Should the transformer be hot when the furnace isnt running? replaced the transformer and it is hot to the touch but not hot enough to burn fingers..


2 Answers 2


What is the size of the transformer? Usually excessive heat in a transformer means it is undersized. Larger transformers actually have there temp increase listed on them. If you get one the same voltage but a larger kw / kva output it will not run as warm if you are concerned.

  • Heat is good..lol
    – JACK
    Sep 22, 2019 at 11:11

Yes, transformer primaries use electricity (and therefore get warm) even when there is no load on the secondary winding. It's usually just 5-10 watts, and that'll be hot to the touch depending on how much thermal mass it has and airflow around it.

It's normal for the typical cheap 24VAC 40VA transformers used in security systems and security camera get hot enough to discolor their plastic shells, so I'd say it's normal for a similar furnace transformer without a plastic shell will get just as hot even though I've never actually touched or measured mine on my furnace.

I've heard of people using 24VAC switching power supplies instead of 24VAC transformers in their furnaces to save wasted energy. That constant 5-10 watt draw can cost you $5-10 a year in electricity. The down side is that they're more likely to fail vs a transformer, which is not something you usually want to risk on a furnace. Also the power from one is "noisy" and might not play well with a furnace controller that wasn't expecting it.

  • Do you mean 24VDC switching supplies instead? Sep 22, 2019 at 15:37
  • A 10w draw at 11.5 cents per kWh is only ~ 1 dollar a year. 5-10$ is crazy high.
    – Ed Beal
    Sep 22, 2019 at 18:00
  • @3Phase - a transformer will put out AC, so since that what the original system expects... I'd stick with that.
    – DaveM
    Sep 22, 2019 at 18:58
  • @EdBeal Double check your math. 10 watts for a year at 11.5 cents is $10.08/year. An easy rule of thumb for 24/7 usage is watts equals the yearly cost.
    – Dotes
    Sep 22, 2019 at 19:52
  • Actually I got that from a power company calculator, that’s the rate it put in I just put the wattage and length of time. Think about it a 100w lightbulb would cost 10$ per month at your number. I have 10 incandescent hard service bulbs on from dusk to dawn in my horse stalls, my home is all electric my total bill is around 150 most months just the lights in the stalls would cost close to 100 a month then the 12 floods in the riding arena that are usually on at least 3-4 hours a day and 4 other dusk to dawn barn lights 1 at each entrance to the arena. That doesn’t even get to the house.
    – Ed Beal
    Sep 23, 2019 at 16:07

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