Troubleshoot Low Voltage Transformers for Heating & Air Conditioning Systems
     

• Low voltage transformers: definition, wiring, troubleshooting, uses - what is a low-voltage transformer, what does it do & where is it found on heating furnaces, boilers, or air conditioners or thermostats?
  • How do you tell if a transformer is bad? How do you test a low voltage transformer?
  • How does a low voltage transformer work & how they are wired
  • How to test the low-voltage heating or air conditioning transformer to see if it's working
  • List of common uses of low voltage transformers in buildings & on building equipment
• REPAIR / UPGRADE LOW VOLTAGE WIRING - separate article
• Questions & Answers about wiring & testing low voltage transformers for HVAC equipment like air conditioners or heaters & in low voltage wiring systems
• References

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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 thermostate 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.

Den,

To test for a bad low voltage transformer you simply need a volt-ohm meter or a digital VOM. See USING DMMs VOMs SAFELY for advice on how to use a volt-ohm meter, and see Guide to Electrical Test Equipment for advice on ow 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 (above left) 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.

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.

So the transformer has 12V wires connected to its inupt 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" switcht) 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 at THERMOSTAT WIRE CONNECTIONS.

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 tor 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 voltge 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 burnup: Finally, by visual inspection you MIGHT see burn marks or evidence of overheating - but I wouldn't depend on visual inspection alone.

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 conditionres, 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.

• Primary control LV transformer: The low voltage transformer found inside of a heating boiler primary control and used to operate circulator or other relays (transformer visible at lower left in our photo at left). The low voltage transformer shown in the lower left in this photo of a primary control is not the thermostat power supply. Other primary controls used on hydronic heating systems and boilers may have two of these relays, one for the oil burner circuit and one for a circulator pump.
  
  In our photo you can indeed see two thermostat control wires connected to the two screws marked "T" just below the LV relay. Those thermostat wires are carrying the "on-off" signal from the room thermostat.
  
  The relay itself operates a 120V relay found just above the LV transformer (center left on the control board) that is used to switch the heating boiler oil burner on or off.

• Low voltage transformer operating one or more hot water heating circulator relays.
  
  The transformer in the primary control of most heating boilers also operates one circulator pump. But in a building with multiple hydronic heating zones that use individual circulator pumps, each additional circulator pump, presumably serving an individual heating zone, will be turned on or off by the room thermostat located in the zone the circulator pump serves. (see our photo at the top of this page).
  
  In our photo at left, you can see at center three red circulator pumps (two different models have been installed). At the upper left corner of the photo in those gray Honeywell labeled control boxes are individual circulator pump relay controls, each of which will be activated by an individual thermostat. We can see that we have a gas fired hyrdonic heating system boiler with three zones, but we don't know from the photo if all three are heating zones - one of those relays and circulators could be supplying heat to an indirect fired water heater.
  
  See Circulator Pumps & Relays and also INDIRECT FIRED WATER HEATERS for details.

• Low voltage transformer operating the oil burner circuit inside of a cad cell relay. Details are at CAD CELL RELAY SWITCH.
• Low voltage transformer operating the control for a draft inducer fan found on flue vents on some heating equipment. See Draft Inducer Fans
• Low voltage transformer in a stack relay or protectorelay that operates an oil burner - schematic at left. This control is detailed at STACK RELAY SWITCHES
• Automatic Vent Dampers
• LOW VOLTAGE BUILDING WIRING that controls lights, electrical receptacles
• Doorbells
• Many other relays, solenoids, and devices that operate equipment found in buildings

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