Air Conditioning - Transformer blown and then blown again

Hi.

So, our AC went out yesterday, and I traced it down to the 120v->24v transformer. 120v in, 0v out ... replaced it, and the system fired right up ... for about 30 seconds and then the new transformer blew.

It still surprises me that nothing seems to be fused besides just the outside circuit breaker ...

So, what do I check next, or am I stuck with calling in the experts?

Thanks,
Sam

Try try again----- I think Id put a new transformer back on. Turn the power to it on. BUT DONT TIE the 24 V wires to it. See if the 24V wires are bad or is it the tstat ????? That could have a short in it.

ED

To save the cost of transformers, I like to install a small (3-5 amp) automotive fuse in the power output line from the transformer. Fuses are a lot cheaper than transformers & groups easier to replace. You can get a small in-line fuse holder from any auto parts store.

To save the cost of transformers, I like to install a small (3-5 amp) automotive fuse in the power output line from the transformer. Fuses are a lot cheaper than transformers & groups easier to replace. You can get a small in-line fuse holder from any auto parts store.

I wondered about adding a fuse, but expected to put it on the 120v side ... since it's 24v/40a on the inside, that would equate to a 8a on the 120v side ... I'm not sure what it *should* draw through it (don't see that spec on the docs inside the panel), but thought about trying a 5a fuse on the hot lead on the 120v side.

Does that sound about right?

Thanks,
Sam

No Like said put the small new fuse on the out or low side of the transformer.

ED

Just wanted to finally followup the the apperent resolution.

I ended up having a pro install a fused transformer (it was actually faster then waiting for greybar to open the next day), and it blew the fuse ...

He replaced the fuse, reconnected all of the wiring connections, and it *seemed* to work, so he left. (tihs was Friday)

As it turned out, I woke up Saturday early morning, and the system fan was *still* on ... I was only able to turn it off at the system cutoff switch (was ignoring the t-stat).

He came back out on Monday and talked with the guys at some parts distribution shop and they determined the controller board was toast ... he drove down, got the board, and replaced it. Reseated all connections, and it *seemed* to work, and went home.

Nice feature: the new board has an onboard 5a fuse! And it's gotten some good exercise.

By Monday night, the fuse was blowing shortly after the system started up to A/C.

Rather than keep paying the pro for debugging time, he was kind enough to give me some tips (I am able to test for voltage and all of that).

Here's what I ended up doing:

* pulled the t-stat from the wall, removed the wiring from the t-stat base, and inspected to eliminate that unit as a source of a short. I even tried just jumpering the wires with aligator clips to start/stop the A/C ... kept blowing fuses. Left the wires capped off at the t-stat location and went downstairs.

* pulled the t-stat wiring from the connections to the control board and jumppered from there with aligator clips ... same results, so it wasn't the t-stat wiriring.

* the fuse on the controller board was the regular sized automotive 5a blade style ($1.58 for a 5 pack at walmart ... bought several packs), so I picked up a compatable wiring kit from Walmart, so I could extrude the fuse from the board and get my ampmeter cliped around the loop. Sure enough, shortly after starting the A/C up, the fuse would go thermonucular.

* with ampmeter in place, hotwired just the fan ... no blown fuse ... let it run overnight (a lot longer than I had planned, but the in-laws went to bed, cutting off my access :) )

* so, at this point, I had determined it was only blowing when the AC was enabled ... I moved the ampmeter to the leads to the outdoor unit and fired it up ... sure enough when it went to fire up the outdoor unit, a huge spike came through the line and blew the board fuse. I wanted to make sure it wasn't the new board that had a short on it, hence this test.

At this point, I wasn't sure if it was the outdoor unit (just over 5 years old, out of warrentee and undersized for our house, apperently ... I didn't install it) or what. I started tracing the control wire and got lucky.

Shortly after the control wire had gone horizontal on it's way out to the outdoor unit, it had been caught on a piece of tin straping holding the wire and the various piping it was wrapped around to the duct above it ... I'm surprised it lasted 5 year, and suspect that *somewhere* before we moved in it had been messed with because the jacket had been cut into by the tin strapping and was causing a short. I am suspecting that it wasn't fully cut into initially, or it would be shorting from day 1 ... but maybe compressed and over time, it melted through the insulation layers.

I had some slack I could work back to that spot and tried to splice the connection to see if that was the only root cause, but the individual insulation jackets had melted together at least 4" in both directions of the short! Home Depot didn't have the same white cable, so I just got some t-stat cable (seems like the same except for the jacket may not be outside rated, but the cable is never actually exposed, it runs through a piece of copper pipe into the A/C unit).

$10 worth of cable later, the unit was ready to run and we had a mild weekend ... didn't get used!

But, it's run flawlessly since then, and I've removed my fuse debugging loop and buttoned everything back up in the unit.

So, the moral of the story is that a short *can* happen without any direct event .... over time. (still surprises me).

Sorry for the long post, but was a long and comical road (if you consider sleeping in 85F overnight heat comical), but you *can* help your pro's if you can do basic debugging technique and just break down the problem.

Thanks,
Sam

Glade you found it. Around here the chipmunks like to eat the insulation off the 24V wire to the units. Can that give you a fit to find where that short is. Like yours it will short just sonetimes.

ED;)

Sounds like you need a new tech.

They can be a bear to find sometimes. I used to have a problem every spring with the wires from the air handler (in the attic) to the condensing unit until I installed armored cable. When I get metal munching mice I'm going to move.