Check there isn't a short in the "R" cable between the jumper and the line. I had what sounds like this very same problem. The jumper was contacting the R line in two places, shorting out the control. With the thermostat set to where it should come on, I just jiggled the wires to find the short and then rewired it.

The issue was the wiring on my system, not the thermostat. After trying as much troubleshooting as I could myself, a professional electrician came in and essentially rewired my air handler to account for the new thermostat.

that may have been switched off by accident?

I assume you have a split system, outdoor condensing unit with indoor furnace...there will be a 24 volt control transformer, typically located in the condensing unit wiring compartment, but could be located within the furnace.

The 24 volt control transformer will have protection in the form of fuses or sometimes an integral circuit breaker with built-in reset switch. The first thing I would check is that there is 24 volt control power present. You can check this right at the secondary of the transformer or between the 'hot' and 'common' wires, typically the red and blue wires at the thermostat. Sounds like you have lost 24 volt control power from the transformer, if so, reset the breaker built into the transformer or replace the fuse just ahead of the transformer, whichever you have.

Just a thought.

I will check this along with the fuses. Thank you.

turn off the power when I installed a new thermostat. After the install, the heat pump didn't work. Turns out, I did blow a fuse in the air handler unit.

If I'm not mistaken, the thermostat is 24 volts. I've never turned the power off to the furnace when changing them. The battery only runs the electronic controls in the unit and doesn't get power from the wiring.

I think it has something to do with the wiring in the new thermostat. I would go back and check the wiring instructions. Thermostats are either 3, 4 or 5 wire. Some require that you either remove a "jumper" from one of the posts to another. Some require the installation of a "jumper." Some even require different colors going to different posts on the back of the thermostat. Check the instructions again and confirm it is hooked up correctly.

I have a red, white, yellow, green, and blue wire. They were all connected to R, W, Y, G, and C terminals, respectively. The new thermostat had those same terminals. The only difference is that instead of a single R like on the original thermostat, the new one has an Rc and Rh, with a jumper connecting the two. The instructions indicated that if you only have a single red wire from the house, which is the case, that you leave the jumper in. That would make sense in order to supply power to both the furnace and the A/C.

I have the single red house wire going into Rh, with the jumper still connected to Rc.

The two most likely things are that you hooked it up wrong or the thermostat is bad. Have you tried putting the old thermostat back to see if it still works? That could at least eliminate any potential problems outside of the thermostat itself.

As far as I can tell most of your troubleshooting isn't really showing much. When you pull the batteries, the thermostat display probably should go dead, as the batteries are what actually powers the thermostat in most cases (not the wires from the furnace). Also, if you're checking voltage across a switch, it makes perfect sense to have voltage across an open switch and no voltage across a closed switch. And those live wires you're playing with are most likely low voltage and not really all that dangerous. But don't put your tongue on them or anything.

So that's not the problem. And by pulling the batteries, I meant to imply that's the only reason it looked like it was ever getting power. The original thermostat did not have batteries (although it had available battery slots), so it was obviously deriving power from the wiring. I think that's what the blue wire (labeled "constant") was for?

Do you think the voltage of those live wires would be enough to blow a fuse?

Your car is only a 12 V power supply, but it has lots of fuses and an electrical system that can handle some pretty serious current.

In your first line..."I installed..."

You screwed it up.

But how do I FIX it?

have someone who knows what their doing install it.