HVAC Thermostat not powering on, blown transformer?

Question

We have recently been having some remodeling done at a currently not-occupied home. One of the things done was a brand new breaker was installed. Before the breaker was installed, our HVAC was working, including a new (Smart with C-wire) thermostat. Now it isn't working at all. I've confirmed there is power going to the HVAC unit, although I'm less certain it is actually working correctly.

In diagnosing the problem, I immediately suspected the transformer could be the problem, as I have had issues with these in the past. I measured the AC voltage across this transformer, measured from the two bottom wires (Black and yellow), and saw 0. I measured it from each of them to the casing ground and measured 120. I saw no voltage between the two in the back (Red and blue).

Bottom line, I suspect one of 3 things is wrong, and I'm wondering if you can help me figure them out, or if it is something else entirely. Either the power was wired wrong to the unit, or the transformer is blown, or I'm measuring voltage wrong and there's a problem in a different area.

The box this was in, with some schematics, if it helps anything.

Answer 1

Somehow, the breaker swap put both hots on the same leg of the service

From the wiring diagram (and the fact the transformer lacks a 120V input), we can tell that this is a 240V air handler. So, it needs two hot wires, one from each leg of the service, to run at all. When the breaker was changed, though, somehow both hot wires got put on the same leg (likely due to improper application of a double-stuff breaker), which put 0V across both supply terminals of the air handler instead. No wonder it doesn't work!

Fixing it requires untangling whatever the breaker installer was doing so that you can get a proper two-pole breaker in there for the air handler, with each hot getting put on its own leg, as it should be.

Answer 2

The 2 black wire are the same electrical point so you measure differential of potential (volt) between point a and b. When you measure between black wire, the results is 0 because are the same point . When you measure from the black to the metal case and you read 120 volts it means you have 120 volts from the breaker that feed the black wire. If you don’t read any voltage between yellow and black, mean you don’t have L2. So your problem is not in the L1 or black wire your problem is in L2 or yellow wire coming back from the load center aka breaker box. If you have a 220V transformers as show in the print, it’s a requirement a double pole breaker so black and yellow are disconnected at the same time if any short circuit or overcurrent event. I strongly suggest to install the double pole and not two different breaker what appears to be your setup.