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I have a heat-only electric furnace. Bypassing the thermostat, the R and W terminals on the control board are always connected. The thermostat is good but something is causing the control board to always have a low setting on the Fan and heater.

I have replaced the 240 and 24V relays and that did not fix anything. What could cause a constant ON even with nothing connected to the control board?

Note:

BREAKER OFF & no thermostat connected: Control board is not shorted, all terminals do not beep with multimeter continuity test

BREAKER ON no Thermostat connected: fan turns on, heating elements have 125V on them, all control board terminals beep and multimeter claims continuity between them.

Voltages: R to GND ~ 13V; W1,W2,G,C to GND ~30V

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R and W being shorted would amount to a constant call for heat. On older furnaces with multi-tap PSC motors, heat would usually use the tap for the low or medium-low fan speed so that explains the constant low speed fan.

Common thermostat wiring reference:

  • R = 24V power
  • W = Call for heat (fan on)
  • Y = Call for cool (fan on)
  • G = Call for fan only
  • C = Common for powering thermostat

Connecting R to any of the other terminals is how the thermostat makes the command to the furnace.

What it doesn't explain is why the heat source isn't constantly on as well. Is there any other wiring that could be controlling the heat strips separately?

What is the brand/model of your furnace?

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  • Yes - but R and W seem to be connected even without the thermostat connected or ON. I replaced the relays and that did not fix it. I’m not sure what else could have gone bad to cause this, no wiring was changed since it worked correctly. I do notice that the fan is not on maximum when it is on without a call by from the thermostat, and when I short R and G for the fan, it kicks on harder as well. I’m not even certain the heating elements kick on when it’s operating on this low setting without a call from a thermostat. It’s like there is a constant floating voltage on the control board.
    – Jonathan
    Aug 4, 2022 at 19:24
  • OK - I misunderstood. I thought you were describing a jumper or something aside from the thermostat wiring. Can you post pictures of the control board?
    – Chris O
    Aug 4, 2022 at 19:25
  • Edited the post to show the control terminals
    – Jonathan
    Aug 4, 2022 at 19:31
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    With the breaker OFF, and no jumpers or wires connected - none of the terminals on the control board are shorted or connected. When breaker is ON to the furnace - the entire control board becomes “shorted” if I use my multimeter and test for continuity on each terminal. I’m not sure if they are supposed to be connected but they are voltage controlled? So voltage levels determine operation, not simply shorting them? Or if there is something wrong causing them all to be connected when they should not be. R-GND = 13V ; W,G,C to GND = 30V
    – Jonathan
    Aug 4, 2022 at 19:40
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    Exactly. That's what is leading me to believe something else is damaged there. The thermostat terminal are basically messengers. 24V is supplied from R and by connecting (through an appropriate resistance device like a thermostat - do not jumper directly) to the other terminals you complete the low side circuit of a relay and switch the high side. Disconnected, all terminals other than R should be at 0 potential to ground.
    – Chris O
    Aug 4, 2022 at 19:57

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