Our new place has a working AC (meaning that the physical box outside spins and can produce cold air) and a working fan (meaning that the fan spins and is able to put air out to the vents in the house). However, both the AC and the fan will not work together.

Through some experimentation, when turning the heat on, the fan will run and output warm air. If I then immediately switch to AC, I will get out about 2 minutes of cold air through the fan until the fan turns off.

The fan in all cases has not been set to 'auto' but rather has been forced to the 'on' mode.

This leads me to believe there is an issue with the control panel, as both components work individually. Any suggestions on how to debug/fix this issue before we have to pay for an inspector to come in?

  • 1
    Can you post a wiring diagram + make/model number for your furnace? – ThreePhaseEel Jun 19 '17 at 22:16
  • I will try to get all make/model of the components and edit the post later tonight @ThreePhaseEel, thanks for your response – Easton Bornemeier Jun 19 '17 at 22:20
  • To be clear, the FAN switch on the thermostat is in the ON position, even when you're switching between heat and A/C? With the FAN switch in the ON position, does the fan still run when the thermostat is OFF (neither HEAT, nor COOL selected)? – Tester101 Jun 20 '17 at 10:47

If this is an older furnace you may have a bad relay in the furnace control box that runs the fan when the a/c is turned on. If it a newer furnace you may have a problem with the printed circuit board. A picture of the control board or the wiring of the furnace would help, along with the model, serial, and age of the equipment


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