On Wednesday, June 21, 2017 at 7:15:34 PM UTC-4, wrote:
On Friday, April 8, 2011 at 6:52:06 PM UTC-6, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing on my
Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of the wiring
diagrams that I also copied to my above flickr site as jpg images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf

As you can probably see in the pictures, there is visible charring of the 115V
leads going into the transformer, and of course the 115V circuit is open
(again). If you didn't see my first thread, this is the third transformer the
unit has blown. In the previous discussion, it was discussed that perhaps the
first one just blew because of old age (6 years), and the second blew because
it wasn't a proper replacement (poor quality, made in China, etc.). This third
unit is most certainly a proper replacement, and it's most certainly indicative
of a real problem I have somewhere else in the unit. I didn't see any such
charring on the previous two units, at least not like this.

I never got a real chance to test out the system after installing this third
transformer. We had cool weather for several days, and I never tried to force
the system to come on so I could monitor it; that was probably a mistake.
Unfortunately, I was also absent from the premises during the extended times
when the unit was most likely operational, so that didn't help either.
However, my family tells me that it WAS working and cooling the house rather
nicely, for at least a day, perhaps two. I'm getting 115V in all the right
places, so it doesn't look like an over-voltage condition to me. Perhaps it's
an overheating condition? It looks to me like the only real load on this
circuit is the blower motor; could the motor be causing this? The blower spins
freely when I turn it by hand. Start capacitor on the motor maybe? Relay on
the control board perhaps?

Our XR90 furnace did the same thing, I installed a 50W transformer instead of the stock 40W just in case it was underrated. I measured the operating output current during cool or heat and it maxes at .6 amps when 2.0 would be the limit. I cleaned up all the sloppy wiring from the installation and so far no blown fuses but with such an intermittent problem you never feel like you got it for sure. The only problem I could find was during heat call the red light would blink for 3 times indicating a problem with the safety pressure switch (enough gas pressure?). I temporarily bypassed the switch then it would latch and the heat would work fine. The only thing I can figure is that it the unit tries many times to turn on the heat and the pressure switch never makes the board commits suicide and deliberately blows the fuse so that you have to call a service man???? Sounds crazy but I wonder. I'll replace the pressure switch and see if the problem ever returns. For now I have a bunch of 2 amp (instead of 5A) fuses on hand to keep from blowing the transformer again. From the many web posts this seems like a common problem.

so i know this is an old thread but i will add this

I've had 24V xformer burn out and it was due to a shorted wire.

There is a lot of vibration in HVAC equipment and if there is a wire touching
a bracket or anything, it may eventually wear through the insulation and
create a short, often these are intermittent and hard to see. The short blows the xformer.

I was lucky to spot the bad wire in my case. I repaired the short, installed a new xformer and added a PTC fuse device in series with the secondary just in case. A regular fuse rated for an amp or so above the xformer rating is a good idea. Also tie wrapped all the loose wires so none were rubbing on anything.
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