Goodman Model GMNT100-4 Auto Power Down and Up until Temp.

check that the replaced thermostat is the same temp degree setting of the last one

your limit switch is opening up, that causes constant fan. Check for flashing codes after flame shuts off and fan continues. Check airflow, filters, lack on return air for possible hi temp limit problems.

You could have a bad overtemp sensor which is activating at too low a tempreture. They should not be that expensive. I would just replace it to eliminate the control if it is bad. It is a small metallic disc attached to the heat excanger. Has two wires coming off it. Approximately 1 inch in diameter. Usually on the front of the heat exchanger. It should have the tempreture that it activates at stamed on it. The temp is probably in celceus. Prior to replaceing it you could determine the activation temp and if you have a thermometer, or better yet an infared thermometer, check it while running to see what temp it activates at. If it is bad it will activate at too low a tempreture and should be replaced. Let me know if you need anymore help.

hardrocko

Thanks for using FixYa

it sounds like the low temp limit switch, the one that cools the furnace down with the burner off and the thermostat open, when it cools to a point the fan stops, the other switch in burner area turns burner off/on, hope this helps

Check if your thermostat is set on 'o fan' setting.
If it is, trey to set it on 'Auto-fan' instead.
since it seem as if the heating does go off,only the fan may be running continuously.
this is possible if the thermostat is incorrectly set or installed.
good luck.

The sequence of events for furnace operation is -

Fan purge for 30-45 seconds. During this time, a pressure switch is actuated by the running of the purge fan, proving the control board the purge fan is running.

After purge and purge prove, the hot surface ignition and or spark begins. After a few second delay, the gas valve will open. The control board then monitors for flame prove through another sensor located near the burners. If there is no flame within 3-5 seconds, the gas valve closes. It will repeat this process, usually 3 times, before it shuts the system down, which requires a power down of the system to reset.

I have seen trash get into the hose between the purge blower and the purge switch. But you could be having other problems.

Check your themostat. The fan switch may be in the on position and not in the auto. If that is not the problem, try turning the power off to the furnace, wait 1 minute and turn the power on. It may have went into a lock-out which causes the blower to run.

There are certain faults like "Flame Rollout" and "High Temp Cut-out" that will cause the fan to run continuously until corrected. If the furnace is working then some power is making it to the "G" terminal at the circuit board. Could be a jumper or miswiring.

Press the *power button* once to start the heater on high power, continuous run. If left at this setting, the unit will run continuously on high power (1500 watts, 12.5 amps) and will not shut off until the unit is manually turned off.
Press the *power button* again, to change to low power, continuous run, (900 watts, 7.5 amps) and will not shut off until the unit is manually turned off.

Set temperature:

Press the temp/timer button until the temperature icon blinks. Use the up and down arrows to set the desired temperature.
The temperature icon will stop blinking and the current room temperature will reappear after the desired temperature is set.

The heater will turn off and on to achieve the desired temperature.

Set timer: (auto off)

Press the timer/timer button until the timer icon blinks. The hour display will appear. The auto off icon will also appear.
Use the up and down arrow buttons to set the heater operation time between 1 and 10 hours. This function will activate once the timer icon stops blinking. The heater will automatically turn off whn the times runs out.

Set timer: (auto on)

Press the temp/timer button until the timer icon blinks. The hour display will appear. The auto on icon will also appear.
Use the up and down arrow buttons to set the heater start time between 1 and 10 hours. This function will activate once the timer icon stops blinking.
The heater will automatically turn on when time expires.

Hope this helps!