Hi,

My furnace recently starts acting weird. After running up to satisfied temperature, the thermostat stops normally, then the blower stops 30 seconds later as usual, but the draft motor sometimes keep on running forever. Turn off then on the power to furnace obviously reset it to normal, removing the thermostat from the wall won't stop it, but switch the thermostat to "FAN ON" then switch to "FAN AUTO" will stop it.

It's hard to troubleshoot since the problem is not consistent, it happens every 2-4 days, sometimes after a long run, sometimes after a shot run.

I check the pressure switch, got 24V when the furnace is running, but got nothing when only the draft motor runs. The drain is not blocked. I just don't know what control the draft motor. I also cleaned the flame sensor, although it doesn't seem to be related.

I can attach some photos of the furnace and the wiring diagram if needed.

Any help is appreciated, thanks.

Do the following test, shut off the t'stat for a call heat, power off the furnace to clear any issues with the control. Now, in the burner compartment, with the power back on, find the main limit switch, which usually located right over the burners ( should have two wires going to it)... take off one of the wires, and see if the main circulation blower starts, or does just the inducer blower start?? If just the inducer blower starts... your problem is that the main limit is trippping during a heat cycle, and the inducer continues to run to cool down the heat exchanger. This can be caused by a dirty air filter, closed or blocked room vents, and / or too slow of a main circulation blower speed, and in some cases, gas pressures not set correctly on the gas valve, and rare but possible defective main limit switch.

thank you so much

I removed one of the wire from the limit switch, the burner fire up as normal except the main blower doesn't run while the inducer blower does run.

When I turn off the furnace and turn it back on (right after the test), the induced blower continue to run again. Guess I didn't remember it right when I stated earlier that turning off then on could reset it.

The only way to turn the inducer blower off is to set the fan to ON (on the thermostat) or set the thermostat to run the heat.

Since it's not consistent (has been running fine for the last two day), I don't think the limit switch is defective. I do notice that when the problem happens, the chamber is still hot, so I guess you are right that the inducer blower needs to run to cool it off. What I don't understand is: shouldn't the limit switch reset/turn off the inducer blower once it's cool enough?

On the limit switch there is an adjuster with values of 80, 85, 90, 95, 110. Currently it set to 110, and the main blower only run for additional 30 seconds after the thermostat reach desired temp and shut off. Can I adjust so that the blower run at higher speed or longer? I'm not sure if I can adjust the main blower speed but the diagram has the setting table.

Since uploading a 750px diagram is useless (too small to read), I uploaded the diagram here just in case:
https://c4w-dev.cla.umn.edu/example/furnace_diagram.jpg

thanks lot for your help

I was so stupid, the settings on the limit switch must be the lower limit for the main blower to stop running. I should have set it to lower number so that the induced blower don't have to do the main blower's work.

I'm lowering the setting to around 85 and see if it will change anything.

You have a control board issue,if inducer runs constantwhen no call for heat. But be sure there isn't a call for heat remove the rw at the conrol board if inducer runs replace the control board

The problem was the relay that control the induced blower, it's too old and keep getting stuck. I read the diagram but it's not there, but I should have thought of it. Anyway, every time it happens, I just go down to the basement, kick the furnace slightly and the relay releases.

I replaced the relay and didn't have any problem within the last 3 months, so that's must be it.