I have a 24V 40VA transformer for my boiler. It powers two zone valves labelled 0.9A each, and two thermostats labelled 1VA each. All of them together would draw about 45VA.

I'm now adding an outdoor reset controller, which will add a small additional draw.

Should I replace the transformer with a 75VA one?

While it seems obvious that I'm already over capacity, the system has worked perfectly for years. I'd like to know if there are any gotchas or special considerations when installing a larger than typical transformer on a boiler control circuit. Should it be avoided? Or is it a good thing to do?

If it matters, the zone valves are Taco T571-2, the stats are Honeywell T4s, it's a Weil Mclain PFG 6 boiler and the new Reset is a Tekmar 256.

1 Answer 1


If you don't go overboard there's no issue with transformers "too big" while "not big enough" does risk overheating. The loads only draw what they need.

What would overboard look like? Hundreds of VA with a draw of 45 - you waste some energy in the inefficiencies of transformers not being 100% efficient, and that waste is proportional to size. Basically you'd be heating a big lump of iron all the time due to the magnetic eddy currents in the core segments. You already heat a smaller lump, and you'll be heating a medium-small lump with your somewhat bigger, but not even twice the load replacement proposal. That's reasonable. 5-10 times the load is unreasonable/wasteful, unless it's a temporary situation where you plan to add much more load.

  • Commonly available ones are 20, 40, 50, 75, and 100VA. Over 100VA puts it in a different class requiring different installation techniques, hence also hard to find. The choice here is really between 75VA or do nothing. This answer pushes me towards 75, since I'm touching this now and going to do thorough testing so now's the time. And I'll be going from 110% of rating to 67% which feels good. I don't know what you do if you have a large number of zones ... I guess you use several transformers with relays for the boiler circuit.
    – jay613
    Nov 6, 2023 at 21:16
  • 1
    I changed 40VA to 75VA. Old one ran at about 130 degrees F, new one at about 80. ✔
    – jay613
    Nov 24, 2023 at 14:13

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