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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 have seen shorted reversing valve coils and contactor coils cause this problem several times. Most of the time it is in the thermostat wiring. This can be a pain trying to find out where the short is located 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.  If the fuse on your control board continues to blow and you are tired of replacing fuses then we have a Lil Popper fuse reset tool that we sell on the following page: Please click here if you are interested is seeing the LiL Popper fuse reset tool. If your furnace or air conditioner does not have a fuse to protect your low voltage circuits then we have a fuse installation kit on the following page: Please click here if you would like to see our fuse installation kit.  I hope you can easily find and fix the problem. Below we have two really good information YouTube Videos made by grayfurnaceman and Word of Advice TV titled “How to replace a transformer without burning up the new one” and How to Find a Short in an HVAC System. Many thanks to grayfurnaceman and Word of Advice TV for making these great informative videos.  If you have any questions please email us anytime. Our email address is: support@arnoldservice.com or comment below.

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

  1. Hello Steve –

    In middle of summer, had a bad storm with power blips. Later that night, 40VA furnace transformer blew. Old transformer.. thought coincidence and age (20+ years)… Replaced transformer w/ fan relay, moved on. Had a tech come out and confirm my work. He did and found no other issues. Also replaced outdoor capacitor and contactor as I had them on hand. Contactor seemed fairly pitted. Capacitor tested OK, but was old rusted… capacitors are cheap.

    A month later, transformer blew again. Had tech come out this time. Installed 75VA transformer with 3.7 amp secondary breaker built-in. Found no other issues.

    Now in winter (HEAT) several months later, transformer blew. Transformer breaker didn’t pop. New tech came. Said it appeared transformer blew on primary side but not totally sure. Replaced transformer, also installed 3amp inline fuse on 24V side. Tech also found incorrect wiring on 24V side related to ignition transformer — unclear of exact issue. Also found burnt 110V neutral connection at the burner override switch (wire nut was melted) and the housing of the override switch itself was broken, allowing 110V wires to move/vibrate a bit within the housing. Unknown when these issues popped up. This tech seemed far more knowledgable than previous.

    Could I be in the clear?

    Considering having electrician re-wire circuit — it is aluminum wiring..

    1. Hi Bob! My heavens you have had lots of troubles with the transformers burning up. Yes, since you have aluminum wiring I would suggest that you have your HVAC system rewired. Sorry that I can not be much help on this. I have heard that it is hard to keep tight connections with aluminum wiring. If you can not keep tight connections then, of course, you have arcing heat and a possibility of fire. I hope that you get your HVAC system fixed soon. Steve

  2. Steve, I too am popping transformers (until I put an in-line fuse), but it seems to happen intermittently. Have an older Rheem heat pump. A few years back, had to replace the defrost board because of corrosion on the bottom…looked like combination of moisture and electricity doesn’t mix. The board mounting seems right against the bottom of the box that covers, and wonder if condensation is culprit. Later on have had other issues that popped the fuse, sequencer relay, fan relay, and thought this last one was the contactor (previous service guy had pulled cover and points were looking rough, so replaced it)…but no joy. disconnected C&Y (X&Y, Rheem) and still popping. Wondering if defrost board is bad again. Visual is iffy. Thoughts?

    1. Hi Scott! I really have no idea where the low voltage is shorting out because the only way to tell would be to visually inspect the thermostat wires to make sure they are not grounding out against each other or the body of your air handler or outdoor unit. If the bottom of the defrost board has a possibility of touching the body of the unit then this could most certainly cause problems. You might want to insert some insulating material under the defrost board so it will not ground out. This could be a control grounding out like a contactor, reversing valve or another component, but man it is hard finding out. You would have to test each component by taking the wires loose and testing for grounds with an ohmmeter. You can also test the thermostat wires with an ohmmeter, but this is quite time consuming too because all wires have to be disconnected from both ends and then use an ohmmeter to test between every two wires. If you get any resistance then the thermostat wires are bad. I hope you can easily find and fix the problem soon! Steve

  3. Hello, I have an older Coleman EB15B furnace that keeps blowing transformers. There is no short that I can find in the low voltage wiring. I have an after market blower fan that is wired slightly different the original fan had which was blue wire high speed not connected, a yellow low speed not connected, ablack wire cooling speed connected to number 4 on the relay a red heating speed connected to number 6 on the relay and an orange common coming from the capacitor to the line in on the transformer. The new fan is a Mars that has four wires red black yellow and blue with 2 brown wires going to the capacitor. So my question would be if I run the black to number 6 on the relay for high speed heating, and the red to number 4 for cooling and cap off the blue wire, where should I hook the yellow wire? Back to the 120v line in on the transformer?

    1. Hi Chris! Sorry to hear you are having this problem. Maybe the high voltage primary wiring is burning the transformer up. You would want your new Mars fan motor to run on high-speed black for cooling and any lower speed probably red for heating. You need to determine by looking at the motor label which wire is common. Most of the time white or yellow are the common, neutral wires on fan motors. Once you determine which wire is common the other wires are just different speeds. Any speed wire that you do not use can be taped with electrical tape and hang loose as long as it can not be sucked into the blower wheel and cause problems. From you telling me the way you had your older motor hooked up the black on the Mars motor should go on #4 for the cooling speed (high speed) and any one of the lower speeds connected to #6 for heating. Maybe you have a different relay from what we sell on the following page: https://arnoldservice.com/product/90-340-white-rodgers-fan-relay/?q=fan%20rel On the White-Rodgers relay 90-340 #4 would be a hot wire coming from a 110-volt power source. When 24 volts is applied to the relay (fan on in Air Conditioning) #4 closes the circuit with #6 and gives 110 volts to the blower motor high speed to run the AC. The 90-340 relay shows how the circuit works on the body of the relay. Please see the following screenshot where I try to explain: https://www.screencast.com/t/A9dZNOsd1 You might have the relay wired wrong. I am sorry that I can not be much help. Make sure you identify what the common wire is on your blower motor and do not wire a direct short to ground (common). Steve

  4. Hello,
    I have an A/C that works when it wants to. It regularly goes through these phases where the compressor will rapidly turn off then back on and then eventually remain off and just buzz while everything else remains on. It might be fine for a week or two, but without fail it will begin acting up again. The contactor and capacitors were replaced. I also replaced the outdoor thermostat wire from where it comes up from a concrete slab to the unit. The thermostat was also replaced. I do not believe there are any low pressure or high pressure switches installed. At this point, I am highly suspicious of the 24v transformer inside the furnace. Recently, I visually inspected the transformer and noticed the terminals on the 230v side have a carbon build up as though it has been sparking. I can’t see the 24v side because the wires are embedded inside the transformer (no visible terminals). What could charred 230v terminals indicate? I can confirm this build up has occurred over the past 6 months because I have pictures from 6 months ago of the furnace and the transformer terminals were clean at that time. After this recent discovery of charred terminals I do not want to run my A/C at all anymore. Any help appreciated.
    Thank you.

    1. Hi Peter! I would suggest that you try to test with a volt meter or do a visual inspection of the contactor to see if the contactor is being energized (24 volts to the contactor coil) when the unit cuts off on ON. If the contactor is kicking off and on when the contactor should be ON with the contacts closed all the time then you have a low voltage problem somewhere. If you think it might be the transformer with the charred wires then I would recommend replacing the transformer with a transformer equal to or with a transformer with a higher VA rating. We sell transformers on the following page: https://arnoldservice.com/product-category/furnace-parts/transformers/ A good 40 VA universal transformer like the TFM4031 or the P201-3401 Bryant Carrier transformer should work well if the 40 VA is high enough. The charred wires usually occur if there is a loose connection on the transformer wires or if the transformer is over-heating. A short in the low voltage wiring can also cause charring. Please inspect your thermostat wires and make sure they are in good condition. Most of the time thermostat wire problems occur on the outdoor unit where the wire is exposed to animals and hot sun. I hope you can easily find and fix the problem. Steve

      1. Thank you, Steve. I replaced the transformer (picked one up locally). The bad news is that the compressor still randomly cuts out. The good news is I have narrowed it down a bit further. I lucked out and was able to get a voltage reading at the contactor while the problem was occurring. Sure enough, when the compressor cuts out the low voltage drops to almost 0. I can say with almost certainty now that, just as you said, it is a short in the thermostat wiring. Although I replaced a short span of it already, it looks like I will need to go ahead and replace all of the wiring. The tricky part will be replacing the wiring that goes under the concrete. If you have any tips for that I’d appreciate it. I will also do some searching around the web on how to do that. I’m guessing I’ll need to buy some fish tape. I’ll also grab a kit that has all the various terminal connectors and re-terminate any old connectors I see. I’ve also got a replacement furnace control board on the way just in case the short is on there. Thanks for all of your help so far.

      2. Hi Peter! Great to hear that you think you have found the problem! If you are trying to run the thermostat wire through the concrete in a slab home then most of the time they run the wire and line sets up through the attic. I do not know if that is possible for you or not. You might want to just run the new wire through your house and out the window to see if that is the problem. I would hate for you to go through fishing the wire, spending all that time and the wire not be the problem. I hope that the thermostat wire fixes your problem. Best of luck! God bless you and your family. Steve

  5. Hi, Steve, My thermostat couldn’t activate the heat/cooling system. I called a technician to fix the problem. After checking with voltmeter, he said the 24v transformer burned; after replacing it with a used one, he continued his trouble-shooting and told me that the furnace control board is bad. He asked me to order a new control board and install it myself. If I fail to install correctly, call him for help. So I ordered a new furnace control board from amazon, and installed absolutely according to its original wiring (which is different from the manufacture installation guide by connecting the motor’s high speed line to the “Cool” site of the control board while the guide connects the “Motor” site instead). After installation, the fan works, then I changed the thernostat setting: switching the fan to “auto” and the system to “cool”. Waiting for a while, the AC didn’t start, then I changed fan from “auto” to “on”, the device still didn’t work. Later, I found the 24 transformer burned out. Thanks.

    After installing the new furnace control board, the fan works, for the thermostat setting is System “off” and Fan “on”; but after I changed the setting: System “Cool” and Fan “On”, the AC didn’t work and the 24V transformer burned out. I checked the wiring of both the new control board and thermostat, no problem with wiring. Please tell me what might cause the trnsformer burn out.

    1. Hi Sam! So sorry the hear that you have had all these problems! Most of the time as I state in the post, transformers burn up due to shorted thermostat wires. Sometimes it can be a shorted control like a control board or contactor, but most of the time it is thermostat wires shorted that cause the problem. I would suggest that you either replace your thermostat wires or test to see if any of the wires are shorting out. This can be quiet time consuming so it is almost easier to replace the wires. I would like to suggest that you leave the outdoor unit’s thermostat wires disconnected and see if the new transformer still burns out. You can do a feel test on the transformer to make sure it is not overheating and burning up another transformer. Most of the time it is the thermostat wires that are exposed to the sun and animals that short out first. I would suggest a 40 VA or higher VA transformer. I hope you can easily find and fix the problem. Steve

      1. Hi, Steve. Thank you very much for your quick response. I don’t understand “…leave the outdoor unit’s thermostat wires disconnected and see if the new transformer still burns out”. My thermostat on the 2nd floor has 4 wires connect the R, W, Y, G of the furnace control board, while control board’s Y and C connect to the outdoor condenser. That is, control board “Y” has both indoor thermostat Y wire and outdoor condenser Y wire. What do you mean: to disconnect the outdoor unit’s thermostat wires: disconnect the control board Wire Y and C that connected the outdoor condenser, or there is another thermostat in the outdoor unit? Really appreciate your help!

      2. Yes, I mean to disconnect the Y and C wires, the two wires that go to the outdoor unit and see if the transformers still gets hot. Most of the time it is the outdoor unit’s thermostat wires that short out because of sun exposure and animals chewing through the insulation. If the transformer does not get hot then you will know that the thermostat wires to the outdoor unit are at fault or control on the outdoor unit are at fault like a contactor. Hope you can find the problem soon. Steve

      3. Hi, Steve, you’re great. When disconnecting outdoor wires (Y and C)from the Control Board, both the heat and the fan-only work properly. while I connect Y and C, the transformer gets hot, and volts from both R and Y (control board) are only about 4.6V rather than the 24V. I will try to check the outdoor unit. Please give me some detailed suggestions or link on how to troubleshoot the outdoor unit including wire and contactor. Thanks a lot!

      4. Hi Sam! I would suggest that you replace the thermostat wires that go to your outdoor unit. This could be a contactor problem, but most of the time it is the wires that are shorted out. I would like to suggest that you search on Youtube on how to do this repair. We have an air conditioner troubleshooting flow chart that I hope will help you determine where the problem is: https://arnoldservice.com/wp-content/uploads/2014/12/air-conditioner-troubleshooting1.pdf We have lots of air conditioning and heat pump troubleshooting questions with answers and Youtube videos on the following page: https://arnoldservice.com/air-conditioning-heat-pump-troubleshooting-questions-answers-index/ We also have an air conditioning and heat pump troubleshooting simplified page that I hope will help you find and fix the problem: https://arnoldservice.com/air-conditioning-heat-pump-troubleshooting-simplified/ We also have lots of troubleshooting advice on the following page: https://arnoldservice.com/troubleshooting/ If you have any specific questions please let me know. I hope you have a great day! Steve

      5. Great Sam! So glad to hear that you were able to find and fix the problem by replacing the thermostat wires that go out to your outdoor unit! Thanks so much for sharing how you fixed the problem! God bless you and your family today and always! Steve

  6. How do you test a fan relay board in a trane heat pump?

    1. I am sorry, I do not know. I would suggest a new board. If you want me to look up the board please send me your air handler’s model number to our email address: arnoldservice@gmail.com. Thanks! Steve

  7. Trane heat pump, has burnt out three transformers. I set thermostat to no heat or cool. Turned fan switch to on . Fan ran about five minutes and blew transformer again. I am taking the fan relay board out, it looks good, no burnt spots. What do you suggest?

    1. Hi Mike! As it says in this post most of the time transformers burn up because of shorted thermostat wires. You might need to first inspect the thermostat wires to see if there are any breaks in the insulation and then test the thermostat wires with an ohm meter to see if any are shorted out. Sometimes it is easier to replace the thermostat wires because the testing can be quiet time-consuming. I would suggest an inline fuse to protect the transformer from getting burnt out. We sell one on the following page: https://arnoldservice.com/product/inline-fuse-holders-zebra-zfh01/ I hope you can find and fix the short. Steve

  8. Dear sirs I’ve been through 3 transformers in recent month and they work for a couple weeks and then bam another one gone no short in wires but I think it’s the reversing valve how can I be sure

    1. Hi! So sorry to hear that you have been through 3 transformers. I do not know if you have already tried this or not, but I would recommend a higher VA transformer. If you have like a 35 VA transformer I would recommend a 50 or 75 VA transformer. The larger the better. You should be able to test the reversing valve solenoid by following the instructions on the following page: https://www.achrnews.com/articles/104419-heat-pump-electrical-component-checks I hope this helps you find and fix the problem. Steve

  9. Hi Steve, I recently bought a new humidity controller for my forced air furnace. The furnace has a White-Rodgers controller that has output pins for the humidifier which I wired up in series with the humidity controller. I also recently replaced the 120 vac to 24 vac transformer with a larger one. About a minute or so after I turned it on, the solenoid (also new) in the humidifier made a loud pop sound and I could see the casing had melted and it was staring to produce a burning smell. I quickly disconnected it but now I’m not sure what happened. I can see if wires had shorted out that the transformer could burn out (I’m gonna put an inline fuse in it after reading your article). But the furnace is working fine so I know that the transformer is still working. But I don’t see how the solenoid water valve could melt from that. The only thing I can imagine is that it got 120 vac somehow. Do you have any ideas or suggestions?

    Jonathan

    1. Hi Jonathan! The only thing that I can think that would cause the 24-volt solenoid to burn up would be maybe that the solenoid windings inside the solenoid shorted out or like you say maybe 120 volts was applied to the 24-volt solenoid. I would check the White-Rodgers controller to make sure that the voltage coming out of the controller is 24 volts and not 120. Yes, I would check with a voltmeter to see how much voltage the solenoid is getting. Hope it is not 120 volts. I hope you can easily find and fix the problem. Steve

  10. Mr. Arnold,
    About six weeks ago, during Michigan’s hottest days of the summer of 2018, my transformer and circuit board blew. I knew they were gone as soon as I open the furnace door. The unmistakable odor of fused circuits emanated from within. I ordered replacement parts, installed them myself. A/C worked fine for a month. Then, transformer blew, again. I ordered another transformer, installed it myself. A/C worked for an hour! I ordered another transformer and started doing some serious research and detective work.
    I read many, many articles from your webpages and deduced that my problem must be in the outdoor thermostat wires to the condenser. Upon inspection of these wires, I found them to be devoid of insulation. The two individual wires were bare and the insulation that would normally cover the two wires, together, was also gone.
    I went to my calendar and discovered that my transformers all blew on a Monday. “Well, that’s suspicious, I thought, the only other thing that occur on a Monday is the lawn crew who does my yard.” As my inner voice trailed off, and before I finished this thought, I realized that a weedwhacker was the menace of evil and mischief that had bedeviled my A/C system.
    I replaced the wires, running them through a three-foot length of garden hose as added protection against the villainous, slicing, dicing strands of the weedwhacker. I also spliced in a five-amp fuse between, yet another, 24volt transformer and the circuit board. Why the A/C design did not include a fuse/breaker, in the first place, I do not know, hmm?!
    Finally, after six weeks of battle, one circuit board at $80.00, three transformers at $25.00 each, additional hi-tech fortification (read: garden hose) of my Citadel of Cool, and much perspiration, the weedwhacking beast of my heated nightmare was thwarted.
    Armed with new knowledge, I now stand ready to defend should this beast, or any other deviant creature attempt to breach my security. Truly, Knowledge is Power! Thank you for your assistance.

    1. Good Morning Douglas!Thank you so very much for taking your time to send us this information about how you found the problem with your low voltage transformer burning up! I sure do appreciate you writing this! God bless you. Steve

  11. Mr. Arnold,
    I did a pretty stupid thing. I have an older IPC AC unit that was freezing up. I called a HVAC service who came and put in some R-22 which seemed to fix the issue, however, since then the compressor would not shut off. We worked around it for a while by cutting it off at the breaker. I read some things online that suggested the contactor was sticking and recommended replacing the contactor and capacitor every 5 years as maintenance. Well, we have been in our home for 12 years and never had to do much with the HVAC. I ordered both a contactor and a capacitor for my unit. I put them both on, here is where the trouble starts. I had not studied how the wiring worked on the contactor and I did not take pictures when I disconnected it. I reattached the wiring as I thought I had recalled it being, which was wrong. The contactor smoked a little and the 24 volt transformer popped. I checked the contactor and rewired after getting a better understanding of how it worked. I ordered and replaced the 24 volt transformer, but nothing. I am getting 24 volt out of the transformer, but nothing coming out of the control module to the outside contactor. The blower doesn’t run. I am guessing that my wiring error may have burned up something in the control module. Am I on the right track or should I look somewhere else? I could not find a protective fuse anywhere.

    1. Hi Joe! So sorry to hear that you had this problem! I would suggest that you start with where you are getting the 24 volts hopefully on the R and C (com) terminals of your control board and test with a volt meter to see where you stop getting 24 volts out. This should be where the problem is located. I made a mistake like this before an burnt up every low voltage control in a customer’s system which included the control thermostat and gas valve. My mistake ended up costing me over $300.00. I hope that maybe you can find a fuse somewhere in your system that protected you other low voltage controls. I learned from that mistake and I am sure that you will learn alot by finding out how to fix your system again. We have lots of air conditioning and heat pump troubleshooting questions with answers and Youtube videos on the following page: https://arnoldservice.com/air-conditioning-heat-pump-troubleshooting-questions-answers-index/ We also have an air conditioning and heat pump troubleshooting simplified page that I hope will help you find and fix the problem: https://arnoldservice.com/air-conditioning-heat-pump-troubleshooting-simplified/ We also have lots of troubleshooting advice on the following page: https://arnoldservice.com/troubleshooting/ If you have any specific questions please let me know. I hope you have a great day! Steve

  12. I am working on a older Trane unit. Mod # BLU108E936B1, that gets no power to the fan motor. I replaced the relay and the fan motor but still no power except 2-3 volts. After bench testing the old motor I discovered that it was in fact turning slower than normal. I thought that maybe I had a bad transformer but it delivers 24 volts to the relay coil. Any thoughts on this?

    1. Hi EK! I see that you are having trouble with a fan motor running up to top speed on an older Trane furnace. It sounds like you might have motor capacitor problem (weak capacitor), a fan relay problem or fan motor problem. Our Trane parts program shows that your furnace should have motor part number: MOT02883 with capacitor part number CPT00330 which is a 4 MFD/uf 370 volt oval capacitor. The relay is part number RLY01014. You might try running the motor on a higher fan speed. The black wire on the blower motor should be the high speed wire. I would suggest checking with a volt meter to see if you are getting 110 volts to the fan motor. If you are not then you could have a relay where the contacts are bad or a loose wire connection. Please see the following link for a screen shot picture showing some of the parts for your furnace: https://www.screencast.com/t/siTZ67tmni5m I hope you can easily find and fix the problem. Steve

  13. I have a question.The 240v- 24v transformer on my heat pump burned out it was the common wire on the 240v side what may have caused this. Thanks

    1. Hi Michael! Most 240 volt transformers do not have a common on the primary side because they have two hot wires that power the primary side of the transformer making 240 volts. Most of the time transformers burn up because of a direct short to ground. Moisture, over-loads, power spikes and old age can also be a factor. Most of the time it is caused by a short in thermostat wires. I hope that you can find and fix the problem. Steve

  14. 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

      5. Thanks for the tip on tracking down a 24v short. I had that issue today and wasn’t sure if the burnt coil on the contactor would affect the transformer’s low voltage to a point that it would burn out the low voltage side of it

      6. Hi E.K.! Thanks so much for your kind words! Yes the contactor coil can cause a low voltage short in a system and cause the low voltage transformer to burn out. Hope that your system has a low voltage fuse in the line so that it does not hurt any other components on your air conditioner. Many systems have low voltage fuses near the transformer, on the air handler or furnace’s control board. I hope you have a great and blessed day! Steve

      7. I have a question nobody can figure out why my fan relay switch keeps going 4 times now I’ve had so many problems thermostat replaced transformer replaced breaker replaced and fan relay replaced and ac still bows breakers and shits off

      8. Hi Tammy! Sorry to hear that you have had all these problems with the parts going out on your unit! This sounds like you might have a short in your low voltage wiring or a weak low voltage transformer. I would suggest making sure that you are getting a minimum of 24 volts AC out of your low voltage transformer between the R and C (com) terminals. If the transformer is not producing enough voltage (under 24 volts) then you will have contactor chatter and it will cause other components to go bad because they can not make a solid connection. Without a solid connection in the fan relay and thermostat you will have chatter, poor connections, over-heating and part failure in a short period of time. You might need to get a stronger VA (Over 40 VA) transformer or if you have a short in the low voltage wires you might need to get the thermostat wires replaced. Please make sure all low voltage connections are good and tight. I hope you can easily find and fix the problem. If you do find the problem please let us know so we can spread the news. Thanks! Steve

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. 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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