DIYbanter - View Single Post - Why does the 115V-24V transformer keep blowing on my Trane XB80?

The Daring Dufas[_7_]

On 4/3/2011 8:18 AM, wrote:
On Apr 2, 3:45 pm, "A. wrote:
On Sat, 02 Apr 2011 14:10:01 -0500, Steve Turner wrote:
A couple of months ago the heater quit coming on in my upstairs Trane
XB80 HVAC unit. It's been off warranty for about 2 years, and I am
enough of a do-it-yourselfer that I'm comfortable with simple repairs.
I found that the LED on the control board was not lighting up, and
subsequently found that I was not getting 24V output from the 115V-24V
transformer. Sure enough, the 24V circuit was open, so I found a local
replacement transformer (not the exact form factor, but same specs) and
things worked fine after I replaced it. Now, I have the very same
problem (only this time it's the A/C that's trying to come on; not that
it matters), and once again 24V is missing on the output side of the
transformer. This time the 24V circuit has continuity, but the 115V
circuit is open. Of course, I can replace the transformer AGAIN, but
I'd sure like to know what could be causing this problem... Any ideas
would be appreciated. Thanks.

Probably should measure the power consumption and see if it is in or out
of spec with the transformer if you can find the specs. If it's in watts
the formula is volts x amps. 24 volts x 500 miliamps = 12 watts and so
forth. If the draw is higher than the specs of the transformer then
something on the 24 volt circuit is drawing too much current. You might
be able to spec the transformer by calling Trane.

We have a winner folks. My vote for the best answer. Instead of
speculating
on transformers, I'd put an amp meter on the transformer and find out
what
the load really is. It's not going to do any good changing
transformers if there
is a failing component somewhere else that is drawing a large
current.. Or
if the load side has been overloaded by wiring something else into it
that
should not be.

You do know that the common lead (often blue colored) for the 24volts is
usually grounded to the metal cabinet of the air handler. A bit of
insulation skinned off a thermostat wire could cause a short circuit
when the bare copper comes in contact with the air handler cabinet.

TDD