Our church has two HVAC units to cool and heat the sanctuary. In cooling mode, they operate like a normal AC system using two compressor units outside. In heat mode, they use hot water provided by a natural gas fired boiler with a heat exchanger inside each air handler to warm the air. Both units are controlled by a single thermostat. This picture shows the two air handlers - they're the ones in the background. The two in the foreground are for another part of the church.

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Each air handler has a 24 VAC transformer in it. When those systems were installed, the outputs of the two transformers were wired together in parallel to provide power for the thermostat. Both those transformers failed at the same time, and thus the thermostat was not working, which mean no heat.


Is it normal, or allowed, to wire multiple 24 VAC transformers in parallel in this type of installation? I'm not sure what it buys you as I think the transformer with the higher output is going to power the load.

  • 1
    not at same time. One transformer failed, then the second one failed due to overload.
    – Traveler
    Commented Oct 26, 2022 at 21:25
  • Wait, so your system alternately heats or chills the water and carries that to the air handler? Wow. Another system I'm very impressed with has the refrigerant cycle happen locally between water and circulated air. In summer it heats the water, and the outdoor unit is a cooling tower. In winter the same water is routed to a gas boiler, district heat interchange, geothermal, heat pump, solar heat, whatever ya got. Commented Oct 26, 2022 at 21:43
  • @Harper... - Not quite. In cooling, it's a typical AC arrangement using an outdoor compressor, refrigerant, and evaporator coils in the air handler. In winter, it uses the boiler supplied hot water (which circulates continuously through the entire building) which, through a separate hot water pump, circulates hot water through a second set of coils in the air handler. So the water circulating is always hot. There is no provision to chill the circulating water.
    – SteveSh
    Commented Oct 26, 2022 at 22:30
  • "I think the transformer with the higher output is going to power the load". Oh, it's worse than that. Any difference in the output voltage of the connected transformers (in magnitude and/or phase) will drive a current that circulates around the transformers, increasing the load beyond the normal requirements of the heating controls. As already noted the solution is to use one transformer with a suitable rating. Presumably the transformer will be powered from one of the air handlers. Note that that handler will need to be switched on for the other handler to work.
    – Graham Nye
    Commented Oct 27, 2022 at 15:47

3 Answers 3


The risk in your plan is that you MUST have the transformers in the same phase or they'll operate like a dead short. I would suggest a single higher power 120-24v transformer that can supply the needed amperage for the two air handler controls and T-stats. BC product recommendations are frowned upon here at SE, just do a search for "hint hint" Google "higher power 24v transformers".

AGAIN: if you go with two transformers in parallel they MUST be phased correctly so that they don't "fight each other" .

  • "... must be phase together". Yes, I understand that. These systems were installed 15-20 years ago, before I came on the scene. And so far as I know (though I can't be sure), that's the way they've been wired from day one. There was some repair done on one of the air handlers back in August, so it's possible the leads from one of the transformers got swapped back then. But if they were damaged from that, then we wouldn't have had any AC.
    – SteveSh
    Commented Oct 26, 2022 at 19:45
  • The service person brought in a single "universal" transformer that is temporarily wired in in place of the two failed parts.
    – SteveSh
    Commented Nov 20, 2022 at 21:57

If they are identical transformers and fed off (the same phase of) the same supply, it will probably work.

But if one fails, that leaves the other overloaded, and it will fail too. Which is most likely what's happened here.

It's not normal, and I can't think why anyone would do it unless that couldn't find a single transformer with a high enough rating.

  • And we're not just talking about identical input and output, but impedance too and that's not usually on the name plate so don't do it.
    – JACK
    Commented Oct 26, 2022 at 18:43
  • I don't think they were wired in parallel for amperage reasons, though I was not around 15-20 years ago when those systems were installed. The 24 VAC powers the single tstat, which then provides the control signals goes back to the two air handlers.
    – SteveSh
    Commented Oct 26, 2022 at 19:49

In my opinion the best solution would to replace both transformers with one large enough to supply both units. The existing transformers have a VA rating (guess 40), add them together 40+40=80. The next size larger would probably be 100VA.You need to keep the voltage at 24V but the VA must equal the sum of both. You should be able to find one but expect it to cost more then the two 40VA units.

  • Actually that's what was done, albeit on a temporary basis. The HVAC tech had a "universal" transformer in his truck that he wired in.
    – SteveSh
    Commented Oct 26, 2022 at 23:42

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