I have seven 24v 2amp thermostats.

The thermostat wires all feed to the same place.

Do I just need a single 24v 2amp power supply to power them?

The thermostats are Tekmar 532. They will connect to a grundfos pump and a zone control valve. zcv is 24v as well. For now I just want to test that the thermos are not doa but I suppose the power supply is going to have to power the 24v zone control valves as well.

  • maybe you don't need any. which thermostats, what are they connecting to?
    – Jasen
    Commented Mar 12, 2021 at 20:07
  • 1
    Thermostats that need 2A themselves (some of the Wifi ones might use that much), or thermostats capable of switching 2A? That'll make a big difference in the answer. What are these thermostats exactly?
    – Nate S.
    Commented Mar 12, 2021 at 20:09
  • Yes what is the wattage of the valves and if the pump controlled by a contactor that needs to be added also (the contactor) all added up then get the next size up.
    – Ed Beal
    Commented Mar 12, 2021 at 21:41

2 Answers 2


That will adequately power the thermostats alone

The full spec page for your Tekmar 532 thermostats states they need a power supply of only 1.8VA for themselves. Since your power supply has 24V * 2A = 48VA available, you can easily power all 7 of them, leaving 35.4VA of capacity left over. The 2A spec is what the internal relays are capable of switching; i.e. your connected devices must not draw more than 2A.

Now, keep in mind that your transformer needs to power both the thermostats as well as the devices they control, and the devices sometimes have larger power requirements. If your devices do have large power requirements, you may want to use separate transformers for each thermostat, or one much larger one. It also doesn't hurt to oversize the transformers a bit, since they will run cooler and last longer that way.


I think you need to have a single transformer! If you try using multiple transformers to operate the same contactor, you have a 50/50 chance per transformer of them being out-of-phase and throwing 48V at the contactor instead of 24V, or creating a circular dead short that sets the thermostat wires on fire.

If you need more than 40 VA of power, use a bigger transformer.

If you must parallel two transformers, put them absolutely right next to each other and keep the wiring dog simple - just the 2 wires, and make your other tie-ins elsewhere - so it is completely obvious that you are paralleling.

  • Oh agreed if they're all controlling the same thing. I read it to mean the 7x thermostats were controlling 7x valves, in which case individual would be fine, but if they're all paralled, then a single transformer would be best. Of course, if they're paralled, the valve will be on when any thermostat calls for heat, so it'd be easy to overshoot the target, and there's probably better ways of accomplishing this.
    – Nate S.
    Commented Mar 13, 2021 at 1:00

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