The HVAC transformer

I discuss the low voltage transformer design for heating and air conditioning systems. The 24v transformer is used on most residential and commercial systems for thermostat power.
This video is part of the heating and cooling series of training videos made to accompany my websites: and to pass on what I have learned in many years of service and repair. If you have suggestions or comments they are welcome.
If you are a homeowner looking to repair your own appliance, understand that the voltages can be lethal, the fuels are highly flammable and high pressures are used. Know your limits.

21 Replies to “The HVAC transformer”

  1. A short on the primary will not “take out” the transformer, if by taking out you mean burn up the primary winding. A short on the primary will cause the overcurrent device protecting the circuit feeding the transformer to operate.

  2. So that's how they make a class 2 transformer current limited. They incorporate the fuse on the secondary side.

    Recently I dug into code for low voltage circuits, discovering the class 2 is considered inherently safe because of voltage and power, i.e., VA/wattage, design limitations and self limiting. (60VA for 12v, 96VA for 24v) A DC power supply can be designed that self limits the current, but I wondered how they did it with the transformers. Class 1 or 3 power supplies/circuits require inline fusing to limit overcurrent.

  3. I was swapping thermostats and I think I hit the blue to red wire. Would that smoke the transformer?

  4. my unit is a 240v system. how do I wire the 240 tap with one wire? this system has no return and only a ground. I have one leg from L1 going to the 240v tap so my com is ground. and my 24v side only puts out 11 volts. so do I hook up the L1 leg and a leg from say T1 or T2 to the 240v tap? thanks. I'm stumped on this one

  5. I lost neutral in my house. Some outlets 175 volts and some had 50 volts ..All my 120 volt primary transformers fried .. Good tip on the inline fuse….They can save from replacing transformer twice ..or until you find the cause…although it would not have saved me from a power company fault.

  6. Are the white and black on the HV side actually interchangeable? Or it the white some how tied to the body or if reversed will it not operate properly or safely? Thanks

  7. I have a forced air gas furnace, the 4 wire transformer is dead. The output wire are colored. I went to the hardware store and they said a "24VAC door bell transformer would work. They told me the output is non polarized so the 2 wires that go to the control board are not polarized and it did not matter which wire goes where. Is this true? Thank you

  8. Should I be reading 24 volts on both leads of the transformer coming out the 24 side? I am checking one leg to ground on the low voltage and reading 28 volts and I checked the other leg to ground and getting 0, but if i check both together i read 28 volts. will the common side of the transformer read 0 if i check it to ground?

  9. I really enjoy your videos, they are on point and very informative. I teach an HVAC Class at a state prison in Texas. Under the circumstances I cannot access the web in my class, but I would really like to show your videos to my classes. However, I am able to take a flash drive in with me. Would you be willing to authorize the downloading and use of your videos as training aids in my class? Thank you very much for your consideration!   

  10. Thanks for the posting

    Did you ever hear of grounding the neutral on a on the output side for boilers?

  11. Where can you normally hook up a resettable fuse to troubleshoot a short? My Heat Pump doesn't have a fuse, but I want to install an inline fuse.

  12. Hey GFM,

    Thanks for the video and explaining  the Volt Amps that helped out a lot.  I wanted to ask, if you could replace a 40VA with a 60VA if that is all you had in your truck at the time or if it needs to be the exact match?  Thanks for your help.

  13. my air conditioner is under 220v and i want to use a thermostat Nest that work with 24v, so what should i do to get both wired without damage, thank you.

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