Control Power In A DGAA07BDTA

[Mike S]

Hey guys,

Bought my place in December and doing a major reno. Not living there at the moment, thank God (as it is only about 15 deg F out now.

The furnace was working ok (minus short cycles, but that is the not the problem I writing about.)

So sunday I ripped out a window to replace it, but ran out of time, so I threw some vapor barrier over the hole for the night, and when I came back the next day, my furnace wasn't running.

Broke out the multi-meter, and found that I had line power going to transformer, but no control power coming out. Great..problem solved.

Bought a replacement, threw it in, and blamo...heat glorious heat. BUT it ran over the set point of the thermostat. I couldn't get it to shut off, and when I finally did get it shut off, it wouldn't fire up again.

Took the multi-meter out again, and found....No control power again! Line power is ok, but no 24 VAC.

I have read somewhere that transformers don't just die. If you find one has gone, troubleshoot further to figure out what is source of the problem. So my question is, before I go and throw in another $40 transformer, what is killing them?

Thanks in advance for the help,
Mike

[Robert]

OK, first let me give you the best kept secret in HVAC for homeowners.

ANYTIME you replace a Transformer, get an Automotive type fuse holder and an ATC 3 amp fuse.

Install it in the hot wire on 24V secondary side of xfrmr.

It prevents ANY future xfrmr blowouts from low voltage shorts.

With that said, you have a short on the low voltage side.

Check ALL low voltage wires to see if any are touching neutral wires (bare) or touching metal.

Look closely at where the t-stat wires enter the furnace through the side of cabinet.

Something ~may~ have gotten in and chewed on a wire also.

Test for 24vac from the neutral side wire and a good ground.

Take care and best wishes,
Robert

[Mike S]

Robert,

It worked. Many thanks, and my apologies for not posting sooner, but I only have internet access at work right now.

The short was in the T-Stat wiring. Because I am in the middle of a reno, some of the t-stat wiring was laying on the ground, and I accidentally stepped on it and crushed it into the threads of a screw.

Also found somebody had replaced the "No More than 3 amp fuse" on the control board with a 7.5 amp. I probably wouldn't have burnt out the transformer if the proper fuse was installed. :S

Anyhow, thanks again for pointing me in the right direction.

Mike

[Robert]

Hi Mike,

You're very welcome, glad it helped you get it taken care of.

Take care and best wishes,
Robert