How to replace a transformer (without burning up the new one) 

THANK YOU! My transformer went bad taking out the AC but the control board fuse was good. 120V to the control board but no indication of power. I got a new transformer from the hardware store that would work, but it didn't and just started to heat up. Same no indication of power with power. Saw your video. Something took the old transformer out. Made me look at other videos and found one showing what uses 24V and to isolate one by one. The AC contactor wire at the condensing unit got a little chewed and shorted. It didn't pull enough current to pop the board fuse, but it pulled the transformer down to 2 volts before I fixed the short. THANK YOU! Your video started me down the rabbit hole to fix my AC on a hot day. The new transformer works and the old one is dead.

Solution - a real big thanks to your many videos. I have spent hours watching your videos and repeated them to educate myself. The hardest part would be to learn how to use the voltmeter - I have one - but have not used it in years. HERE is the solution. You have another video where you tap what appears to be a thermocouple - like a button battery on a tuning fork - I found this component near the top of the Rheem Prestige Super Quiet 80 and tapped it as you did. The furnace has cycled 5 times now so my hopes are high this was the fault. If the furnace fails again - your further ideas would be appreciated.

I'm surprised you didn't show brushing the 24v leads across each other to look for a spark, showing a short without the fluke amp meter. Show shortcuts that save time! Love you man!

That would be the best way. I had some resistance because of cost but if I (like you) sold them as a lifetime warranty, both customer and service tech would win. Thanks for the comment. GFM

Thanks for the support GFM

this video is gold. nobody talks about this. impossible to find what you just stated in public forums for quick troubleshooting.

Thank you for another best-in-HVAC-instruction video!

Yeah, the mini breakers are a great idea. GFM

Thank you for posting this great information. Your videos are 5 stars and a wealth of great information.

Well, the reset button you are referring to is the flame failure reset. When you run out of oil, garbage is drawn thru the pump and many times plugs the nozzle. You will need to replace the nozzle. Checkout the oil furnace playlist for some ideas. Let me know how you come out. GFM

Many people do not even know the purpose of these transformers. The purpose is to step down the voltage from the 120Vac/240Vac to low control voltage24Vac which is what runs the air conditioning and heating systems. Multi tap transformers are adaptable to more than one source voltage where the primary coil will have three wires (L1, L2, L3) with the centre tap and L3 being usually set for single phase 120Vac.

Thank you for the video. I learned a lot.

Thanks will try isolating tomorrow, Schematic is hard to read. I see pressure switch in series with limit switch and fusible link (is fusible link the rollover), No Rollover switch in diagram. Could other circuits affect this short such as gas valve? Ignitor circuit? Limit switch for vent motor? Thanks again.

Trane XE90 xformer burnt two years ago after a new tstat was installed. Replaced the xformer and put a fuse in but continued to burn fuses. Replaced the Tstat and had no issues until this year when turning on the heat. Disco'd all tstat wires and wires to the Outdoor AC unit. Measured 27.1 volts on secondary of transformer. When jumpering white to red to turn the heat on, the fuse tripped. You mention to look for relay or contactors. Are these relays from wires on Control board?

Here's a question for you. I have a two furnace/AC setup that has a common return trunk line (don't ask). I just switched over to NEST thermostats and the installation was easy. The 2nd floor is working like a charm. The first floor seems to have fits every once in a while and reports that there's no power to the thermostat. First thing I eliminated was I made sure the condensate pump was 100% clean so I I did a thorough cleaning job and reinstalled. Same problem persisted. Next thing I did was replace the Emergency Switch with a new one and I grounded it to the system (it wasn't grounded). Same problem within hours. Next thing I did was replace Breaker with a correct 15A one. Reason I did the Switch/breaker was it seems that flipping them reset everything and it _started_ working again. Now I'm wondering about the transformer. Its connected to _both_ furnaces and I'm guessing its about 20-25 years old. Not making any noises, showing about 1.6v at the connection. When the system is running, I'm seeing 3.0v at the thermostat connecting wire. Its Christmas Eve and tomorrow I'm gonna bring the nest unit down to the furnace and run a 24" jumper to confirm that the wires from furnace to thermostat are OK. You have any other ideas? Bring your suggestion of elimination!

Burned up 2 transformers trying to start A/C this year, and I kept hearing the condensing unit contactor was probably at fault. Replaced both, and noticed transformer buzzing, ready to blow again. I disconnected low voltage wires at contactor, still the same, so I disconneced same wires inside furnace, and no more buzzing. The wire going outside was shorted out, rubbing last 10-15 yrs on a metal part of furnace. Replaced, now have A/C!

Thankyou for Great Help!

Thanks, I'll definitely do that!!

Only if R shorts to Common. In all other cases, the thermostat is only a switch. Hope this helps. GFM

wow, thanks for responding! this is where I am at. Saw your other video....put amp meter over secondary wire ( red going to Terminal board), turn on breaker to air handler, then getting 5 amps. I have lv wires to outdoor HP disconnected and No calls on Stat. What could be drawing the amps on secondary wires with nothing on but power to Ahandler. I put fuse in line, trnfmr hums, but not loud, fan does come on with Fan switch and cool call. Replaced Seq as well, ohmed out all LV wireing, doesn't seem to be a short in them, comments?

Can a short circuit in a thermostat cause a burn out in a transformer?

Shorts can be anything. The fusible link is probably the rollout. Any load in the low voltage could be the problem. GFM

great video, I like it

Just wanted to say. Awesome videos!

thaks, for your fast responding, i will do that, have a nice night!

my oil furnance tranformer reset button keep popping up usually this happen when the oil runs out but there's oil in the tank, there's a reason why this keep happening?

hi, thanks for video. really helpful i have question, how did you get number 1.6A? step down transformer(120v->24v) always have 1.6A on secendary? what if i use transformer (240v- - 24v)? im wondering how to get the number 1.6A

My heat pump is 11 years old, noticed 1 month ago that I had no 24 volts at thermostat, checked 3 amp fuse, blown. Replaced 3 amp fuse, ran for one month okay, then failed again. Now Transformer secondary side has no voltage, but replaced fuse is still okay. Is it possible that the transformer got a power surge which crippled it a month ago, also blowing the fuse, but had a little bit of life left in it? I guess I was expecting the fuse to blow again, but now transformer is the only thing down that I can tell. These parts are generally cheap, so I have on order a couple of transformers, contactors, and capacitors just in case. Hope it's not any of the boards, those are really hard to find.

I know this is a bit off topic but I had a cordless screwdriver and it was a cheap one I pluged in the charger and it started popping sizzling then boom right on my hands it exploded! Lol thanks for the demonstration

Hello, I got a question on testing.Do I have to have the wires connected or can I ust test the prongs of the transformer with an ohmeter? Ac volts correct? Where do I place the black and red when testing ? Thanks in advance!

Would a voltage surge burn one of these up? We had some issues with power coming into the house, and half the circuits were getting 240 v, including the furnace. It didn't blow the furnace fuse, but the thermostat is no longer getting any power, but I've confirmed the system is getting 120.

I've got a question: i got a old radio/alarm clock that I want to replace a transformer for i got one thats rated at the same wattage but how do i find voltage? i mean the voltage that comes out of it i dont want to burn it out this clock is special it was made by my grandad in the 70's who was a electronics repairman who is long long dead now. another question does it matter if you use one rated at more of wattage? Thanks!

Almost all 208v nominal systems I work on are incorrectly tapped to the 240v nominal lead. They often work fine, but it blows my mind that the installers didn't tap it correctly...

120-v is your hook up but is there any voltage in the 208/240 wiring pig-tails, I noticed you have no wire nuts on the wires.

The main power source coming into the unit the primary while the contactor is like a secondary or substation supplying power to all the other electrical devices in an HVAC.

Hello Grayfurnaceman, I came across a tech who installed a 40va transformer in place of an original 50va transformer. The old 50va unit had a built-in push button reset. So the tech added a 3 amp fuse to the 24volt side as added insurance. The AC unit starts up and works fine. Tech was paid and everyone was happy. But I'm scratching my head. I have 2 questions. What happens in the winter when unit calls for heat? Will the 40va rated unit be too weak to handle the amp draw, and burn out prematurely? Or will the 3 amp fuse be sufficient to protect it? The ac unit mentioned is a rooftop commercial unit. The tech while wiring the primary was second guessing to wire it to the 208 or 240. So he measured the 2 wire's power (the 2 primary wires going into the old transformer) and it was 210volt So he wired the 1st wire to common and the second wire to orange (208) on the transformer's primary. He didn't open up the disconnect to check phase to phase to determine 208 or 240. Is this adequate? I don't think "rounding the 210volt to 208 was the right thing to do. What happens to the transformer if the correct wiring had been 240volt, and the non-seasoned tech wired it to 208v? What should be the normal procedure to do a correct install on this? Thank you grayfurnaceman.

Which side of the transformer is primary , and secondary / = is red - green 208 - = and both reds 240

Can a short between the leads at the thermostat cause the transformer to fail? A thermostat was being changed out when the leads were touched, now I have a transformer to replace. Thanks

These general all about control transformer TS videos I’ve seen concern only the secondary. How about a fuse on the primary, so, as when the primary has a tenancy to burn out with the quickness, a control transformer is not damaged, ie. the primary shorts/burns up on power up. Any ideas on a primary fuse size to protect during TS???

I have a AC which keeps tripping the reset button of 24V transformer. Once it is reset the AC will run for several hours and then trip the reset button again. Any idea which part has short? The contactor, thermostat? Happy New Year

There has to be a short in the heat circuit. The key to this is remove all circuits. You have removed the stat and its wires. Now you know it is not outside the furnace. You probably have something shorted to chassis between W and C. Eliminate limits, rollouts etc each in turn. This is not easy. It takes time, and attention to detail. let me know how you do. GFM

Always replace a bad XFMR with a breaker type ocp when trouble shooting.

Hey there had a question for you. So I'm looking to get a Nest thermostat as the gas company is offering a rebate on them. I believe I have a C wire as it is connected in the furnace, however the C terminal seems to be connected to the R terminal on the transformer and the C terminal on the transformer goes somewhere else. Is this normal and will it affect whether or not the thermostat has a constant 24 volt power supply? Your feedback would we welcome. Thanks!

I'm getting a lot of humming from my furnace when it's not calling for heat. Starting to suspect transformer. Do you have a video on this?

Hi GFM! The 1st thing I ever did was replace my 24 v power supply on my furnace 20 years ago. The old one did not have 24v common grounded to chasis. I put it back like it was. Is that the norm for the 24v common not to be grounded to chasis?

I am troubleshooting my Bryant KAV383 furnace. I have 120 volts into the transformer (probing black and white wires) and then I have 26.8 volts coming out from it (probing blue and red wires at the same time). is 26.8 too much? my trouble code is a rapid blinking light which is for "line input voltage reversed" apparently. I was thinking somehow the circuit board (21 year old furnace and all appears original) is going bad, but then saw some videos on here and decided to check the transformer. $300 for a circuit board vs $25 for a transformer.. but I am UNSURE which direction to go for the repair. .. thoughts? thanks !

I have a question sir. My ac unit is not working. The thermostat is turned off and has no power coming to it. I checked the breaker box and its fine. I checked the 5 amp fuse on the air handler control board and it is fine. I am getting power everywhere except the control board and the output of the transformer. I have 240 going into the transformer but less than 1 volt on the output which is supposed to be 24 volts. I checked my contactor and it does have power. When I press the button on the contactor the AC unit will turn on. So I'm not sure if it could be a bad transformer or a bad control board or something else?? Your advice would be greatly appreciated! Thanks!

Hello! So I found mine burnt One question I have a 240 volts system On primary wires What voltage the common wire supposed to have or is it consider the neutral ?

I have a problem with my heatpump. I am not getting any low voltage, but I do have primary voltage. I checked the fuse on the board and its burnt, I still haven't put a new one in but I also think the transformer might be burnt also. If it is the transformer, what can be the cause of it on a heatpump?

79 coleman furnace in a mob home, california. fan motor comes on. runs for a minute and shuts off. a minute later it comes back on for a minute, then off again. the furnace heat is on the whole time. no juice coming from 24vac trans. i will replace trans. what else needs replacing

hey man question. I bought a incubator from china which has a 110v and I plugged it in to my uk pains with a uk cable 240v and it blew up how do use that incubator in the uk? do I get a step-down converter for uk and buy a new 110v transformer

i replaced my gas valve on my miller cmf80-AG and installed a new thermocoupling , but when i turn the thermostat up to light the furnace it does not ignite and it also puts the pilot out , any ideas?

is my 24v xformer shot if i'm only getting 21v at the contactor coil???????

I have an ac unit that has burnt up 2 transformers on the primary side in the same day. Can a short to ground on the secondary side cause the transformer to burn up the primary side?

Hi, mr. grayfurnaceman! i had that problem and this is the second transformer blow out, it happens just on the heat mode, so what should be the problem? thanks for your help

thanks...

I have a problem with wedge bulbs burning out every second day in house number eluminated plague. What could be the reason. Bell knob stays eluminated 24x7 with no problem House numbers bulbs keep burning. Any sugestions?

Hey GFM, working on a duomatic Olsen, transformer/relay center smoked, then pressure switch quit transferring voltage. This possible? Where could my large amp draw coming from?

can I test the voltage of the doorbell by removing the outdoor button and using a multimeter on the 2 wires?

Question, my heater/fan wont do anything, i have 220v into the transformer, but no 24 volts coming out. When i took my T-Stat off i noticed the wires were all bare so im sure thats what fried my transformer.. My question is why did it NOT blow my 3 amp fuse but fried my transformer? Thanks.

Hi Gary, I have a step down transformer that received 240 V from the source, but there is no 24 V coming out from the secondary wires. The 3 amp fuse on the circuit board wasn't burned. At first, I thought my transformer was fried by a power surge, but when I replaced it, the new transformer burned right out. I didn't notice any burned or loose wiring. Do you have any suggestions where should I troubleshoot? Thank you for your time

Blown mosfets can short to ground, producing a huge amp draw and destroy a secondary winding on the transformer. Allways check the mosfets adjacent to the transformer. They could be shorted to ground. It happens in DC->AC inverters.

My furnace transformer has a green ground wire coming off of it but the wire is capped off. My transformer failed (I think) because my Bryant 90 plus heat went out but AC works fine. I have no voltage to the R wire, no voltage to the pressure switch either side, no 120 voltage to the hsi but the gas valve opens when it's time to open. The red light works and flashes a code. I tested the transformer and a good 120v going in but it looks like a negative coming out because I use a needle multimeter. What I mean by negative is my needle will bounce to like 1v and then backwards and start clicking. Any suggestions?

What kind of transformer is that? I got one that looks almost exactly the same from my old audio boombox/cd playee… it has two black wires on one side with a small toroid core wrapped around it… and 2 red wires on the other end… im curious about it - how would you categorize mine also btw; it seems there may be many ways If i provide the two black wires w/ 120V(AC), im able to measure 11V(AC) RMS on the other winding at the 2’red wires. It does step down, but ive seen visually similar transformers also categorized as power/audioOutput/Tube/SingleEnded&Control… Great work btw. Thx!

Hi ...whst could cause a gas furnace 24vac transformer to catch fire / melt on the 120v side ?

How much are transformers for central air/heat unit and how much to pay to have it replace?

I have 24v transformer with reset button which keeps tripping the reset button. Are the thermostat wires shorted?

Question: The transformer in my air handler has normal input voltage (240v) but the output is 31.15v instead of 24v. Would that or could that cause my low voltage fuse to blow?

have complained about the humming in the air conditioning unit that's in the bathroom in the ceiling it has harmed for many years and it keeps breaking down they say if units so old can't even buy parts for it anymore what do I tell the landlord

Curious: Furnace failure due to what looks like a dead control board (power to board but nothing works, no transformer, lights, etc) This has happened in conjunction with a house electric problem (floating neutral, voltages all over the place) that is being addressed. I pulled the transformer, the neutral input is visible under the translucent side cover, flashed black. Could the bad house neutral cause a transformer to go bad? *not a furnace guy, just a fix it guy.

Thank you so much, I appreciate your channel so much. I am having to replace my transformer which started smoking. I have watched and read everything I could. My new transformer is duplicate and has no fuse like previous (board or anywhere). Therefore, I am planning to install an inline fuse on secondary wires which are both the same "color". In looking at the wiring diagram and the control board itself, I understand where the wires should connect on the control board terminals; one on C and one on X. (Sec 24 VAC Yellow Yellow) I don't want to make a mistake again as I think/know it was a direct short when I was checking the inhouse thermostat via jumpering and made an error! In following the diagram It seems like I would place/start the inline fuse on the X terminal continued wires to molex as the C goes on to other areas. Am I right or wrong? Sorry, forgot to say this is a Rheem rooftop packaged unit. The other unit I did all the cleaning, checking voltage, ohms, wirings and everything ... it's doing very well; and I also plan/will install an inline fuse on it also. Sorry to be so verbose as my previous HVAC owner and contractor (20 years service) retired; and sold his business and moved out of state. The company he sold name to unfortunately are rip-off-artist... serious as my previous taught me a lot when he did work. Two units are about six years old, installed by previous, and I messed up! Just want to get it right. Any help appreciated if possible.

can the transformer go bad. I short it out at the condenser contactor with 120v by mistake and I do not have any power at the control board. nothing works

My furnace is 15 years old. I replaced my blower motor about a month ago. everything ran fine. just the other day I noticed that the inducer fan kicked on, my igniter lit up but the gas valve didn't turn on. could that be a transformer issue or does it sound more like a bad circuit board

hay Gary Furnus guy I have a greenheck model M34625 and a teckmar boiler 4 stage trying to see where to hook up I have a ca Alarm contact on the Teckmar D264 I have no end switch need a. diagram if you have one

I have a ICP oil furnace from 90s, I m not getting 110v to 24v transformer, does 110 come from fan relay?

What is the main cause while i start package acu,,,after i start compressor the transformer will tripping off

I have a question. The outside unit was only getting 21 volts to the contactor. The technician said he replaced the transformer. I couldn't watch him and I wish I had but I had an appt. Anyhow, when I came back he was gone and the Co sensor and Thermostat were blown. He and the company told me the Co sensor was bad and when he replaced the transformer it blew the Co sensor and thermostat which meant the Co was bad. He left me an estimate for $1520.00 for a new Lennox Comfort 7500 thermostat and a New Honeywell Co sensor. I questioned this and I was bullied into believing it was the Co sensor. I don't believe it. And I do not want to spend 1520 on these two parts. I just can't prove what he did. I can get the parts a lot cheaper myself and wire them in if this is truly the problem. But what I am worried about is blowing the new ones. He left the disconnect on when he left and the thermostat was Hot to the touch. I turned off the disconnect. My company is questioning what happened also and why the huge expense... Any thoughts? I am proficient in A/C repair. I called this company because we have a contract with them. I am a facilities manager for my church this unit is for the choir room.

How do I get the smoke back inside of my transformers

I just had to replace my transformer on my peerless boiler. My first clue that something was wrong was that one of my thermostats started smoking when it was turned up. I pulled the thermostat off the was and it was black, and smelled burnt. I replaced it. The boiler still didn’t run. I was doing voltage checks and had 120v coming into the transformer primary and 0 v on the secondary side. I tried to find a transformer but couldn’t find one. I had to call a plumber who had one one his truck. He also replaced one of the taco three wire control valves. He said the valve shorted which caused the thermostat and the transformer to blow. Does this sound right or did he replace the valve for nothing? He didn’t use a meter to check anything. He just went off of what I told him I found. Oddly enough I was charged a diagnostic fee. Thank you for your videos. Not only did you help me with this one, but I also watched and learned how to clean my boiler thanks to you. I really appreciate it.

If a transformer lasts a few months before blowing.. what do you suggest? Happens on cool and heat, but months of time apart.

thanks MR furnus

I put my transformer 9-0-9 in reverse an it blew. Help me

I think I can used ca Alarm contact not shore

Trying to find a problem in my air handler : I have blown a few transformers. The Fan Relay is buzzing loudly, i turn power on only a few seconds, but after a few trys the Tnsfmr does not make 24 volts. I am on my 3rd Fan Relay. The original is Hnywll 1043, now discontinued, and using others with same amp ratings. The Pins are pretty easy to understand for correct hookup, 1-3 is open..goes to Motor, 4-5 is N.C. also goes to motor, then the coil runs on LV from Trnsmr. The original problem is the outdoor HP is not coming on a Cool Call. I am inside at the stat and Air Handling wires looking for failure. The red and yel LV wire jumped kicks HP contactor on, but through stat it will not. I am on 3rd stat for elimination purposes. You recommend looking for shorts, I will do the amp draw at the secondary, then maybe ohm each LV wiring running through wall from TB to to stat. Things were working fine until leak cause outside unit to not come on.

It is probably the transformer. Some of them do hum and the only way to stop it for sure is replace the transformer. You can make sure that is it by removing the transformer mounts and letting it dangle to see if that stops it. Do not leave it that way. Hope this helps. GFM

I disconnected all of the wiring in the 24V circuit including the blower, I ohm'd everything out but could not find any shorts. I put all the connections back to the 24V circuit and now everything is running fine.

Recently have furnace issues, absolutely no power to the circuit board, so I figured it was the transformer. My question is could the electric company working outside on lines connected to my house be the culprit? Silly question I know but I don’t believe in coincidences. By the way love your videos

this video is for testing not replacing correct?

Question following. I have watched your lessons and very much appreciate the guidance. Lessons are well said and easy to understand. I have a Rheem Prestige Super 80 - 2 Stage - installed 2008, just outside of warranty. I used your suggestion and put a screwdriver against the transformer to listen - transformer is humming loudly to regular hearing too - used the screw driver to isolated the sound- transformer is very warm almost hot to the touch. The furnace works, then does not sometimes. Unplugging and letting it wait half and hour, then start and works for a few days. At trying to light again the two green lights are on but the amber does not - then later it may kick in with amber light on solid. Where is the ground for this? Ideas? Start with Transformer then work from there? It is the transformer on my left - # 46-24124-03. If you have someone near Moreno Valley, CA, you like fine, but have had issues in the area with several companies. At minute 5:55 you say pull the contact - do you mean if the transformer stops making noise it was that contact? Then the whole circuit board needs to be replaced? Any ideas are appreciated for this furnace issue. PS - many years ago in high school the electronics teacher said house ground was to return electricity and your explanation is so clear it is not.

This guy is great but if he talks any slower my brain is going to explode

but once again this just happen on the HEAT MODE. when is in cooling mode is just fine all the time.

Looking at a 30-40 yr-old RHEEMS forced air unit. Blower motor works but every wire-nut popped, and the fan control (with the 8" sensor wand going to the plenum) wire that stabs in left side top burnt (primary voltage, I believe)..both wire nuts with 3 wires ea. on the 120v. in to the transformer fried,,transf. is not outputting nothing, ohms across 120 v. input wires=64 and secondary is 10 ohms..wire-nuts to the fan-motor fried (I replaced the motor 3 years ago). I stabbed a solid wire in the fan-control hole that burnt and wire-nutted it to the old wire, got no change, i.e., the main gas opens and fires when you plug in the transformer with wall control ON but when the sensor is satisfied, it shuts down and no blower-motor happening. I just bought a trans., but am loathe to install&burn that one , too, without discovering the root cause..which would be what?? (..do u think) thanks, cris PS:the guys at the real repairs place only asked me a bunch of pesky questions but would not discuss matters, leaving me with a lot of doubts as to my personal qualifications to be doing this..A-hem!

I said it has hummed

keep smacking... moving on

nice vid but it took you almost 9 min. to say what?

Slurp slurp

Good job. But, just being critical, you tend to make lip smacking or teeth sucking noises, alot.

Jesus Christ dude spit it out

this guy puts me to sleep he is a terrible public speaker