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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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My UD080R furnace no heat, fan can run I can run the fan

Customer Question

my UD080R furnace no heat, fan can run
I can run the fan continously with no problem from the switch on the thermostat. As soon as I set the thermostat to "heat" the control panel fuse blows. The fuse will blow even if the thermostat is set to 60 degrees and the thermostat indicates the temperature is 68 degrees.
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.


Try unplugging one component at a time from the printed circuit board, until the fuse does not blow...the last item was the one with the short and needs to be replaced... if you get all of the plugs pulled from the board, and it still blows a fuse, the the board is bad.

That would be the laymans way of doing it, you will need 4 or 5 spare 3 amp fuses.

If you have a multi meter you can set it on Ohms, and unplug each item at a time and use the ohm meter to see if there are any dead shorts... that takes a bit of skill with the ohm meter.

Let me know what you think, we can go from there without a time limit, however I am off to bed shortly, if I miss you tonight I will pick up your response in the morning.

If you want to get deeper into the trouble shooting, attach a well focused, close up photo of the wiring diagram glued inside the service panel,you can use the paper clipicon at the right end of the toolbar at the top of this dialog box,Include a picture of the thermostat wire connections to the printed circuit board in the same area.

If there is no paper clip icon, look for the '+attach files' link at left of your send button.

I can mark it up with trouble shooting instructions. You will need a multi meter ($30 at a hardware store) and some experience working with appliances, let me know a lot about your skill sets in those regards.

We can go from there in the morning.

Customer: replied 1 year ago.
I have measured it out with my Ohm meter (past electronic technician). I see shorts associated with switches on the thermostat when the batteries not not connected. The batteries skew the readings of the Ohm meter.
I think I already unplugged any optional equipment (AC and Humidifier).
I had a couple people double check the wiring at the furnace and the thermostat.
I was hoping it wasn't the control board, but I am afraid it is the control board, your comments almost confirm it.
Are there any places I can check voltages to confirm a problem with the control board?
Expert:  Phil replied 1 year ago.

Hello again, the gas valve, blower motor, inducer fan and igniter relays are all mounted on the circuit board with plugs connecting each of those items to the board... you can unplug them one at a time and check for a dead short.

The problem is not likely to be at the thermostat.

I am off to bed in a few minutes, I will pick up the results of your tests in the morning.