Low Voltage Transformer (C) Daniel Friedman Low Voltage Transformer FAQs

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Low voltage (12-24 VAC) transformer repair questions & answers:

Diagnosis and repair of low voltage transformers used on air conditioners, boilers, water heaters, doorbells, etc.

This article Series explains how low-voltage transformers are used on heating, heat pumps, & air conditioning systems to provide power for thermostats, zone valves, and other relays and controls.

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Low Voltage HVACR Transformer Questions & Answers

Low voltage transformer for heating or air conditioning (C) Daniel Friedman

Question: Can a transformer be tested for proper output voltage while connected and furnace running?

(Feb 24, 2013) Brad said:
Can a transformer be tested for proper output voltage while connected and furnace running?

I have a Carrier Weathermaker SX 58sxa080-101gg; the system won't hold temp set, it completes one heat cycle stops then won't restart to keep the set temp.

The transformer is behind the blower door; when I cycle the power to the furnace or remove and re-install the blower door, the furnace resets and works again for another heat cycle.
When I test for 24v T-Stat voltage while running I get 0v, I get 26v with the transformer disconnected on the 24v side.

Will a bad transformer reset and work for awhile after the 110v power has been cycled?

How can I test the components behind the blower door during a no-start situation when everything reset to working condition after the power is cycled (when I remove the blower door)?

[Click to enlarge any image]


I would disconnect the wires from the low voltage terminals and check the output with a VOM to confirm the output voltage level.

Question: where to suspect an intermittent short-circuit on a Lennox furnace 24VAC control

x(Nov 14, 2014) Carl said:
Any suggestions of where to suspect an intermittent short-circuit on a Lennox furnace 24VAC control voltage.
Fused the 24V circuit to protect the transformer.
Have replaced the Control Board, so seems to be external to the board.
Short seems to be temperature sensitive (higher ambients or if the furnace runs for longer periods; blows 24V fuse after 15- 20 minutes of continuous operation). Furnace restarts normally once the fuse is replaced.

(Nov 23, 2014) john said:
shouldn't a 24v transfomer put out 24v? i'm trying to install a nest thermostat but only getting 18v (apparently it needs at least 20) from a brand new transformer.


Thanks for the question John, I've moved your inquiry into the article above so as to have more space for a detailed reply.

Comment: The frequency is NOT changed by running an AC signal through a transformer.

(Jan 15, 2015) Kurta said:
"Low voltage transformers convert 50Hz or 60Hz (cycle) line voltage (120 to 240V) AC (alternating current) to low voltage typically 12, 18, or 24 volts AC at a high frequency Hz, possibly as high as 20 to 50KHZ, that's 20,000 to 50,000 Hz."

This is simply wrong. The frequency is NOT changed by running an AC signal through a transformer. In general, transformers are step-up (less current, more voltage) or step down (less voltage, more current). But the conversion takes place at the same frequency and either in phase or 180 degrees out of phase depending on which way you connect the secondary terminals.

Switching power supplies (DC) have high(er) frequency components - but there's a LOT more going on there than in a simple transformer.


Thank you for your comments, Kurta; we'll review the article as well as our original technical sources (also cited at REFERENCES for each article).

Here is a direct quote from low voltage transformer experts at just one of our sources over at Pegasus lighting:

The output of an electronic low voltage transformer, unless it is a DC low voltage transformer, is high frequency (20,000-50,000 Hertz). This means that there can be a substantial voltage drop if the wires carrying the high-frequency current are long, thin, or far apart.

If you want to be identified as a contributor or have other remarks that you want to suggest you can indicate that here as a comment or you can contact me directly by email - see our CONTACT information at page top or bottom.

Thanks again



(July 8, 2015) steve said:
if the 24vdc transformer is bad will the air handler still come on


Maybe not - if that particular transformer controls the thermostat that turns on the air handler or is one that powers a control relay in the unit.

Question: sometimes it seems to hum

(June 13, 2015) mike said:
mine has 2 fuses the glass type sometimes it seems to hum


Humming buzzing transformers are discussed in the article above: I'd replace the unit.

Question: no low voltage to the thermostat

No low voltage to thermostat. Will not come on, simple 7 wire. Simple heat, cool, and em settings, plus fan on or off. Is there a fuse or reset button in the system? AMERICAN STANDARD, HERITAGE 11, manufactured Sept, 2000



Check for voltage right at the low voltage transformer, then


with millions of readers, I'd sure be grateful if you'd post a question in just one place.

Question: I disconnected the LV transformer by mistake

(Oct 6, 2015) Fernando said:
I think I disconnected the LV transformer by mistake, thinking it had to do with the phone line. Now when I turn on the boiler the thermostat only registers the heat but does not control it (house keeps getting hotter). Please could that be it?


If you disconnected the transformer controlling a thermostat it won't call for heat; it won't work.

Question: using the furnace LV transformer for the air conditioner?

(Nov 21, 2016) Anonymous said:

We had a repair man rig / hook our A/C up to our furnace transformer to get us A/c this past summer. He said the low voltage transformer went out for the A/C, but he could rig it so we would have A/C through the furnace transformer.

He was to look into replacement cost for A/C transformer & get back to us, but never did & I didn't care because we had air.

Well now we are trying to light the pilot light for the furnace & he must have taken the furnace wire off the transformer. He said he wasn't going to & for us to make sure the outside A/C unit didn't keep running once we started the furnace, but we can't get pilot light to even light. How do we look or tell if he took the furnace wire off from the transformer?


You would follow the low voltage wiring from the control board or from the thermostat back to see which transformer is powering those devices.

The repair you describe may be just fine, as long as both transformers provide the same voltage AND provided that the in-use transformer isn't being overloaded by asking it to support too many devices.

Question: low voltage transformer was burning up and now blows a fuse

(Nov 20, 2016) AUTHOR:Anonymous (no email)

COMMENT: I have airhandler that was burning up the transformer. I put a fuse in and took all low voltage wire loose. What is causing the fuse to blow


If the circuit blowing the fuse is for sure the air handler, not the outdoor compressor/condenser, I'd be looking for

For 120V circuits:

1. a shorted or loose wire connection
2. a fan or blower motor that is seizing up
3. a shorted relay or control board

For 24VAC or similar low voltage circuits, look for shorted low voltage wires or shorts at a low voltage relay

Low Voltage Wiring Articles


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