How to test whether or not a breaker is powering a zone valve and thermostat?

Question

I have a fuel burner (fuel oil) that heats water and provides my home with baseboard heat. I have two thermostats and zone valves, both by White Rogers (and both pretty old): one set for the downstairs and one set for the upstairs.

I assume that each zone valve + thermostat pair is part of the same branch circuit and thus, controlled by the same breaker. If that's not the typical setup, please begin by correcting me!

But assuming I'm correct on that, I'm trying to figure out what breakers control both zone valve + thermostat pairs. My main service panel got upgraded last year but the electrician did a terrible job marking everything and did not mark where these were.

As opposed to lightswitches and outlets which are easy to test (turn a breaker off and see if a certain light goes out or if my electrical/volt pen doesn't detect any voltage in the outlet), I'm not sure how to test whether a particular zone valve and thermostat have power. Here is a picture of what they look like:

Can anyone give me a recommendation for how I could:

1. Turn a breaker off; and then
2. Check whether or not there's power coming to the zone valve and thermostat?

I have many breaker "candidates" for which ones could be controlling these valve/thermostat pairs, but currently I'm not sure how to test whether the breakers are/aren't supply power to them.

I suspect this will involve buying a multimeter. I used one in college for a class so I'm not a total stranger to them and wouldn't mind buying one if its the right tool for the job.

Answer 1

The Zone-A-Flow 1311 runs on 25V AC. That is also typical for thermostats in general - though many "dumb" thermostats don't even need any power at all. So what you are looking for is a transformer (possibly one for each zone). A typical White-Rodgers from Amazon:

but brand doesn't matter, voltage matters.

Typically these will be powered by the same power for everything else in the HVAC system - e.g., electronic ignition, pumps, blowers, etc.

The binary method of figuring this out is:

• Turn off 1/2 your breakers
• If the device is still on, turn off 1/2 the remaining breakers
• If the device is off, turn the off breakers on and the on breakers off
• Lather, rinse, repeat

Since "dumb" thermostats have no indicator lights, a multimeter will be very helpful here in figuring out when your power is on/off.

Answer 2

I would think a simple non-contact voltage detector would tell you whether there's power running to the wiring.

• While the power is on, determine which wire(s) are hot.
• Turn off a candidate breaker.
• Check to see if the hot wire(s) are still hot.
• Lather, rinse, repeat until you find a breaker that kills the power.

Once you've determined which breaker(s) it is, double check with an actual meter, just to be sure.

Answer 3

Multimeter would do the trick, and is a useful tool that does not cost much unless you go for the high-end ones.

Check for 24VAC (usually) on the thermostat wires (either disconnected, or with the thermostat turned down so it's not calling for heat.)

Usually there's just the one transformer providing the low voltage for all the thermostat(s) & controls, so if you find that breaker, which is usually also the one controlling all power to the "boiler" (which is what fuel-burning water heating devices are usually called unless they are "water heaters" for plumbing type water, even though most are not making steam these days) it should be all of them, but you can also check for 24VAC at the zone valves (usually with the thermostat calling for heat.)

In a code compliant install, the boiler circuit breaker should not be shared with any other circuits, so if you know (or find while looking for it, and improve the labeling) that a breaker turns off something else, it should not be the one for the boiler. So identifying which breakers control various other lights and outlets would both allow you to improve your labeling, and reduce the number of possible candidates for the boiler breaker.