What is this thing connected to my HVAC

Our condo was built 30 years ago by expert corner cutters. I keep discovering new ways they cut corners, or I get curious and investigate one I’ve noticed before but never looked into.

Today I pulled the electrical tape off this thing in the utility closet which appears to be connected to the HVAC system (electric heat pump), and which probably once upon a time had a cover that someone lost and couldn’t bother to replace with anything other than a few strips of tape.

Anyone have an idea of what it is?

Google Photos

It’s a 24VAC transformer for powering the thermostat and controller.
You can see the control wire connected to it - the other end will be attached to the thermostat or inside the HVAC unit.

Good chance it is an ac to dc converter to send power to your thermostat.

No DC - just AC.

Doh, you’re correct.

Ah. That makes sense. If it’s 24V though, why does it say 10V/5VA between the two exposed contacts?

LOL, that might actually be for your doorbell and just happens to be near your HVAC. I didn’t bother zooming in on it before. But the 10v 5va transformers are usually for doorbells.


Our doorbell doesn’t work anyway. The bell button is next to the front door to the lobby, but we have our own entrance in an alley, and the connections have been severed at some point during previous owners renovations.

It’s one of these. Doesn’t look like it has a cover available.

As others have mentioned it’s a transformer meant to run a doorbell, but I thought I’d add that furnaces generally supply low voltage to run the thermostat from their internal circuitry without an external box like this.

There’s no cover because 10 volts is not a shock hazard, these are generally placed in basements or utility areas where there’s no need to make it look nice, and it’s unlikely to get shorted out mounted on the wall like that.

If you ever have an electrician or a friend that’s comfortable working with mains voltage around, I’d have him or her remove it and put a blank cover on the box because if it’s still connected to the mains, it’s drawing a small amount of electricity without doing anything.

I have one of those in the cellar of my very old house. It is connected to the doorbell.