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Thread: Connecting the "Fan "Only" on Thermostat

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    DIY Member skoby's Avatar
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    Default Connecting the "Fan "Only" on Thermostat

    Right now I only have two wires coming from the control board on my furnace to the thermostat in the living room. "R" and "W".

    I want to be able to run fan only. Do I only need to connect a wire from "G" on furnace control board to "G" on back of thermostat?

    Also, whoever hooked it may have wired it wrong. The red wire on the control board goes to the "W" on the thermostat along with the white to the "R".

    It's a Rheem oil burning furnace. I can give more details including schematics and pics if needed. Thank you for any information

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    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by skoby View Post
    I want to be able to run fan only. Do I only need to connect a wire from "G" on furnace control board to "G" on back of thermostat?
    I believe you are correct there.

    Also, whoever hooked it may have wired it wrong. The red wire on the control board goes to the "W" on the thermostat along with the white to the "R".
    That should not make any difference since the voltage is AC, but I prefer matching things properly at both ends.
    "Great minds discuss ideas, average minds discuss events and small minds discuss people." --Eleanor Roosevelt

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    DIY Member skoby's Avatar
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    Do you know if I can just hit the red oil burner switch off and back on when working on this?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    When you are only using two wires in the thermostat, it normally doesn't matter which one goes to which - the thermostat just connects the two together when it calls for heat. But, it won't work properly when you throw in a second function, as power is only on the R wire normally, and it needs to be connected to the G if you want the fan to turn on separately.

    Most of the time, the 24vac control voltage comes from a transformer in the heating unit, so shutting down power to it should kill the power. There are situations where the 24vac could be supplied from another source - when working with electricity, it's always safer to check.
    Last edited by jadnashua; 12-11-2013 at 12:07 PM.
    Jim DeBruycker
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    DIY Member skoby's Avatar
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    Quote Originally Posted by jadnashua View Post
    When you are only using two wires in the thermostat, it normally doesn't matter which one goes to which - the thermostat just connects the two together when it calls for heat. But, it won't work properly when you throw in a second function, as power is only on the R wire normally, and it needs to be connected to the G if you want the fan to turn on separately.

    Most of the time, the 24vac control voltage comes from a transformer in the heating unit, so shutting down power to it should kill the power. There are situations where the 24vac could be supplied from another source - when working with electricity, it's always safer to check.
    Ah ha, that makes sense with the wire switch. Since I'm putting in new wire anyway I will switch them so the red and white are correct.

    Another question. The thermostat has a cool setting also. Will the fan to come on if I program it to or does there need to be some type of central A/C unit attached? Thanks

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    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by jadnashua View Post
    When you are only using two wires in the thermostat, it normally doesn't matter which one goes to which - the thermostat just connects the two together when it calls for heat. But, it won't work properly when you throw in a second function, as power is only on the R wire normally, and it needs to be connected to the G if you want the fan to turn on separately.
    Yes, and I thank you for that clarification. I was thinking of transformer connections while talking about a thermostat.

    Quote Originally Posted by skoby View Post
    Another question. The thermostat has a cool setting also. Will the fan to come on if I program it to or does there need to be some type of central A/C unit attached? Thanks
    If you put a jumper between "G" and that "Cool" connection on the thermostat, the fan should run whenever the thermostat tries to cool the house. But other than that, you would need some kind of controller at the other end.
    "Great minds discuss ideas, average minds discuss events and small minds discuss people." --Eleanor Roosevelt

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    DIY Member skoby's Avatar
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    Quote Originally Posted by leejosepho View Post

    If you put a jumper between "G" and that "Cool" connection on the thermostat, the fan should run whenever the thermostat tries to cool the house. But other than that, you would need some kind of controller at the other end.
    Alright, I'm going to give a shot tomorrow. Thanks

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