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I've spent last few days trying to figure out how to remediate my short circuit without luck, and i'm reaching out here in hopes of any clues for next step(s).

I have a radiant heating system, divided into 3 separate zones, and each are connected to their own thermostat and something called an actuator (the thing that controls the heat flow). They're connected to some sort of low voltage power source that's not connected to my circuit breaker. As I was installing a new thermostat one of the wires (Rh) crossed with another (W1) and I saw a spark (hence short circuit) and thermostats for all three zones have lost power. I tried turning off all switches inside circuit breaker and turning them back again, and that did not help with the issue, and I cannot for the life of me figure out if there's any parts in this system with a fuse on.

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Has anyone seen a situation like this? I'm trying to figure out of all my 3 actuators have gone short circuit at the same time, all 3 thermostats have gone out instead, both, or if there's another piece in this system I've overlooked. Any tips or suggestions are welcome!

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    "some sort of low voltage power source that's not connected to my circuit breaker" - That's what you need to find. It almost certainly IS connected to a circuit breaker. Typically this is a 24V transformer, commonly used for doorbells, thermostats, etc. Something like this - Could be almost anywhere, often slapped to the side of a junction box, but the key is that if you trace out the thermostat and control wires you will eventually find it.
    – manassehkatz-Moving 2 Codidact
    Commented 2 days ago
  • What's in that electrical box in the bottom left of the pictures?
    – Mark
    Commented 2 days ago
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    you killed the 24v transformer
    – DIY75
    Commented 2 days ago
  • the box in the bottom is a housing for all the wire twister caps
    – Henry Lee
    Commented 2 days ago
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    Crossing a wire with another and caused a spark is a good reason to make sure all power is off to that circuit/system. Sometimes when this happens you get lucky and just blow a fuse. Other times you are not so lucky and burn out thermostats and/or other stuff.
    – crip659
    Commented 2 days ago

2 Answers 2

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It's common for the thermostat transformer to be on the same circuit as the boiler (or other heat source) and often it's physically bolted to that device, inside or out depending on design; or to the wall or ceiling near that device.

Wherever it is, it will be found by following the wires.

If you're lucky there are fuses there, so your error is the cost of a box of replacement fuses (so you have spares for the next time) rather than a whole new transformer. If you're really lucky you'll find a box of spare fuses already there, but that's unlikely.

Shorting out the transformer would not be likely to cause the circuit breaker supplying the boiler and transformer with power to trip. Without fuses it might well fry the transformer though. There are usually fuses.

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Look for a zone valve control box. Some models of zone control have their own built-in 24V transformer, a master fuse and separate fuses for each zone. Look for a blown fuse. The box may look something like this:

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Photo credit: Taco, Inc.

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    @HenryLee Note that the fuses may not be the glass tubes that you've seen before - they could be surface mount devices (SMD). The way to identify those is to look on the silk screen printing on the circuit board for something like "F1", F6"- something starting with "F". You can then check if they are blown by using a multimeter to test for continuity.
    – Andrew Morton
    Commented 2 days ago

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