filter removed and furnace panel installed: 0.45” wc vacuum + 0.60” wc pressure = 1.05” wc static pressure
Case 3: Filter removed and furnace panel removed: 0.05” wc vacuum + 0.95” wc pressure = 1.00” wc static pressure
I suspect the builder skimped and installed the minimum ducting, etc. which I believe is fairly common. The filter grill is only 14” by 30”. The manual says the minimum size filter is 24” by 25”. I can easily install a 20” by 30” filter grill which gives the same surface area as the 24” by 25”. However, this alone will not solve my problem because the heater is currently overheating with NO filter in the filter house.
HELP-HELP-HELP. I really don’t want to replace the ducting or furnace if possible. Can I do something with the existing system? What am I missing? Sorry for the long post.

 

Welcome to the forums.

Oh.... this is a long post. I thought it was a novel.

I don't like that the roll outs tripped. That is not a good sign. Those stats rarely go bad.

The only thing you didn't address was.... what fan speed is utilized in heat mode ?

 

Thanks for the quick response. It's been a long journey working on my furnace. I think the heat cycle is on medium speed. The blue wire is on M1 and pulls 5.4 amps when heating. The black wire is connected to "cool" on the mother board. The red wire is connected to "heat". There are zero amps on black and red leads when heating. There is no wire connected to M2.

I also checked the voltages on the various leads on the motherboard during the heat cycle. The cool lead to neutral was 102 volts, heat lead was 120v, and M1 lead was 142v.

I did the same during the cooling cylce.
cool was 120v, heat was 139v, and M1 was 142v.
The motor pulls 7.2 amps on cooling.

I don't understand why all the voltage readings for the heat cycle are lower. Maybe I messed up when I took the readings.
Mike

 

Correction to my previous post.
When cooling, the black wire supplies power to the motor. This is the high speed setting.

When heating, the blue wire supplies power to the motor. This is the medium speed setting.

I live in northern CA and it's now too hot to mess with the furnace. I will wait until the Fall to do more testing. In the interim, I will replace the existing 14" by 30" intake filter with a 20" by 30" filter to reduce the pressure drop in the system. When I can test the furnace this Fall, I will switch the motor to the high speed setting. Hopefully, these two changes will stop the high temp cycling.

I will update this thread in the Fall if it's still open.

 

Sometimes if the fan isn't spinning at the proper speed then the heat radiates off the heat exchanger and will trip the high limit switch. It sounds more though from what you described a return air issue. Maybe get a second opinion from another company as we'll to make sure the exchanger has no cracks or separations.

 

The thread will remain open. Bookmark it and post in it when you return.