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streatherg · Nov 2, 2006, 11:34 AM

I have a goodman GMNT080-4 furnace. It's about two years old.

When the thermastat first turns it on, it runs great... for about an hour, then it just stops, even though the thermastat is still saying system on.

If you reset the thermastat, sometimes it fires right back up again, but only sometimes, and of course at night when I am not awake to reset the thermastat, the house gets very cold!

Any ideas on things I could try before I pay for a serivce call?

NorthernHeat · Nov 2, 2006, 03:58 PM

Sounds like it may be overheating. Make sure filters are clean and all registers are open. The PC board should give you a diagnostic code count the flash's through the viewing hole in the blower door, next time it does this.

streatherg · Nov 13, 2006, 07:25 PM

Originally Posted by streatherg

I have a goodman GMNT080-4 furnace. It's about two years old.

When the thermastat first turns it on, it runs great...for about an hour, then it just stops, even though the thermastat is still saying system on.

If you reset the thermastat, sometimes it fires right back up again, but only sometimes, and of course at night when I am not awake to reset the thermastat, the house gets very cold!

Any ideas on things I could try before I pay for a serivce call?

OK, It fixed it for about a week, then intermittently does the same thing again. Tonight I finally got to look throug the viewing door at the flashing light after it went off. It flashes just once, with about two seconds in between single flashes. Any ideas what this code is telling me?

NorthernHeat · Nov 13, 2006, 10:24 PM

On the blower door or somewhere, there should be a label that tells you what the codes mean.

streatherg · Nov 14, 2006, 06:30 PM

OK, so it says "gas flow, gas pressure, gas valve, flame sensor". What is strange is that it will run for sometime two or three hours before it shuts down, then will not try turn itself back on, until you either turn off the power to the furnace or leave it a while and reset the thermostat. I have read some of your other comments to other questions and they suggest that a flame sensor problem would probably cause it to try to re-start, so I guess that leaves me with the other three.

I have another furnace running off the same gas line that has never cut out, so don't think it is gas flow or gas pressure, which would leave gas valve. I'm assuming this is probably not a "self-do" action.

Thoughts. Is it time to pay for the service call?

NorthernHeat · Nov 15, 2006, 03:46 PM

Originally Posted by streatherg

OK, so it says "gas flow, gas pressure, gas valve, flame sensor". What is strange is that it will run for sometime two or three hours before it shuts down, then will not try turn itself back on, until you either turn off the power to the furnance or leave it a while and reset the thermostat. I have read some of your other comments to other questions and they suggest that a flame sensor problem would probably cause it to try to re-start, so I guess that leaves me with the other three.

I have another furnace running off the same gas line that has never cut out, so don't think it is gas flow or gas pressure, which would leave gas valve. I'm assuming this is probably not a "self-do" action.

Thoughts. Is it time to pay for the service call?

Most furnaces will lock out the heat if it notices a problem 3 times. A flame sensor will do this too. Try to clean it.
I would have guessed the problem to be a faulty main limit since they get weak after opening and closing to many times because of low air flow.

ed s · Nov 16, 2006, 01:49 PM

Originally Posted by streatherg

I have a goodman GMNT080-4 furnace. It's about two years old.

When the thermastat first turns it on, it runs great...for about an hour, then it just stops, even though the thermastat is still saying system on.

If you reset the thermastat, sometimes it fires right back up again, but only sometimes, and of course at night when I am not awake to reset the thermastat, the house gets very cold!

Any ideas on things I could try before I pay for a serivce call?

I have a goodman... 120,000.. it was doing almost thew same thing... this week I put a new pressure switch on it... it is the round black vacuum type... it was a vertical or horizon mount heater... this case it was mounted horizonally.. which put the switch on the bottom... allowing water to get into the pressure switch.. I put a new switch on... mounted up top... running great... no water gets in switch now... it is ashame goodman does not tell installers this...

NorthernHeat · Nov 16, 2006, 05:11 PM

Is this a 80 or 90% efficient?

ed s · Nov 17, 2006, 01:21 AM

Originally Posted by NorthernHeat

Is this a 80 or 90% efficient?

It is a 90%

NorthernHeat · Nov 17, 2006, 05:35 AM

Make sure the condensate drain and trap are clear.

handyone · Nov 21, 2006, 04:52 PM

I just resolved this problem with my own furnace. I had the same symptoms and it did in fact turn out to be the flame sensor.

Two screws and a piece of sandpaper was all it took. Remove the sensor and clean it before you pay for a service call. What can it hurt to try?

jfv · Nov 29, 2006, 09:21 PM

Depending on how mechanically inclined you are, there is a flame sensing rod that is located inside the burner area that most likely has a build up of a white residue on it. This needs to be cleaned. If your are not real sure about going in to this part of the unit, it would be best to have a professional do it. The problem being that if you touch,break or crack the glow coil at the other end of this assembly you will have bigger problems. Let me know if you have any more questions.

JFV

Winger · Dec 8, 2008, 08:35 AM

Hi,

I have a goodman GMNT080 furnace. It's about 5 years old.

When the thermastat first turns it on, it runs great... for about an hour, then it just stops, even though the thermastat is still saying system on.

If you reset the thermastat, sometimes it fires right back up again and at night when I am not awake to reset the thermastat, the house gets very cold!

I read the articles and I am a novice to the furnace. Can you send me the Goodman GMNT080 pictures for the parts you have indicated so that I can start inspect them? I am afraid I might pull something off in error, but with the pictures at least I feel better. Especially the pressure switch.

Thanks.

hvac1000 · Dec 8, 2008, 09:01 AM

Originally Posted by Winger

Hi,

I have a goodman GMNT080 furnace. It's about 5 years old.

When the thermastat first turns it on, it runs great...for about an hour, then it just stops, even though the thermastat is still saying system on.

If you reset the thermastat, sometimes it fires right back up again and at night when I am not awake to reset the thermastat, the house gets very cold!

I read the articles and I am a novice to the furnance. Can you send me the Goodman GMNT080 pictures for the parts you have indicated so that I can start inspect them? I am afraid I might pull something off in error, but with the pictures at least I feel better. Especially the pressure switch.

Thanks.

You are posting to a very old post. Here is the flame sensor.

Winger · Dec 9, 2008, 07:01 AM

Originally Posted by hvac1000

You are posting to a very old post. Here is the flame sensor.

I cleaned the flame sensor and mesaured the pressure switch voltage to the frame ground. I got 24 volts on the pressure switch. But my furnace still does not kick in when calling for heat. The thermostat drops down 2 degrees. It occasionally fired up but it warms up by 1 degree. What is my next step?

hvac1000 · Dec 9, 2008, 08:19 AM

Start by eliminating the thermostat. In the furnace remove the R and W thermostat terminal wires. Then using a jumper bridge the R and W wires. This should cause the furnace to operate in the heat mode. If using alligator type clips on the jumper you easily remove the jumper off one terminal then reattach. By doing this you are simulating furnace operation with out the thermostat. If it starts and stops over a period of the test then the thermostat or thermostat wiring is the problem. If it still acts up as before during the test then it is another component causing the problem.

Winger · Dec 10, 2008, 07:40 AM

Thank you so much hvac1000. My problem is fixed. It was caused by the thermostat. I took it, reset it and it worked so far. I will monitor it for a couple of days and see if it holds up. Again thank you for your advices.

moekneeq · Oct 2, 2009, 10:48 AM

I turned my furnace on yesterday for the first time since the summer. I set the temperature and added water to the furnace. It came on for a hour and then it just went off and won't come back on. Do you think I need to replace the thermostat. My furnace has never given me any problems and it is 20 years old... Please advise.

bigsexie · Nov 16, 2009, 08:04 PM

I don't know what is going on but I am having a problem with my system kind of the same but mines comes on for about a minute and turn off and it is cold my light blinks once and it says on the panel that my system is locked out what can I do does anyone have any ideas please.

Joshdta · Nov 16, 2009, 08:07 PM

Start a new thread, with your make and model of your furnace. How long does the flames burn before they drop out?