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airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8253
Experience:  Furnace, A/C and heat pump specialist.
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When I turn thermostat fan function to auto the fan won't

Customer Question

When I turn thermostat fan function to auto the fan won't run. It will only run on the "on" function
JA: Just to clarify, do you think this is a larger HVAC problem, or something specific to the thermostat?
Customer: It's a brand new thermostat. Problem I think is on the furnace
JA: Do you plan on doing the work yourself?
Customer: Yes
JA: Anything else we should know to help you best?
Customer: That's it
Submitted: 9 months ago.
Category: HVAC
Expert:  airheatman replied 9 months ago.

OK most likely a relay or control board in furnace. Do you have a multi meter so we can troubleshoot?Also I need model on furnace to give you specific guidance

Customer: replied 9 months ago.
I don't have a multimeter. It is a trane 90
Expert:  airheatman replied 9 months ago.

Can you give me complete model from inside furnace in burner compartment so I can provide guidance with this specific unit.Model will be on a label just above burners on right or left inside panel

The 90 is just a series containing several models

Customer: replied 9 months ago.
tu1cd120a9601ae is model #
Expert:  airheatman replied 9 months ago.

Thanks for that information. I would really want to verify by checking to make sure we do not get voltage to motor on call for heat between terminals. H-Heat and CIR- N on board before diagnosing, but the fan is controlled by relays built into the board. So since it runs in on position (which is controlled by a different relay on board), but not in auto, I am comfortable in saying the relay is stuck open and not telling blower to come on in auto mode.

This require replacing the board. If it is something you want to do yourself and need help locating a board let me know.

If you have further questions, please let me know.

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Expert:  airheatman replied 9 months ago.

To verify you can obtain a meter and check for 120v between HEAT-H and CIR-N for 120v 30 seconds after burners come on.If no 120v board is definitely the culprit.