Combing 24v and 120v systems on one thermostat

Discussion in 'HVAC Heating & Cooling' started by DeweyBeach, Dec 2, 2011.

  1. DeweyBeach

    DeweyBeach New Member

    Dec 14, 2008
    District of Columbia
    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. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA
    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.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    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.
  5. leejosepho

    leejosepho DIY scratch-pad engineer

    Nov 23, 2006
    disabled-retired industrial fabricator
    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.

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