# Blown Transformer



## A Little Short (Nov 11, 2010)

What would cause a 240V/24V transformer to blow in a air handler for a heat pump?

I'm going to break the 2 sentence rule here to add the details.

I'm not an HVAC guy but I was asked to look at the thermostat wiring for a heat pump. The customer tried to replace his thermostat and thought he might have broken a wire when changing it. So I agreed to check that for him.

The wire was good so I thought I would look in the manual for the new stat to see if he had it wired right and programmed. First thing I saw was it said the stat was not for a heat pump. So I picked up a new one that was for a heat pump.

I installed it and turned the power on and nothing worked. So I went to the air handler to see if there was a fuse blown or anything obvious. I checked the secondary side of the transformer and didn't have the 24V.
I then checked the primary side and didn't have the 240V either.

The AH had two 60A feed through breakers and one of them was bad. So I picked up a new one and installed it. Still nothing worked, so I checked the secondary side again and still no 24V.

I figured the transformer probably had went out while the guy was trying to get the stat to work. He said he had been swapping wires around. Anyways, I picked up a new transformer and installed it. Still nothing worked. I checked the primary and secondary. I had the 240V, but no 24V. 
I thought maybe I got a bad transformer so I took it back and swapped it.

I checked it this time with no loads connected. I had the 24V. I then went back to the stat and checked the wiring according to the manual and everything looked correct.
I hooked everything back up and still nothing.
Checked the tranny again, no 24V! 

It blew the transformer again.
I told the guy to call an HVAC guy and I wished I had told him to do that to start with. If I hadn't known the guy I would have passed on the job to start with.

Any ideas on what is blowing the transformer?


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## HARRY304E (Sep 15, 2010)

> The customer tried to replace his thermostat


That will do it,,,,,,next!:laughing:


Most likely there is a splice in the tstat cable hidden so when anyone changes the t stat it will short the transformer,and it worked now they will call the HVAC GUY and he will take the money..


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## bkmichael65 (Mar 25, 2013)

If it's blowing as soon as you put a load on it, it's most likely a direct short. It could be wires switched around or making contact at the t stat or if there is a control board in the air handler, sometimes they go bad and short out there


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## A Little Short (Nov 11, 2010)

bkmichael65 said:


> If it's blowing as soon as you put a load on it, it's most likely a direct short. It could be wires switched around or making contact at the t stat or if there is a control board in the air handler, sometimes they go bad and short out there


The wires might be switched around at the stat but I made sure none were touching. There were two unused wires and I made sure they weren't connected in the air handler either.

I didn't see a control board in the AH. I didn't even look at the outside condensing unit, there probably is a control board there.


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## bkmichael65 (Mar 25, 2013)

Little-Lectric said:


> The wires might be switched around at the stat but I made sure none were touching. There were two unused wires and I made sure they weren't connected in the air handler either.
> 
> I didn't see a control board in the AH. I didn't even look at the outside condensing unit, there probably is a control board there.


Some HVAC guys use all different color schemes on their wires and that can lead to some confusion. Usually the common is blue, sometimes black, sometimes ??? It can be real easy to switch things up if you didn't see what way they were hooked up to begin with


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## A Little Short (Nov 11, 2010)

bkmichael65 said:


> Some HVAC guys use all different color schemes on their wires and that can lead to some confusion. Usually the common is blue, sometimes black, sometimes ??? It can be real easy to switch things up if you didn't see what way they were hooked up to begin with


Yep, I figured it would be real easy to mix them up if you don't know what they are supposed to be tied to from the stat to the other units.

Unfortunately, I didn't see where they were connected on the old stat.
I asked the guy if he knew but he didn't.
He said he saw they were color coded and saw the colors listed on the new stat so he just took them off without writing them down.:no:

Oh and he also said he was swapping the wires around to see what each one fed and the unit kicked on ONCE, and that was it!:laughing:

That's probably what blew/shorted everything to begin with.


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## dmxtothemax (Jun 15, 2010)

If the tranny works ok with no load,
But not with the load, 
then the load is faulty or wrongly wired !
Check for incorrect wiring by getting a wiring diagram for the unit,
And also check that the tranny,s wires are correct also !


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## A Little Short (Nov 11, 2010)

dmxtothemax said:


> If the tranny works ok with no load,
> But not with the load,
> then the load is faulty or wrongly wired !
> Check for incorrect wiring by getting a wiring diagram for the unit,
> And also check that the tranny,s wires are correct also !


I've already "threw in the towel" on this and told him to get an HVAC guy.
I did asked him to tell me what he finds though!:thumbsup:


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## LARMGUY (Aug 22, 2010)

bkmichael65 said:


> Some HVAC guys use all different color schemes on their wires and that can lead to some confusion. Usually the common is blue, sometimes black, sometimes ??? It can be real easy to switch things up if you didn't see what way they were hooked up to begin with


Just common sense here from a service tech. There is always a reason for electric stuff not working. Sometimes a combination. Sometimes it is random and intermittent. Here, you have your reason, the transformer died because he swapped wires also possibly letting the magic smoke out of a board or two. 

So what is their reason for mixing up the colors given they have instructions that come with the air handlers and T stats? 

Old bad habits? 

Colorblindness?

Screwtheguybehinditus?

Wannabe lecktrishuns?


Wish I could invent a magic smoke repacking tool. I'd be rich!


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## buddhakii (Jan 13, 2011)

You say you checked new trny and had 24v, then hooked everything up and went back and checked and you had 0v. Did you happen to check the primary on the tranny at this point? Maybe there is a bad connection somewhere on the line side coming into the heat pump.


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## A Little Short (Nov 11, 2010)

buddhakii said:


> You say you checked new trny and had 24v, then hooked everything up and went back and checked and you had 0v. Did you happen to check the primary on the tranny at this point? Maybe there is a bad connection somewhere on the line side coming into the heat pump.


Still had the primary voltage.


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## CADPoint (Jul 5, 2007)

The front door bell is jammed! :001_huh:



> ... I didn't even look at the outside condensing unit, there probably is a control board there.


And more wires...


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## don_resqcapt19 (Jul 18, 2010)

Many Class 2 power supply transformers have an internal non-replaceable fuse that will blow if the secondary is short circuited. 
(you can cut the outer paper off and find the fuse and solder an new one in, but not worth the time or trouble)


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