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Thread: Combing 24v and 120v systems on one thermostat

  1. #1
    DIY Junior Member DeweyBeach's Avatar
    Join Date
    Dec 2008
    District of Columbia

    Default Combing 24v and 120v systems on one thermostat

    Here's my situation: I manage an older semi-commercial building, one part of which has hot water heating (radiators) as well as regular a/c with a compressor outside and an air handler in the attic. The a/c system runs on the usual 24 volts, but the hot water thermostat is 120v (the thermostat starts and stops a water pump on the boiler). The thermostats are next to each other on the wall.

    The problem is that sometimes the systems get crossed up and people end up turning on the heat and the a/c at the same time. In addition, I'd like things to be configured such that when the boiler circulation pump is turned on, the attic air handler comes on to help circulate the heat more evenly.

    Is there any reason I couldn't configure a regular 24 volt thermostat (like a Honeywell FocusPro) to control both systems and hook up a relay such that when the thermostat called for heat, it would turn on the fan and also trigger the 120 volt boiler pump circuit? This would make it impossible for the two systems to be on at the same time.

    Thanks for any advice.

  2. #2
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    The contacts or switching circuit in the thermostat may not be rated for that much current. More importantly,the 24v transformer in you system is typically rated at 40 VA, and ONE relay coil uses most of that.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Properly engineered, yes it could work. You might need to upgrade the 24vac transformer, and, if you used a double pole relay, you could just need one relay, to turn both the 120 and 24vac stuff on. Now, whether it would be an approved installation, I'm not sure, but technically, it could be done.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    DIY scratch-pad engineer leejosepho's Avatar
    Join Date
    Nov 2006
    200 miles south of Little Rock


    I once installed a larger 24V transformer in my furnace so I could add a 120V relay for a fan in an exhaust-heat exchanger, and that worked fine. Just take the time to add up all the numbers and be sure the transformer can provide what is needed overall and that the thermostat and relay/s can handle their respective loads.
    "Great minds discuss ideas, average minds discuss events and small minds discuss people." --Eleanor Roosevelt

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