Honeywell AT72D Transformer override protection.

Discussion in 'HVAC Heating & Cooling' started by Ray McAllister, Dec 19, 2018.

  1. Ray McAllister

    Ray McAllister New Member

    Joined:
    Dec 19, 2018
    Location:
    19438
    I have 2 Taco zone valves powered by 1 24-vac transformer rated at 40 va. Transformers keep failing. I'm thinking that there might be some kind of wiring problem that I can't identify, or that sometimes the zone valves are all on at once and use more than the rated 40 va. I am considering a Honeywell AT72D transformer. It has something they call "override protection". Does any body know exactly what override protection is? If it is something that protects the transformer from damage when more that 40 va is being used, than I think it may solve my problem.
     
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
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    Retired Systems engineer for defense industry.
    Location:
    New England
    The spec sheets for things powered by that transformer should list their power requirements. Add them up. If it's not enough, buy a larger transformer. The zone valves may have a tag on them. This place has some that are larger, as would most any HVAC shop. https://www.supplyhouse.com/Transformers-282000 Some are double what you have, but a 40va one is fairly common. Note there will be some load from the control box itself so you won't have the full 40va available for external things. The spec sheet should say what's available.

    According to the Honeywell spec sheet, that transformer has a thermal fuse on the 240vac primary wiring. Don't know if it's a self-resetting one, or would require replacement. But, their spec sheet does list larger VA ones available https://customer.honeywell.com/resources/techlit/TechLitDocuments/69-0000s/69-1641EF.pdf The AT87A or AT88A are 50 and 75VA respectively.

    Are you sure you have it wired up properly?
     
    Last edited: Dec 19, 2018
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  4. Ray McAllister

    Ray McAllister New Member

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    Dec 19, 2018
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    Honeywell's sales literature says it has "override protection". The spec sheet describes "overload protection" for the 208c/240v models. I have a 120v model, so there is no overload protection described by the spec sheet, yet the sales literature says it has "override protection". I do not know what override protection is. I believe everything is wired correctly. Everything has worked fine for years, all I did was change one thermostat. Things were then fine for a week or two, and then the transformer went bad. I had the transformer replaced, and then it went bad after a few days. I just bought and installed a Honeywell AT72D transformer yesterday, and so far it is working. I am wondering if the new thermostat I installed draws more current than the old one, and the sometimes it exceed the capacity of the transformer when all 3 zone valves are open. The aquastat has its own transformer, so the only load comes for the 3 zone valves and thermostats. I can't find anything that says how much current the thermostats draw.
     
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    A zone valve that is getting stiff could draw more power to activate. Unless it’s an electronic thermostat powered by the transformer, it doesn’t draw any power except whatever it takes to activate the relay.
     
  6. plumber69

    plumber69 In the Trades

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    Jun 20, 2014
    Location:
    Prince Rupert, British Columbia
    look for any wires that look burnt or are touching hot metal spots. Take a pic of your transformer setup
     
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