Goodman HVAC system control problem- blown transformer

[dm440c]

Hi, we have a 4 year old house and the upstairs AC thermostat suddenly lost it's display. I switched the thermostat with the downstairs one and the problem stayed with the upstairs unit. I received a tip that the 240v to 24v transformer might be the problem, so I went up to the attic where the air handler is and tested the leads on the transformer... sure enough, 240v in is OK but nothing is coming out on the 24v side. So, I got a replacement transformer and wired things back up. The display came back on, the unit switched on and started blowing through the vents and I thought i had fixed it.

BUT about 5 minutes later i went up to check on it and the display had gone back out, so something is tearing up the transformers. I'm not an electrical engineer, but I can't figure out what would burn up a transformer without tripping the main breaker. Any ideas?

[Don Young]

Defective transformer, transformer voltage or current improperly rated for the service, mis-connected, or overloaded. These transformers do not present enough load, even when the secondary is shorted, to trip the main breaker.

Try another transformer, making sure it is for 240 volts and you have it so connected. Some transformers have different leads for different voltages. Also make sure the VA or current rating is at least what the original transformer was. Monitor it carefully and if it seems to be overheating check the secondary current to determine why it is overloaded.

Don Young

[Rick-Meister]

Well, in most cases I'd wonder whether the transformer was the proper VA rating. But in your case, I'm guessing you have a short between the transformer and the upstairs T stat. That will blow a transformer without tripping the breaker.

[Moe Jones]

When you replace the transformer again this time wire a inline fuse from the transformer so when it shorts out again it will blow the fuse and not destroy the transformer. Where you buy the transformer they should be able to tell you what size fuse you will need. Then you can start to find what is blowing your fuse. It does sound like when the A/C starts it blows then I would check the wiring going to your A/C outdoors. You wouldn't believe how many times I have seen a weed wackier had cut into the control wiring. Good Luck.

[Stormin Mormon]

Yes, I do. Check the low voltage wiring. Also, put a fuse in, so you're changing fuses not transformers.

[John Gilmer]

"Some?" Try ALL.

He should:

1) Ensure that the "next" transformer is nameplated for 240 (or whatever the air handler is wired for) volts input or if it's dual voltage that he uses the right wires and tapes off the unused wire(s). Many air handlers use both 120 and 240 volts so he should determine which voltage the transformer sees. It's quite common to use 120 volt fans even when all the heaters are for 240 volts. If the fan motor is 120 volts, it's likely that the control transformer is also 120 volts but nothing is certain. 2) Install a fuse on the transformer output rated for 80% of the nameplace output current of the transformer.

Clearly "something is wrong" here.

The clamp on amp meter work just as well at 24 volts as they do at line voltages. That might be a start.

It's quite possible that the transformer is underrated. It might have enough juice to power the fan relay and the changover relay and the compressor contactor. But if it's an especially COLD day and the system calls for "suplemental" heat, it may have the energy to power the thermal relays that bring on the heat strips. The supplement heat strips and "sequenced" and the load (including the load on the 24 circuit) increases in steps. If there is a short on the input of the last thermal relay in the sequence you would never know unless the demand for supplement heat lasts long enough to sent 24 volts to that last relay.

But I would definitely put a fuse on the 24 volt circuit while figuring things out. It's better to replace a $.50 fuse than a $25 transformer.

** Posted from **

[Tony Hwang]

Hi, Checked fuse on 24V AC control voltage?

[Bubba]

You really dont have a clue, do you Don? This happened in the A/C mode and the guy has a short........a simple short. Finding it is the fun part. Bubba