Low voltage transformer for heating or air conditioning (C) Daniel FriedmanTroubleshoot Low Voltage Transformers for Heating & Air Conditioning Systems

  • POST a QUESTION or READ FAQs about wiring & testing low voltage transformers for HVAC equipment like air conditioners or heaters & in low voltage wiring systems

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Low voltage (12-24 VAC) transformers diagnostic, installation & repair guide for HVAC & lighting systems:

This article 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.

We explain how a low voltage transformer is installed and wired, where the device is usually located, and how to test or troubleshoot & repair low voltage transformers using a VOM or DMM.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How to Troubleshoot Low Voltage Transformers for Heating & Air Conditioning Thermostats

Low Voltage Transformer (C) Daniel FriedmanArticle Contents

Question: How do you tell if a transformer is bad? How do you test a low voltage transformer?

How do i tell if a transformer for thermostat is bad ?- Erwin 5/12/12

How do you test a low voltage transformer? - Den 7/16/12

Reply: how to test a low voltage transformer for heating or air conditioning

Erwin if you disconnect the low voltage wires connected to the transformer terminals and use a VOM you should find voltage, usually 14V. If the device looks burned, hot, or smells, don't even bother testing. If it has been buzzing that's another clue.


Remove all of the the low voltage wires from the two (or more) screws on the top of the transformer. Usually you'll see a small gauge red and white wire simply connected under two screws on the upper surface of the transformer itself.

Some heating transformers for thermostats that use more than two wires will have more than two screws and more than two low voltage wires connected or there may even be multiple sets of thermostat wires connected if the transformer is powering more than one room thermostat. (photo above left).

Check for output voltage: with power to the transformer "on", using the VOM on a low voltage scale (say 0-24VAC ) check for voltage between the two screw terminals.

If the transformer itself is receiving 120V power but there is no voltage between the two low voltage output screws, then it is not working.

To test for a bad low voltage transformer you simply need a volt-ohm meter or a digital VOM.

See DMMs VOMs SAFE USE OF for advice on how to use a volt-ohm meter,

see TEST EQUIPMENT, ELECTRICAL GUIDE for advice on how to electrical test equipment safely. And of course you'll have to be sure you have found and are testing the right low voltage relay, as we will explain in the details below.

In our photo you can see a low voltage transformer mounted onto a 4-inch electrical box that is in turn secured to (and powered from) a gray-covered electrical panel. But as there are four circulator pumps shown in the photo, you can figure there will be more LV transformers and more circulator relay switches nearby, connected to other room thermostats in this building.

Why? A typical residential-grade low voltage transformer cannot handle the total current draw (in Amps) to support multiple circulator pump relays or other controls.

Variation in Output Voltage from Low Voltage Transformers

Reader Question: 11/24/2014 john said:

Shouldn't a 24v transformer 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.



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.

There are other common low voltage output levels, for example 5-6V AC on many device chargers and there are also voltage converters that may produce DC rather than AC current.

What that means to you is that before replacing your possibly defective low voltage transformer there are some things to check:

  1. Check the equipment low voltage requirement specifications

     on your HVAC system or low voltage lighting or other low voltage equipment controls to see what voltage is required. Also confirm that you are using the proper type of low voltage transformer or power adapter. For example, if the equipment being powered requires 5VDC don't use a low voltage transformer or power supply providing AC.
  2. Look at markings on the low voltage transformer

     itself to see its output rating. Yours might be an 18V unit.
  3. Check the power connections to the low voltage transformer:

    Check that the input wires (normally heavier gauge wires) to the transformer are properly connected to the line voltage supply (typically 120V or 240V in most buildings). Check that the output or low voltage terminals on the low voltage transformer are connected to the low voltage wiring.
  4. Measure the output voltage at the low voltage transformer

     (using a VOM or DMM) after removing the low voltage wiring that was connected to it - thus eliminating any effects of the wiring itself. Most HVAC thermostats and controls operate at 24VAC. Low voltage lighting systems typically operate at 12V or 24V but other equipment or systems may require different voltage levels.

    Note: For a typical magnetic LV transformer used on HVAC systems a typical DMM or VOM will work adequately. But warns: "Since most voltmeters give misleading readings when applied to high frequency currents the voltage on an electronic low voltage transformer can be measured only by using a "true RMS" voltmeter with a sufficient range."
  5. Determine the type of low voltage transformer you are using:

    electronic low voltage transformers may not behave identically to magnetic low voltage transformers.

    For a typical 18-24V transformer used on HVAC equipment, the LV transformer is a traditional magnetic type. But if you are using an electronic low voltage transformer instead of a traditional magnetic low voltage transformer, the high frequency but low voltage produced by the transformer can leave the circuit vulnerable to a more significant voltage drop that occurs on the lower-frequency higher voltage wiring in the building.
  6. Check the low voltage wiring:

    With the low voltage wires disconnected from both the low voltage transformer and from their control-end, check the wires for shorts or breaks. Also note that if you are using very small gauge wires or a very long wiring run, depending on the power (watts) load on the circuit there could be a voltage drop. Pegasus lighting ( points out that "there can be a substantial voltage drop if the wires carrying the high-frequency current are long, thin, or far apart."
  7. Check the line voltage level

    to be sure it's within specifications at the source of power to the low voltage transformer, typically 120V in residential buildings.

How your low voltage thermostat and its low voltage transformer work & how they are wired

Low Voltage Transformer (C) Daniel Friedman

It's usually pretty simple to understand the wiring and function of a low voltage relay on HVAC/R equipment.

The low voltage transformer changes (transforms) house current (at 120V AC) to a lower voltage (usually around 14V to 24V AC) used to operate the room thermostats and possibly other heating or air conditioning controls.

The low-voltage transformer shown at left, a reader-contributed photo, is feeding two pairs of low voltage 24V circuit wires (red and white); the 120V power wires feeding the transformer are the heavier wires in the center of this device. Click to enlarge this or any image.

So the transformer has 12V wires connected to its input terminals (not visible in our photo at left as by code the 120V wires have to be inside of that 4" metal electrical box). The low voltage thermostat wires that bring operating current (14-12V VAC) to the thermostats are on the exposed surface of the transformer.

In the simplest applications, two wires, typically red and white, are connected at the transformer.

A "C" or common terminal wire on the low voltage transformer is connected to the control relay that operates the heater or air conditioner, and

an "R" terminal on the transformer is connected first to the thermostat (which acts as an "on-off" switch) and from the thermostat onwards to a thermostat terminal found on the heater or air conditioner's primary control or circuit board.

Details about thermostat wiring and low voltage control wiring are found


Reader Question: how many devices can a standard 24VAC transformer support?

2016/03/16 Peader said:

I have a 40 year old house that has been remodeled over the years with additional multiple heating zones, modernized thermostats with wireless computer access, intercom, etc. My 24vac control transformer has started buzzing very loudly and I plan on replacing it.

With this new demand on the 24vac side should I consider increasing the existing transformer capacity of 40VA to something larger? Is there a way to determine the output capacity of the transformer?


Thanks for the question Peader.

For other readers, 'VA' stands for Volts x Amps. Volts multiplied by Amps is also the classic definition of a Watt. If you hook up more devices drawing more current than a transformer can provide, it'll burn up - fail.

Also to be clear for other readers, the OUTPUT VOLTAGE of a replacement transformer should not be changed - that is if you are running 24VAC devices you want 24VAC supplied by the transformer. You wouldn't change to a 12VAC transformer (for example).

Here is a typical 24VAC transformer product specification using Robertshaw data as an example:

Class 2 Transformer, 40 VA Rating, Input Voltage 120VAC, Output Voltage 24VAC, Multi Mounting, Protective Device Fuse, Primary Connection 8 In. Leads, Secondary Connection 8 In. Leads, 1 Phase, 50/60 Hz, Not Rated Temp. Rise, Open Enclosure Type, Enclosure Rating NEMA 1, Enclosure Finish Bare Steel, Height 2.75 In., Depth 3.25 In., Width 2.44 In., Standards UL 1585 File Number E110657, E92807, E99697, CSA File Number LR107325-1, NEMA

A 24VAC transformer rated for 40VA (volt-amps) means that it can support 40 / 24 = 1.6A or 1.6 amps of current draw (load). That is its output capacity and the answer to your question.

Add up the current requirements (amps) of the devices you want to support. E.g. one zone valve might want 0.3 Amps. so the most you'd put onto one 40VA transformer would be 4 such zone valves - though I've seen a few working installations at which five or even six zone valves on a single transformer.

The VA rating of the transformer should be greater than the load of all the devices connected to it.

So how did they get away with wiring so many zone valves onto one transformer? Luck. Usually not all of the zone valves are being turned "on" by their respective thermostats at the same moment.

During activation the zone valve motor runs for less than a minute. But eventually, on a very cold night, and with bad luck, all five or six thermostats try to turn on all five or six zone valves at the same time. Maybe the homeowners came back to a cold house and ran amok turning up all of the thermostats in the course of a minute. Hiss. Buzz. Dead transformer.

Typically an older wiser, HVAC installer adds additional transformers rather than installing one giant one serving everything in the house. That can also avoid having to snake long runs of low voltage wire - depending on where your various devices are. For example an installer hooking up 6 zone valves would install two 24VAC 40VA transformers and wire 3 zone valves to each.

I'd guess that your wireless computer access is talking to a wireless router that is plugged into a 120VAC wall receptacle so that ought not be a concern.

I've hooked up three NEST thermostats in a test building, all working just fine off of the original transformer that originally supported the old Honeywell round units.

See NEST LEARNING THERMOSTATS - the manufacturer probably took care to be sure that their new product would work in older homes without requiring homeowners to change out their thermostat transformer or add additional ones.

Usually an intercom system has its own separate transformer as does a traditional home doorbell and as does the receiver for a remote operated garage door operator.


Testing the Low Voltage Heating or Air Conditioner Transformer

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

When we think that there may be a problem with getting power to a thermostat (if it needs power) or to other low-voltage-operated controls in an HVAC system, the "test" of the low voltage HVAC transformer involves these very simple steps:

  1. Find the low voltage transformer

    that is operating the thermostat or other control device. Usually the transformer is visible, mounted on an electrical box in the utility area where the heater or air conditioner blower unit is installed (blue arrow in our photo, above left), may be mounted inside of the furnace or air handler itself (photo at left).

On some modern HVAC equipment the low voltage transformer may be harder to spot as it may be incorporated into a more complex circuit board or component.

In all cases, however, if you follow the small gauge low-voltage wires from your thermostat back into the furnace or air handler cabinet you can find wires of this same dimension connected to a transformer.

  1. Check that all of the wires and connections 

    are in fact intact, un-damaged, and properly connected. If you suspect that a thermostat wire has been damaged between the transformer and the thermostat, you can try a simple continuity test - disconnect the thermostat wires and connect them together at one end of the run - then test for continuity between the wires using a VOM at the other, disconnected end of the pair.
  2. Check that the low voltage transformer is is providing power 

    using a VOM we test for voltage between 12-24V AC at the transformer's output screws. Here are the procedural details for testing a low voltage transformer for live output power:

    Remove all of the the low voltage wires

     from the two (or more) screws on the top of the transformer. Usually you'll see a small gauge red and white wire simply connected under two screws on the upper surface of the transformer itself.

    Some heating transformers for thermostats that use more than two wires will have more than two screws and more than two low voltage wires connected or there may even be multiple sets of thermostat wires connected if the transformer is powering more than one room thermostat. (photo above left).

    Check for output voltage

    with power to the transformer "on", using the VOM on a low voltage scale (say 0-24VAC ) check for voltage between the two screw terminals. If the transformer itself is receiving 120V power but there is no voltage between the two low voltage output screws, then it is not working.
  3. Listen for buzzing

    Also before a low voltage transformer fails you may hear it buzzing - a condition that can continue for some time but ultimately is likely to lead to failure.
  4. Inspect for overheating or burn up Finally, by visual inspection you MIGHT see burn marks or evidence of overheating - but I wouldn't depend on visual inspection alone.

Reader Comment: Buzzing low voltage transformer

(Mar 3, 2015) Raymond Rackley said:

Thank you for this website.I learned so much about what I have been trying to say to every electrician that has come to solve the problem of buzzing/vibrating transformers. They all say it's normal but when it keeps waking you up when the thermostat calls for heat in the middle of the night, it is far from normal.

You have taught me to stick to my guns and hire someone who will listen and understand that I have already found and isolated the problem. Now I just need them to either replace the transformer or check the thermostat that just clicks at will. Thank you for breaking things down in English to a novice in heating and electricity.

Now I need a service technician.

List of Low Voltage Transformers Found on HVAC/R Equipment and in Other Building Applications

LV relay used to operate a boiler primary control relay (C) Daniel Friedman

Watch out: there may be more than one low voltage thermostat, for example one controlling heating and another controlling air conditioning in your building.

Or there may be additional low-voltage thermostats providing power to individual hydronic heating zone circulator pump relays and to the thermostats that control each of those zones (see our photo at the top of this page).

Watch out: there are other low voltage transformers found on air conditioners, boilers, furnaces, circulator relays, fan relays, etc. that are not the units providing low voltage to the room thermostat and main on-off HVAC/R controls.

Circulator relays (C) Daniel Friedman

Protectorelay oil burner control schematic

Low Voltage Wiring Articles


Continue reading at LOW VOLTAGE TRANSFORMER FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Suggested citation for this web page

LOW VOLTAGE TRANSFORMER TEST at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Ask a Question or Search InspectApedia

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher - Daniel Friedman