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# question about line thermostat

Member Posts: 230
I went to a customer today and when I put the meter of the two leads of the line thermostat it gets 36 volts when I go downstairs and I put my meter on the two leads I get 68 volts when I make contact with the two leads by pump I get 120 volts does this make sense. Is this how its supposed to work.
Thank you
Joseph Hardoon
HHI Services LLC

## Comments

• Member Posts: 17,810
Possible. Those sound like "phantom" voltages -- a voltage induced in one wire by a nearby one. Back in the old days we didn't have to worry about them, as our meters had too low a resistance. Nowadays...

If you were measuring between the two terminals on the thermostat, try also measuring between one terminal and ground and see what you get.

Another slightly risky trick is to rig up a 110 volt test light (with a fuse!) and alligator clips or probes, and probe each terminal to ground. If the wire is "hot" the light will light. If it's a phantom, noting will happen at all -- but if you measure the voltage to ground it will now be zero. Take the test light off, and it may go up again.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 11,172
if it's a 120 volt line voltage thermostat you should have 120 v between hot and neutral (or a good ground) going into the stat.

With the thermostat hooked up you will get 0 volts on the two thermostat wires if the stat is calling for heat. If the stat is not calling you should get close to 120v across the thermostat wires.

Stat calling you should get 120 volts to the pump
• Member Posts: 230
Sorry was posted a little unclear.. when i put meter to 2 leads by thermostat get 36 volts.. at same time by hot an neutral at pump get 68 volts...when i thermostat wires together then i get 120 volts by pump and it goes on?
• Member Posts: 17,810
Keep in mind that the thermostat is just a switch -- it's not powered. Usually (not always) a line thermostat is switching a hot (black) wire, and the live side should be 120 to both neutral and ground (neutral may not always be in the box, but ground has to be on current wiring).

The two wires at the thermostat, however, are connected only at one end. When you put a multimeter across the two at the thermostat, the voltage you read can be really rather odd, and depends on a number of factors.

That said, a truly hot wire -- unswitched -- should always read about 120 to both ground and neutral. A neutral should, of course, never be switched, and should read 0 or close to it to ground. A switched wire, with the switch open, may read other than 0 or 120, depending on the way the wiring runs, the impedance of your meter, and the impedance of the load.

If you are concerned and for some reason cannot cut power to the circuit, the most ultimately reliable test that I know of starts with a test light -- either contact or non-contact -- which should light near a hot wire and not near a witched or neutral. Then, if you are still unsure what you have, as I mentioned, rig a test light (I said alligator clips, but I meant alligator on one end, probe on the other) consisting of the probes, wires, a suitable fuse, and a 120 volt light (preferably incandescent -- I use a night light socket and bulb). Connect the alligator to ground, and probe the hot terminal. If it's hot, the light will light. If it isn't -- even if it's showing a phantom voltage -- it won't.

In all this kindly keep in mind that you are playing with live electrical circuits, and they can kill you, or you can short something out with lamentable results. Even very experienced sparkys approach this with a good deal of caution.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 11,172
If you hooke the two thermostat wires together and the pump started then the stat is probably bad. You can get one at the Big Box store . It's the same stat used on electric baseboard
• Member Posts: 230
Thanks... but any reason for the strange voltage or is it like Jaimie said some sort of ghost  voltage
• Member Posts: 11,172
Depends on the meter your using. Some digital meter put no load on the circuit and will give you a ghost voltage when there is nothing their.

If you put your meter on the thermostat wires without them being connected to the thermostat and with the power on you "should" get 120 volts. 1 meter lead is on the hot wire coming into the stat, the other wire goes to the pump motor, through the windings to the neutral connection so with no load on that circuit you should get 120 volts

with the stat wires disconnected and the power on and you check the incoming hot wire to ground you should get 120 volts to ground.....that's if you have a good ground.

from the other wire (the one feeding the pump) you should get 0volts
• Member Posts: 4,351
If the circulator does not turn on but you see a significant voltage across the contacts of the t-stat, the contacts are probably burned and have high resistance. Some current flows through the contacts and the circulator but the voltage drop over the burned contacts means that there isn't enough voltage at the circualtor for it to run.