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.

"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.


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



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