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# Line Voltage Thermostat

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Member Posts: 4,076
I was testing the wiring on a boiler replacement and got a tic tracer beep with the thermostat both on and off and couldn't understand until I realized that they split the neutral at the thermostat instead of the hot.

My meter reading with the thermostat on was 110 volts and 56 volts with the thermostat off. Why am I not getting zero volts with a disconnected neutral?
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab

• Member Posts: 8,157
edited February 2022
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I can't be 100% sure without looking at the job, but the windings on the motor or resistance coils on the heating unit that thermostat is operating is allowing the voltage to back feed to the thermostat junction box. That resistance is the reason for the voltage drop on your meter. An old Simpson analog meter will be more forgiving and realistic with that kind of voltage reading. (never did like them con-found digital meter contraptions)

OR

You have a DIY Homeowner wiring something off that circuit because it was the closest electric he could find to make the impromptu wiring hack.

Disconnect the wire at the source and see if there is still power at the thermostat. If there is, then it's the DIY Hack

Stay safe. You may want to know that many of my old customers were SHOCKED to find out that I wasn't the best electrician

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 15,764
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Are you sure they split the neutral? Assuming a two wire line voltage stat they would usually just run 1 cable with a black and a white wired like a switch loop.

Whatever they did I agree with @EdTheHeaterMan digital meters can and do give false readings sometimes.

An analog meter puts some load on a circuit, a digital does not so the digital picks up transient voltages sometimes

I had a flourecent light fixture once that the installer did not ground the fixture, it was mounted on a block wall, the power supply came in the back of the fixture with no ground wire and no metal connection to a conduit or flex.

Emt came out of the light fixture down to a metal switch box. Checking with a digital meter showed the EMT was "Hot".

An analog meter showed nothing. You could touch the EMT nothing there.

Picking up some transient off the ballast. We ran a scrap piece of wire to a conduit that was grounded to check against
• Member Posts: 222
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My newer digital Fluke has a setting that puts a little load on the circuit, its optional of course.
Old retired Commercial HVAC/R guy in Iowa. Master electrician.
• Member Posts: 5,874
edited February 2022
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Why is there a neutral at the thermostat? Is it wired from the panel to the emergency switch to the thermostat to the system? Probably not.
• Member Posts: 8,157
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HVACNUT said:

Why is there a neutral at the thermostat? Is it wired from the panel to the emergency switch to the thermostat to the system? Probably not.

Why not? It would work.

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 4,076
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Yes, that would work, but as I said, the hot wire is always energized at the boiler; thermostat on or thermostat off.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab
• Member Posts: 2,479
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I'm kinda dumb. I don't quite know what you did or what was the situation in how you were measuring. Was it a 110 V or a 220 V thermostat? I suspect a 110 V thermostat.

AC is a two way traveler on a conductor. Measuring a hot lead to ground will give you a 110 V reading, measuring a "grounded conductor", erroneously more commonly referred to as a "neutral", will give you a reduced voltage in line with what you saw.

That's my take on the issue without further understanding.
• Member Posts: 22,382
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Bad t-stat? Is it a mechanical or digital stat?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 222
edited February 2022
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If you read across an open switch, you will (might) get a reading because the device is in the circuit. My guess its not on a grounded box nor is the neutral available, that is what he is seeing.
Old retired Commercial HVAC/R guy in Iowa. Master electrician.
• Member Posts: 23,554
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If the hot wire is always energized at the boiler, but the thermostat will turn things on and off... there may be a serious wiring error: the neutral should NEVER be switched. It sounds as though it might be. Another possibility is that someone wired an extension leg -- like the lower arrangement in @EdTheHeaterMan 's diagram -- and neglected to mark the white wire of the two wire cable they were using with black (or red). That's another no-no. You can use a two wire cable -- white and black -- that way, but only if the white is marked.

I'd be mightily inclined to trace back all the wiring -- not just the cable -- and find out who did what. Or at least what had been done.

Phantom voltages are a first class bore. I have a nice little test light (made it myself) consisting of a wire with an alligator clip, a 5 amp fuse, a 4 watt night light bulb in a socket, and another wire with an alligator clip. Made it because I got tired of scratching my head over phantom voltages on ungrounded BX cable (don't ask and I won't tell). Clip one end to the perplexing wire and the other end to ground. If the wire is genuinely hot the light will glow cheerfully. If it doesn't glow, now measure the voltage -- it it's a phantom, the rig will ground it out to zero.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 2,646
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High impedance digital meters can do that....

I have a Klien clamp on meter with a separate "LOW Z" position on the dial. It mimics what @Jamie Hallis talking about. The meter was \$120,  no need for Fluke expensive meter for that.

Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
• Member Posts: 8,157
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Yes, that would work, but as I said, the hot wire is always energized at the boiler; thermostat on or thermostat off.

This diagram would explain your statement Alan

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 4,076
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Yes, that does. Thank you, Ed and everyone else!
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab