[Mrod500]

Newbie here... Thanks in advance

Two units, upstairs and downstairs, house built around 1990 and original equipment YORK vertical unit blower/furnace as closet units and two condensers outside. Added Emerson Sensi WiFi unit downstairs and connected C wire as instructed. No issues. Works wonderfully.

A week or two later, did the identical process upstairs but this time, the thermo never fired up and I smelled slight electrical burning from unit, specifically the transformer. The breaker never tripped. I put the old thermo back on (battery unit) and now nothing fires up as the unit (blower) is dead - no heat, no fan, and no AC. The transformer reeks of burnt electrical.

This is an older unit with no main motherboard (that I can see) and no fuse (please see pic). I found the replacement transformer online and have sent the (bad) thermostat back for a replacement. A few questions:

1) Could this be a short from the new thermo since it never fired up? (I tried to put the upstairs thermo on the downstairs faceplate to see if it would fire up but it wouldn't snap on. Something wasn't right as the pins would not line up on two identical units.
2) I found the new new transformer online. Is this something I can install myself? Will all the terminals be the same?
3) Can I add an inline 3-amp fuse to the C wire in case there's a short there?

I've done numerous repairs myself such as new capacitors and a new fan blade motor but nothing like this. Thanks for your help!

Mike

 

[bfrabel]

Was the power to the furnace shut off when you were playing with the wiring? If not, that's probably what the deal was. Or you might have nicked one of the wires or it could just be a bad t-stat. It happens.

They make transformers that have resettable breakers in them. That's what I would recommend. Your little inline fuse idea works too, but you would want it on the "R" or hot wire, not the "C". The common wire is actually supposed to be shorted out to ground, so I don't think a fuse would help on that wire. It's kind of like the neutral on your 120V stuff, it's also bonded to ground and not protected by fuses or breakers.

 

[57TinkerMan]

Probably not related to the OP's problem, but I cannot remember (and I know it will get worse) what else on an old furnace would require a constant 24V source as shown in the pic below. Spark ignition module?

 

[bfrabel]

Probably not related to the OP's problem, but I cannot remember (and I know it will get worse) what else on an old furnace would require a constant 24V source as shown in the pic below. Spark ignition module?

View attachment 604413

gas valve and blower relay.

 

[57TinkerMan]

In the pic the gas valve and fan relay are powered by the W and G terminals and it appears that the common sides of the coils are connected to C. As said, I just find it curious but don't know the answer off the top of my head. I would want to know what it does before replacing the transformer.

 

[Mrod500]

Thank you both for your input. Yes, the power was off at the breaker as well as the condenser. However, I turned the power back on when all was done being wired/connected and that's when I started smelling the burning smell. It is coming predominantly from the transformer.

Currently, the unit is dead. I have my old thermo hooked up and when I cycle through the AC/blower fan/heat, nothing clicks on at all. I've got a new transformer that will be here Monday. I'll start with the transformer and go from there. I'll add the fuse to the "R" cable. That makes sense.

 

[bfrabel]

Yes, the power was off at the breaker as well as the condenser.

By "the breaker", do you really mean BOTH of the breakers, or at least the single pole one that powers your furnace? The 2 pole breaker and the disconnect for the condenser usually don't have anything to do with the 24V side of things.


If you left power on to your furnace, then you likely shorted it out when you were messing with the thermostat wires. This is a common rookie mistake.

 

[Mrod500]

Well, good point. So I shut off the 15amp to the 'upstairs furnace' as well as the breaker outside that control the condensers. Is there another one I'm missing?

 

[bfrabel]

Well, good point. So I shut off the 15amp to the 'upstairs furnace' as well as the breaker outside that control the condensers. Is there another one I'm missing?

As long as the "upstairs furnace" was the one you were working on, and the breaker was labeled correctly, I guess you should be good.

 

[Missouri Bound]

Invest in a cheap multi tester.....cheaper than a transformer.
Guessing the power is off is not a wise move.