I recently replaced my 24 volt HVAC control transformer. The outside unit started up including the air handler and ran for maybe 10 minutes. After the units shut down I noticed that the transformer was hot to the touch and no longer is working.

What causes this and how can I fix it?


Underpowered transformer

Sounds like you may not have gotten an equivalent replacement. When looking for a replacement transformer, you should be concerned with

  1. Input, or primary voltage
  2. Output, or secondary voltage
  3. Volt-amperes (VA) rating.

If the transformer is not designed to supply the amount of current needed by the components on the circuit, it will overheat and likely die. Current generates heat. The more current that flows through the transformers coil, the hotter those tiny wires get. Get the wires too hot, and they will break. Heating and cooling cycles of the transformer can cause the wires to become brittle, which can also lead to failure over time.

If you've recently added components to the system that draw power from the transformer (WiFi thermostat, electronic dampers, etc.), you may have to upgrade to a transformer with a higher VA rating. If you replaced an old transformer with a new one that had a lower VA rating, that could explain the overheating.

Short on transformer secondary

A short somewhere in the circuit on the secondary side of the transformer, could also lead to overheating. Often times the secondary circuit has no overcurrent protection (fuse), so it can be easy to overload the circuit and not know until it's too late.

  • Sweet answer. I have concluded that i must have a short because my replacement. Specs were the same as what was removed.(24v 49va) so now i need to find out If the short in within a component. Or in the wires. Thank you for your help. It was much appreciated. If you or anyone else could offer any more guidance on troubleshooting or testing the components it would be gladly accepted. Thank you
    – user261702
    Jun 7 '14 at 13:31
  • If you have an ammeter, you could clamp it on one of the wires and see how much current is flowing. That would tell you if you're overloading the transformer.
    – Tester101
    Jun 7 '14 at 16:17
  • Thanks for all your help. I found the short in the outside unit low voltage wires leading outside. I disconnected all the wires from the furnace board. I then replaces the low voltage transformer with in line fuses. The fuses would prevent burning out the transformer again. Then i started reconnect the t-stat wires while. Checking the amps being pulled on the transformer. When i reconnect the outside unit the amps went up. That told me the short was on the outside. Unit. I examined the wires and found that the wires were taped together. The short was within the taping.
    – user261702
    Jun 14 '14 at 16:56
  • 1
    @user261702 That's great. Glad I could help. I don't know why the manufacturers don't fuse the transformers secondary, guess the extra cost is too much for them.
    – Tester101
    Jun 14 '14 at 20:41

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