How to troubleshoot the transformer 

Just started school for hvac. Awesome to have your videos to help me understand what I learned in class

HVAC student here about to graduate. i have been obsessed with your videos for hours now. great videos

Thanks. The ohms test helped me diagnose mine and determine I had a high voltage issue. New transformer installed and my a/c was back to work.

Thank you for this lesson. Good job on explaining the troubleshooting process.

Great video. Thanks. I was getting no voltage out, but wanted to verify it was the transformer. My primary had failed, like yours

Great job,I thought your video was very easy to understand and follow. Now I'm not afraid to check mine because you gave me knowledge, Thanks

Truly excellent videos, grayfurnaceman! I have an older RGDC Rheen gas furnace from the 1980's (I'm in SoCal so furnace literally only runs 10-20 hours per year). While cleaning the burners last week, I broke the tiny tube on the mercury pilot switch that is part of the ignition module (SP710A). Of course those switches are no longer made so I installed a modernization unit from Robertshaw. All seemed well but then the transformer failed on the 24V side. Based on your vids I investigated all wiring and connections. I replaced the fan unit (only $40 for the unit including transformer and relay that bolts to the 4x4), and I also installed a 5amp tiny circuit breaker on the hot (red) wire as it comes out of the secondary side of the transformer. I can only speculate that the wiring shorted against the chassis - because all I did was re-do all wire nuts related to the new ignition module including addition of electrical tape. I verified about 26V voltage and about 0.6amp current. Your logical approach to troubleshooting helped me to think about these things and the furnace is working nominally now!

I’ve learned so much from your videos! Thanks

Opps, I did not follow up on that one. Thanks for the reminder. I will be doing that one probably tonight. It will be also located on the "control transformers" playlist. GFM

Generally, transformers fail from shorts in the system or overloads from things like contactor armatures sticking. The transformer itself is very dependable. GFM

Take a look at "How to replace a transformer without blowing up the new one". And you are right, transformers seldom fail. Thanks for the comment. GFM

thank you for your expertise...

I like your videos a lot,you get two thumbs up in my book young lad :)

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

Thanks for the video!

our furnace a n.g Goodman has failed twice this winter. The furnace is 4 years old. I was getting funky voltages at the thermostat so I check the transformer and noticed what appeared to be a cut wire on the boot where it attaches to the secondary of the trans. , so I put a new boot crimp it and put it back on trans. and it worked for about 8 weeks and then happened again this time to the other side of the trans so I repeat and the furnace is currently working but in the back of my mind I'm concerned bcuz I didn't notice any heating of the wire and it was a nice clean shear so I don't think slight vibration would cause this but can I get some professional opinions out there, I'm curious, thanks.

Usually a short in the system if you find a blown transformer and some shorts can be a pain to locate

If you are testing with the ohmmeter, the finger will not upset the readings. GFM

Also Gray I had a question for you is their any way to test Transformers that are attached to a pcb board? my question is I was replacing aboard in a Monitor heating unit kind of like a rinnia space heater. I replaced the board left got a call back still didn't work. Felt the transformer in the newly installed board and it was super hot. Is their some way you can test electrical components on PCB boards etc... so you don't have to swap out to check if the board is bad?

they also have fuses under that tape worth check be for scraping it

Connecting primary to 120VAC... No load on secondary... Aren't you creating a dead short? Or, is the magnetic field the load at this point? Thx.

Hey chief! I have a question if you touch with your finger while you checking?is it gonna give you a wrong reading ? That's why I can't do my testing.

liked the video .... you mentioned a followup video, so as not to keep blowing transformers.

how can we calculate the capacity of a transformer? Like 750 Watts or 1000 watts, i am asking this because i bought a transformer from a place for my Power inverter. now the issue is the shop i bought it from claims its a 1000Watts transformer but i checked it with two other person they say no its a 750 Watt transformer. Now how can i calculate and be sure?

This video will eliminate lots of service call. Great job.

Thanks for the "young lad"

OL is over limit on resistance scale. Infinite resistance air or open circuit. At least that was my experience when using a meter back in the 70s.

What’s that white board in the back called the one with the holes

what should the primary side have for resistance if it is working? I've tested my transformer and it has 2ohms resistance on the secondary side and 25.5ohms on the primary. Thanks

As transformers seldom fail, I would guess there is a problem with the board that is causing a short. If the board changes the low voltage power to DC, the diodes in the rectifier could have shorted. You can look for bad solder joints on the components if the board just does not work, but if it is shorted you would need knowledge of semicouductors. Hope this helps. GFM

I recently replaced the contactor on my goodman heat pump/ A/C out side unit, the contacts were melted together, I replaced the contactor and the heat pump worked again while the weather has warmed the A/C wasn't coming on after checking many possibilities I gave the transformer a smack with a screw driver and it's started working again, hopefully for good, not sure if that was the cause or I just wiggled the right wire some where

I have a goodman 24 volt transformer I replaced. I disconnected the old one but on the new one on the output (secondary side) it doesn't tell me which tab connector is hot and which is common. The 120 side is clearly marked. Is there any way to figure this out? I'm afraid I'll reverse the polarity and cause a problem.

I have humidifier's transformer burning out in few months of running. The water valve solenoid draws 1.2 amps at 24 VA it's 29 watts. Transformer is 40 watts. The voltage to primary winding is connected (as installed by technician) to blower connector; one neutral and L1 (120 volts)so valves opens when blower operates. Any suggestions why this happens, maybe spike from blower's capacitor, any sugestions

hello, love your videos, you've taught me alot which I've applied in the field.. can you make a video about ghost voltage? how and why does ghost voltage occur. appreciate you alot. thanks very much!!

Excellent video! Thanks for posting this! I have a question: what is the acceptable voltage range coming out of the transformer? I hear people in other videos talking about 27 volts. My thinking was as long as it is at or above 24v it should be ok. Can you chime in on it please?? Thanks!

It goes without saying, but I'm not a technician.. I do however manage to fix more things I try to work on than not, so while at my grandfather's house on Thanksgiving I thought I'd try to figure out why his heat works intermittently. Older model Tappan by the way. I was told a few good wacks on the side of the air handler usually gets it going. Obviously points to a dying relay of some sort - I do know that much. I found a relay but as I'm looking over and thinking through how this thing works I try turning it on with the fan "on" instead of in "auto". It works fine, instantly when you switch to "on" every time. It's a single relay, obviously it isn't bad - I think. As they say, if you reach for a bigger hammer and that doesn't work, your problem's electrical. Onward. I pulled the leads that are coming from the transformer I found and get about 26vac when the fan is set to "on" but only about half that when in "auto". This is where I dead end. The fact it's about half the voltage isn't lost on me but I can't work out why, or if that's just a red herring of sorts I should just ignore. 13 vac just isn't enough juice to energize the 24V switch is my conclusion I'm confident of. Any thoughts for further troubleshooting and potential home based repair? Note, he lives in the middle of nowhere, which is why I'm not just running up to the local hvac shop and picking up a new transformer before I head home. Thanks

Hey Gray, i asked a question awhile back about the only problem I couldn't figure out. I had 120v in the primary, but zero coming from the secondary. I figured it was the transformer and I replaced it. I put in a new transformer, but the same issue. I checked resistance and transformer was fine. It ended up being an eletrical issue elsehwere that caused it

Can't find anything on testing for mA AC. Specifically,a transformer with an output of 15v AC/500mA AC rating.

I have a York 025-28671-000 HVAC Transformer. Should the transformer be hot to the touch of the finger when I have all breakers turned on but the ac unit turned off?

You know what I have imo a really great service were I found a broken transformer. Let me share this with you gray. Service call for a heating system the homeowner said the system came on so loud it would vibrate scream etc... so badly that the neighbors could hear it. Went and look at the system had no power on the low voltage side. It was coming in but not relaying 120 to the 24 volt side turns out the transformer actually shook off the board! making it work intermittently it broke the sauder

Great vid but it would be helpful to put a link to the next video either on the screen or in the comments

Cool story! GFM

Hello my Ducane furnace operates normally except it has one problem I'm concerned about. When I plug it in I hear a faint electrical humming sound, the led light also stays on. Is this normal ? or would I have to change out the transformer?

GrayfurnacemanI burned 2 transformers , and all the time it happens on primary side (120vac) when I found the short on low voltage side . I did all the research but nobody came with definate answere . Would you please explain the theory of transformer?

So the Transformer is not showing any power at all on the 120. so i started to look at the capacitor so could that cause the main feed to not reach the transformer if it had failed?

My problem is high voltage. I have 247 volts coming in and am putting out 29.6 volts. I am having too rapid failure of sequencers as a result.

That piece in the beginning of the video that was what we were looking for. Looked like a voltage converter and not a transformer.

does the Vo of the Transformer always read 24V in order to turn the unit on? what if it reads only 22v?

Can a faulty transformer cause humming and vibrating in the house?

Is it possible that the 24v would be intermittent? I mean T stat is on but as soon as it kicks either AC or heat it will go out (AC, fan, heater fan and t stat goes blank) but it 4:57 comes right back.

Hello, i want to ask. My secondary transformer continuity is 0. Is that mean my secondary transformer is broken?. And my transformer is step down( 240 to 24v), when I check the output voltage. The voltage that I get is 32V. Its normal?

To check my 24 volt out put what do i set my meter on volts or AC volts thanks

I accidentally found out that the ground wire in one of the switches that was causing GFI tripped is hot. What is the cause of it?

Hello, having a problem in cooling mode for a Heil AC, the inducer fan and blower fan are running at the same time but the condenser doesn’t not kick on. Thought it was the contactor but found this problem after. Any idea.. If I push the plunger in condenser fan and compressor comes on

I have a 24v transformer but the output was only 20v,,it was the reason why transformer tripping off

So when you checked the two wires going to the transformer when it was shut off that was suppose to be 120?

What if I DO have continuity on both the primary and secondary side but when I hook the thing up I still get no voltage on the secondary side? Clean 121v on the primary side... I've gone through ur whole series and I'm still at a loss. I'm on transformer #3... I have no continuity on on relay side (with transformer unhooked) in the open position (like it should be) and now I have 3 seemingly dead transformers so I can't test for amperage on my equipment side either.... I'm mostly just ranting... This is frustrating. Thanks for the help u did provide....

Could a transformer work intermittently or does it fail completely? Seems I'm getting intermittent power to thermostat. When I test transformer contacts (where thermostat wires are attached) I get no reading. I open the box the transformer is mounted on (so jiggling the transformer), reinstall and get power to thermostat - off and on, but (fingers crossed) on for now.

Also what does it mean if I plug in the common and hot line then both of them are reading 120v off the ground? I’m assuming that it means something inside the transformer made a bad connection

Good video slurp slurp

MY dad has an old electric furnace and the blower motor stays on all the time. I unhooked the thermostat wire still runs. it doesnt have a circuit board that i see. the only thing i see is a transformer. What could keep it running all the time?

Wires from the low voltage touching together would short the transformer out . Thats most cases right . But I bet there are others too ,like it just getting old . One thing I found out is if they get real hot , something is wrong from the start .

Is the 24 volt output end polarized?

What is the problem if I have 2 identical units and both transformer read 24 volts at the secondary on unit #1 but both read zero volt on unit #2. Both units has 120 volts going to the primary of the transformer. Any idea?

My gas heater isn't working, my transformer is only reading just under 11 volts coming out. Does this mean my transformer is bad or is it likely something else? Thanks for the video

Great video! I am 99.9% sure my transformer is bad. I'm in s. Texas and was using an inverter to power the furnace during the storm. In the middle of the night the blower motor would power cycle at very low rpms. In the dark I accidentally shorted the switch receptacle (what i used to power it). Now if I put 110 direct to the motor it doesn't power cycle but VERY low rpms. Control board and thermostat is not connected. Am I looking at a transformer and a blower motor and maybe a control board?

When I test my transformer, when it is not connected, the meter reads resistance on the high side and the low side. Like 18 ohms on the high side and 1.6 ohms on the low side. The circuit board has no power when I connect the transformer. I've got 120 v at the lines that go into the circuit board. And 120 v coming out of the board through the wires that go to the high side of the transformer when I push the door switch, but no voltage coming out the low side of the transformer. (And no red light on the board. Nothing works) So is the transformer bad, even though there is resistance (continuity) when the transformer is not connected?

Can a transformer go bad as in trying to output more power than it suppsoe to? The Fuse on my circuit keeps blowing. Somtimes it will turn on but if i unplug it and plug it back in the fuse blows instantly. Nothing on my ciruit board looks shorted, so i'm pretty sure its the transformer. From what i know is the transformer intakes ac and converts it to dc. It has three wires on the dc side and i have no idea how to test it. I just cant firgure out why somtimes it works but then goes out after i unplug it and plug it back in. Please help

It’s toast🔥🍞

Got one from China ..complete Dud . think they would test before???

GFM, is there anyway to ask some questions. I am perplexed with a couple of issues at work.

I am dealing with a Carrier RTU that had a bad inducer motor. Turned disconnect off several times in the process of replacing motor. WiFi t-stat of course loses connection when disconnect is turned off but comes back on within a minute or so after re-energizing disconnect. 3rd time turning disconnect off and then back on however and WiFi signal never did reconnect. Modem is fine and working for other units just not connecting on this one. I checked transformer primary and secondary voltages and while primary is good to go, my secondary is reading 22 volts under no load. This is a 208/230 system running at 208 volts. Have not been able to get back to it today due to constant rain. This transformer btw, has a built in circuit breaker protection and it is not tripped. I am wondering if turning disconnect off and back on several times had something to do with transformer now only putting out 22 volts with no load even on it and also loss of wifi stat connection to unit.

Plz full connection transfer

thanks for this video - it's pretty funny, but i couldn't figure out what i did wrong because the furnace wouldn't fire up...didn't have the door closed so it wasn't powering up (doh)

TLDR: If you notice electrical items dangling from the rafters in your attic that do nothing, go ahead and disconnect them asap because they could possibly do bad things. And it seems I'm also building things with dud parts. D'oh. We had one of those transformers pretty much exactly like that one wired up in the attic for a doorbell that had been removed in probably the mid to late 70's... At some point in the last decade, the old low voltage doorbell wiring that was strung all over the place shorted, and apparently the transformer got so hot it melted the primary into a dead short, which then melted almost the entire length of its dedicated 14 gauge wire to the breaker box before it finally went open. How the house didn't burn, I have /no/ clue. Meanwhile, I'm working on a homebrew thermostat setup (12ga power cord, thermostat module, 24v/40va transformer, 40A contactor for overkill, 20A outlet, all squeezed into a project box) to use on a window a/c, space heater, turn a deep freeze into a fridge, etc. It works great, but the practically brand new transformer gets hot enough with no load that I can't hold my fingers to it for more than a few seconds, and it shows 20 ohms on the primary... Pretty sure I'm going to need to get another transformer, wouldn't you think? :p

Why do you only get 12 volts when reading your 24 volts to ground?

The problem is that someone let the smoke out of that transformer

Putting wires "here" testing "there" ifc...

give 120 and coming 24v

(: thanks

I bet its one of those fkin hampton bay transformer

Fake news