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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.
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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.
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:
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.
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
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:
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.
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.
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.
Continue reading at LOW VOLTAGE BUILDING WIRING or select a topic from the More Reading links shown below.
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