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Problem: My 24 volt transformer continues to burn up. This is the second transformer. What could be the problem?

Problem:  My 24 volt low voltage transformer continues to burn up. This is the second transformer. What could be the problem?

Answer: I have encountered this problem several times. Most of the time the problem ends up blowing the fuse on the control board, (if your control board has a fuse) instead of the transformer. In most cases the thermostat wires have shorted out somewhere and the thermostat wires are the problem. I have seen other components inside the air conditioning system with coils and electronics cause the transformer to burn up.  I had a shorted out reversing valve coil cause this problem one time. Most of the time it is in the thermostat wiring. This can be a pain because it can be so very time consuming. In order to find the problem I would turn off all the power, take all the low voltage thermostat wires loose from the thermostat, air handler, and outdoor unit. I would take a digital meter set to ohms and check between all the wires with the meter. You should not get a reading between any two wires if the wires are in good shape. If the wires are good you would need to test the components from each wire or terminal to ground. The meter should not move. This is a pain sometimes finding the problem. Many times I would find the thermostat wires shorted where the wires were installed going through the furnace body. The vibration of the furnace or air handler over time had torn through the wire insulation and caused a short. Sometimes animals will chew through the wires and cause a short. Sometimes sun light on the wires over time will rot the insulation and short out the wires. If you need a transformer we sell transformers on the following page: Please click here if you are interested in seeing the low voltage transformers we sell.  I hope you can easily find and fix the problem. Below we have an information video made by grayfurnaceman titled “How to replace a transformer without burning up the new one”. Thanks grayfurnaceman for making this informative video.  If you have any questions please email us anytime. Our email address is: support@arnoldservice.com or comment below.

22 thoughts on “Problem: My 24 volt transformer continues to burn up. This is the second transformer. What could be the problem?

  1. I install a new Transformer in the inside unit this Transformer had a fuse on the primary side I also install a fuse on the secondary side be was a 5 amp fuse primary side5 amps Brock primary side will blow about 2 minutes after the fan starts I have no idea why unhook all thermostat Wire

    1. Hi! Sounds like you have a short or a loose connection on your primary side since it is blowing the fuse on your primary side. Please make sure you do not have any loose connections on the high voltage side (high voltage power coming into your furnace). Make sure the transformer primary (high voltage) is not installed in-line with the high voltage blower motor. Since the blower motor draws over 5 amps this would cause the fuse to blow if hooked up with the blower motor. The transformer should be connected to the control board usually labeled P1 and P2 or P1 and neutral. If it continues to blow the primary then you might have a short to ground in your blower motor windings. I hope you can easily find and fix the problem. Steve

      1. My heat works fine but when I turn it on to air my transformer instantly blows. It never gives a chance for my outside unit to come on? What could be causing this. I’ve been through three transformer already.

      2. Hi Michael! Most of the time the problem with burning transformers up is caused by shorted thermostat wires that go out to you outdoor unit. If you do no want to do the test with a meter to see if it is the thermostat wires that are causing the problem then I would recommend running new thermostat wire to your outdoor unit. The following is the test that I was talking about in the post: In order to find the problem I would turn off all the power to the unit and furnace, take all the low voltage thermostat wires loose from the thermostat, air handler, and outdoor unit. I would take a digital meter set to ohms and check between all the wires with the meter. You should not get a reading between any two wires if the wires are in good shape. If the wires are good you would need to test the components (contactor reversing valve solenoid) from each wire or terminal to ground. The meter should not move. This is a pain sometimes finding the problem. Many times I would find the thermostat wires shorted where the wires were installed going through the furnace body. The vibration of the furnace or air handler over time had torn through the wire insulation and caused a short. Sometimes animals will chew through the wires and cause a short. Sometimes sun light on the wires over time will rot the insulation and short out the wires. I hope you can easily find and fix the problem. Steve

      3. Also on my thermostat there is a brown wire that’s not hooked up? Does it need to be hooked up?

      4. I would not recommend hooking up the brown wire since it has not been hooked up on your thermostat before. The brown wire is probably just an extra wire that does not connect to anything. I hope you have a great day! Steve

  2. Hi Steve,
    Here is the problem:
    The control board (240V) keeping the 24V 40VA transformers (controlling both heat pump and emergency heating element). Frist the thermostat display was blank, I replaced the blown fuse with a new one in the control board, and the display came back. It was on A/C mode. A/C did not immediately run as I figured out it would take 5 minutes to reset after any shut down. I walked away from the unit. Sometime later, I went back and checked, the display on the thermostat was blank again. Checked out the 24V transformer, the primary side was burned (strong odor and over heating discolor on the coil wrapping tape, open circuit as well). Replace the 24V transformer with a second identical working one. It got burned too, and no display on the thermostat ever. The 5A fuse on the control board did not blown (why?). After watching your video, the cause may be some short between the thermostat (bad?) and the control board, and need to check out each thermostat wire one by one (by measuring the resistance between it and the ground, right?). If you can further comment this burn out to resolve the issue will be highly educational and appreciated. Thanks

    1. Hi Joe! Sorry to hear that you are having all of this trouble with your transformers burning out. I have no idea why the short did not blow the fuse on the control board other than a possible short to ground before the fuse on the control board. We have a good video on troubleshooting this problem on the following page: https://arnoldservice.com/fuse-control-board-blows-constantly-find-low-voltage-short/ You would take all your thermostat wires loose from their connections both inside and outside at the unit. You would measure the resistance between each thermostat wires. You should set your meter on Ohms and you should not see any resistance between any two of the wires. If you do there is a short in the two wires that show resistance. I hope you can find and fix the problem soon! Steve

  3. Hi everyone
    I have one pblm in my control 240 v AC contractor and 25 VA transformer always burn in few weeks so please suggest how to solve these fault

    1. Hi Nagesh! I would suggest that you make sure that none of the thermostat wires are shorted out by doing the suggested test with an ohm meter in the post above. I would like to suggest that you install a higher VA transformer. Most AC systems require a 40 VA transformer. If your transformer is not producing the minumum 24 volts AC at the contactor coil it will cause the contactor contacts to chatter and quickly burn up the contactor. If your 25 VA transformer is being over-worked then it will burn it up right away. We have a real good universal transformer on the following page: https://arnoldservice.com/product/tfm4031-universal-low-voltage-transformer/ I hope you can easily find and fix the problem. Steve

  4. Do you know what would cause a voltage reading higher than 24V at the contactor? When I first turn on the AC unit, the voltage is right around 24V. The unit does not come on, the contactor does not pull down as it should. I am pretty sure the coil is bad, based on a higher than normal ohms reading. Anyway, the system is on and voltage started at around 24V, but within seconds the voltage started creeping up and getting higher. I assume the transformer is bad, but I have searched and read for hours and could not find anything like my issue.
    Thanks,
    Jared

    1. Hi Jared! I really do not know what would cause a higher than normal voltage reading at the contactor unless some thermostat wires are shorted together. Normal voltage at the contactor with the contactor energized should be between 24 and 28 volts AC. Most of the time when a contactor is energized the voltage goes down slightly. The voltage should never go below 24 volts or you will have possible problems with contactor chatter or the contactor not closing at all. You should be able to test to see if your transformer is good by measuring the voltage between C (com) and R at your furnace or air handler’s control board. If your transformer is good you will read between 24 to 28 volts. If you want to check the coil on the contactor with an ohm meter I would recommend turning the power off of course, disconnecting the low voltage wires at the contactor coil. I tested two of our new contactors and they measured 11.5 and 18.3 ohms. If your contactor coil is what out of this range then I would say that you need a new contactor. I hope you can easily find and fix the problem. Steve

  5. My HVAC repair person had the same problem with the transformer on one of my units. He ended up having to replace an outdoor 2 pole contactor, and then replaced the transformer with a low voltage transformer.

    1. Thanks you so very much for your post! Yes, sometimes a low voltage control like a contactor coil or a reversing valve coil can ground out and cause the transformer to burn out. Steve

  6. Can I burn out transformer hooking up thermostat wrong

    1. If you have a thermsotat which includes a common wire then, yes you could possibly burn up a transformer if you hooked the wires up wrong. If you have a normal 4 wire thermostat hookup with R, W, G and W terminals then it would be hard to burn the transformer up because you are dealing with only one hot wire the Red (R) and there is not a common wire to short the thermostat out. I hope you I have answered your question. Steve

  7. Can you please explain why primary side of the transformer keeps burning up rather than secondary , when the shorts were found in the contactor coil of the condenser

    1. Hi! I have no idea why the primary side of the transformer would burn up instead of the secondary on a transformer when the short was actually in the low voltage contactor coil. You would think there would be a short somewhere on the high voltage side of the transformer? If any one else can explain why please comment. Sorry that I can not give an answer. Steve

  8. Hi Arnold,

    My home inspection for sale yield a burned out transformer when switched from AC to Heater. I replaced the transformer, Fan manually turn ON/OFF OK, AC running OK, but when I switch to heater, transformer gets really hot and burned out before I could switch it OFF.

    The home inspector did mention when he switch the AC to heater, he heard an distinct POP, and nothing works after that. But all I could find is a burned out transformer.

    Do you have any suggestions for me to look for the problem? Thanks for your help. Meanwhile I am trying to buy another transformer.

    1. Hi! Most the time the thermostat wires have a short to ground and this causes the transformer to ground out and burn up. I would suggest that you inspect the thermostat wires to see if you see any breaks in the wires. One time I had a thermostat wire that was grounded out where it was wrapped around a nail right behind the thermostat. Sometimes it is easier to replace the thermostat wires instead searching if you can not find the short. This is time consuming but in order to test the thermostat wires for shorts I would disconnect the wires from the thermostat and furnace at both ends then use an ohm meter between all the combinations of two wires. You should not get an ohm reading between any two of the thermostat wires. If you do there is a short in the wiring. The transformer burning out could also be caused by a gas valve solenoid that is shorted out or a control board relay that is shorted. Most of the time it is in the thermostat wires. Best of luck in finding and fixing the problem. Steve

  9. Good day folks I just put in a 40 amp transformer in my ac its the right size, I have been using about two a year and when I put this one in, woks great but when I turned off the ac, but left the power on I went to check on it the transformer was so hot I could not touch it, do I need one thats 50 amps with a reset button
    or what do I need to check next any help my reversing valve was shorted out so I just unpluged it.

    Robert

    1. Hi Robert! This can be a very time consuming problem to try and troubleshoot. You might want to try a higher VA transformer like you mentioned a 50 VA transformer. If the transformer has too many components drawing current from the transformer, then this can cause a transformer to over-heat and burn up over time. A loose wire, loose fuse or a shorted low voltage component like the shorted reversing valve solenoid can also cause the transformer to over-heat and burn up. Most of the time when transformers over-heat it is caused by shorted thermostat wires. You would need to take the thermostat wires apart on both ends and test between the wires with an ohm meter. If any resistance is shown between the wires then there is a problem. This could also be a loose connection in the control board or other low voltage control. Loose wires and components can cause high amp ratings and cause a transformer to over-heat. I am sorry we do not sell a 50 VA transformer. Only 40 VA transformers. I hope can easily find the problem and get it fixed. Steve

      Steve Arnold, https://arnoldservice.com

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