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Do you need a G wire from the thermostat to control the blower during the heat cycle?

carlc Member Posts: 4
HI folks, my coworker says you need a G wire from the thermostat to the furnace in order for the ECM blower to run during the heat cycle. I know that with a standard furnace (PSC) furnace, Only R and W are necessary to run the furnace (and blower) during a heating cycle. So, the situation was, a customer had an Ecobee thermostat with only 4 wires coming from the thermostat to the furnace, so no G wire, only R W C Y hooked up. It was one of the first gen Ecobee wifi stats i believe, non touch screen, with no batteries. The issue was that the ECM blower was not coming on during a call for heat and the furnace was limiting out. I jumpered R to G at the board and the blower still did not come on. I replaced the blower motor and ECM module, but the same issue still happened. My coworker believed it was due not having a G wire between the furnace and thermostat. I've always thought that the G wire is only necessary to have to control the blower by itself, and for air conditioning. He said it's necessary with an ECM blower. Is this correct?

We net replaced the board and the thermostat with just a basic 1h/1c battery operated thermostat. hooked the wires R W G Y. I wanted to replace just the board first and test the theory with the same thermostat in, but my coworker said it may "take out the blower again" so we replaced the control board and thermostat at the same time and it corrected the issue.


    JUGHNE Member Posts: 9,763
    I have never seen any of these components but, I think that all gas furnaces control their fan with a control board that brings on the fan after the burner fires.

    Usually need a green for AC though.

    I would remove the tstat wired and then jumper R &W at the furnace and see if fan turn on.
    HVACNUT Member Posts: 4,594
    Your reasoning is correct. No G is needed for the heat cycle. Like @JUGHNE said, fan control is done through a time delay relay in the board.
    You won't however have blower operation in A/C or fan on modes.
    You can jump Y to G in the board for A/C but must leave the thermostat in auto for the fan. If the thermostat is switched to fan on, the A/C will run.
    In the future, diagnose rather than throwing parts at it. It'll be cheaper.