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furnace fan wiring improvement

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  • furnace fan wiring improvement

    Hi all,

    Currently we have a multi speed direct drive fan in our furnace. And we have a heat pump for AC in the summer and light heating. When it's cold we use oil. The thermostat has a switch for fan auto or on. It works fine, and I believe it is wired for highest speed only for both setting.

    I would like the system to work as follows:

    in Auto mode when using AC: fan either off or high speed.
    in Auto mode when using heat: fan either off or lower speed.

    in On mode when not heating or AC: fan on lower speed.
    in On mode when using AC: high speed.
    in On mode when using heat: lower speed.

    The reason for high speed on AC is because cold air is heavier than hot air...therefore requires more power to move it.

    I'm not sure how many speeds the motor has at this time. I will investigate further.

    Any other suggestions or ideas are welcome.

    What would I need to buy? And a wiring diragram.
    I don't mind having a manual switch to swith between summer and winter mode if necessary.

    We have an Aube thermostat model TH146 (multistage) and the control module: CT280-2H1C
    manual here:


  • #2
    Re: furnace fan wiring improvement

    Typically, when we wire an air handler, we use the second speed on a three speed fan. The low speed usually doesn't move enough air and the high speed is usually noisy from the air flow. You want to be careful that you don't cool the house off too fast, by satisfying the thermostat and failing to remove the excess humidity. You will wind up with a cool, clammy house. Don't over-think it, if it's not broken don't fix it.


    • #3
      Re: furnace fan wiring improvement


      We recently moved into the house. The fan is hard wired to high speed. Up here in Montreal the outside temp swings from -30C in the winter to +30C in the summer. While it may seem that the controls I propose seem like overkill, its more for tuning. I am hoping that after one complete year I will find optimal setting for each season.

      Further, some rooms are really close to the furnace so they adjust quickly, and others are far away (50 feet of ductwork). The temperature of the air in the ducts changes alot from one end to the other. It's quite complex to tune the vents for even distribution further complicated by summer/winter requirements (eg: We close the vents in the basement in the summer).

      I guess I will have to come up with a system of relays and switches to set this thing up.

      Again, all feedback welcome.



      • #4
        Re: furnace fan wiring improvement


        Check out this drawing I just made. I beleve it ensures that no two (or more) connections to the fan get charged and any time.

        Don't do this if you don't know what you are doing! Be careful! (disclaimer). The drawing and assumtions could be wrong and you could blow up yourself or the entire Universe
        Click image for larger version

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        This design is for a 24volt thermostat and a 115 volt AC 3 speed fan.
        The furnace fan controller will send 24V to a relay with a 24v coil.
        When no fan is requested the normally closed (idle) is connected to the common.
        The blue wire is charged and it goes to S2.

        Switch S1 is used to control whether we have high or med speed when heat/ac is requested.
        Switch S2/S3 are used to control whether we have med, low, or off speed when the system is idle.

        When the thermostat requests the fan to run then the coil in the relay is charged and power goes to the normally open connection connected to S1.
        Turn switch S1 up and we have high speed fan when fan service is requested.
        Turn S1 down and we have medium speed on the fan when fan service is requested.

        Turn switch S2 up and we have med speed fan when no service is requested.
        Turn S2 down and power is sent to S3.
        Turn S3 up and we have medium speed on the fan when no service is requested.
        Turn S3 down and the fan will not run when no service is requested.
        Last edited by tls1; 09-01-2012, 08:55 AM.