I was changing my thermostat and I attempted to make the swap live without turning power off. When I connected new thermostat, the fan kept running.

Ruled out bad thermostat and wire short by disconnecting those wires from the board and turning furnace on, fan still turned on.

Ruled out bad board by buy a brand new one. Fan still runs

Ruled out burner buy unplugging it from the board fan still ran.

Look at wire lay out and the fan has 4 cables, one to a on/off low speed fan switch, one to a number "2", one to a "cool", and one to "heat" I tested it by unplugging the "cool" and fan still turned on. I then unplugged "heat" and the fan didn't run. I then left heat unplugged and tested thermostat and it worked like it should, turning the fan on and off when I told it to.

As far as I can tell. The "heat" cable goes directly from fan motor to the board. What could cause the fan to run?

  • What's the make and model of the furnace? What's the make and model of the thermostat? Are you sure you wired the thermostat correctly? Is the thermostat configured properly? – Tester101 Mar 11 '16 at 20:00
  • Does the thermostat have a fan switch that switches between "auto" and "on"? Is it on "on"? – Harper Mar 11 '16 at 20:41
  • I am unsure of the model of the furnace. The sticker inside only pops up the oil burner. But it is made by York. Thermostat is Honeywell. Old round one with mercury. Yes thermostat is wired correctly. It's the 4 wire one green red yellow white. To corresponding letters. Thermostat is set to off and fan is set to auto. It does have the switch that can toggle between auto and on and is set to off – Tico Mar 11 '16 at 22:18

Tico,the fact you swapped out the thermostat with the power on, may have been to your detriment. It is always best to remove power before working on electronic controlled equipment, one for your safety and secondly to avoid the risk of sparking which electronic circuitry doesn't like. You don't have to see a spark for damage to occur. My first thought was a control board of some type,but you say you have purchased a new one, which may rule that out. My second thought is an electronic switch, external to the control board but controlled by the board,which may have a short, possibly to ground or your negative supply. For this to be confirmed, you will need to access a circuit diagram or schematic. Hope this assists in some way.

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