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  • HVAC Thermostat

    I'm wiring up a pair of ceiling fans in my shop so they automatically turn on when the thermostat calls for heat. The fans only need to really run when the heat is on. My thermostat can call for heat, cooling, and also can just call for the fan. I'm thinking of turning on the fans with a standard 24VAC relay using the Fan relay on my thermostat and setting the "FAN" to Auto mode. I've tried to find what Fan "AUTO" really does with typical thermostats (e.g. Honeywell programmable). My shop heater just needs a call for heat and it automatically turns on the fan when the heat exchanger is ready. I'm wondering what Fan "AUTO" does when in heating mode, nothing? I'm hoping to use the fan output so I can also just turn the fan on from the thermostat in the summer time. It seems like the fan output only automatically turns on when in cooling mode and not heating mode on thermostats.

  • #2
    3 Phase Lightbulb,

    I'm no expert on "T" stats, but the auto setting is a feature on older stats that has the fan come on only in the call for heating or cooling as a separate output, the rest of the time the fan is off. So if you are trying to use a "T" stat that has this feature I would say connect it to the heating mode output to run the heater. To try and get fan only to run on the unit heaters you will need to rewire the controls of the unit heater to have a separate fan control. The unit heater offten only has indirect control of the fan, it see's a on command from the heat in the heat-exchanger that tells the fan to come on and turn off when the temp drops below a fixed set point.

    Some units have a fan bypass switch on the unit heater, maybe you can tie into this for your summer control, but it might not be 24v, could be motor voltage.

    I'm sure J Tiers or others will come up with additional information to guide you to the best application for your situation.

    Good Luck and keep us updated as to the final fix.

    TX
    Mr fixit for the family
    Chris

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    • #3
      I figured out what the problem was. There is a setting in the "Installer setup" options to enable fan control when in heat mode. The "auto" fan mode was only working in cool mode and not heat mode for some reason.

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      • #4
        If you want the fans to run only when heat is on, then the logical connection would be to the line that calls for heat.
        Paul A.
        SE Texas

        And if you look REAL close at an analog signal,
        You will find that it has discrete steps.

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        • #5
          AUTO runs the fan ( furnace blower ) only when the thermostat calls for heat ( or cool ).
          ON runs the fan continuously, which might be what you want in the summer to de-stratify the space and provide a little breeze.
          This holds true only if the fan is wired to the fan terminals of the thermostat.

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          • #6
            Originally posted by Paul Alciatore View Post
            If you want the fans to run only when heat is on, then the logical connection would be to the line that calls for heat.
            I wanted two modes of operation. In the winter I want the ceiling fans to come on when calling for heat, and in the summer if I want to turn on the ceiling fans I want to just set the fan to "on" on the thermostat and just have the ceiling fans run and not the fan in my heater, etc.

            The problem was the "Fan" relay on my Honeywell thermostat only operated when you set the fan to "ON" or in "Auto" mode but only in cooling mode. The fan relay would not automatically turn on when in heat mode. There was a "installer setting" where you press some special keys and it lets you change parameters in the thermostat -- one of them was to enable Auto fan operation in "heat" mode.

            I'm all set now. I've got my ceiling fans installed and working from the thermostat using the fan relay.

            The only problem now is I wanted to slow the fans down a little bit so I also installed a pair of Westinghouse fan controllers so I could customize their speed: https://www.amazon.com/gp/product/B0161VECOQ

            But unfortunately the high speed is way too fast, and the next slowest speed is way to slow. It's as if the 4 speeds are 100%, 25%, 10%, and 5%. So now I'm going to try a pair of these instead to see if I can get the speeds I want from each fan: https://www.amazon.com/gp/product/B007YA2YLC

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            • #7
              Originally posted by reggie_obe View Post
              AUTO runs the fan ( furnace blower ) only when the thermostat calls for heat ( or cool ).
              ON runs the fan continuously, which might be what you want in the summer to de-stratify the space and provide a little breeze.
              This holds true only if the fan is wired to the fan terminals of the thermostat.
              That's exactly what I was expecting, but on my Honeywell themo, the AUTO setting does nothing when in heat mode -- unless you go into the custom installer setup and change/enable it.

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