Mystery wire on line volt thermostat


I have line volt thermostats for each of the baseboard heaters in my condo. The thermostats are old analog and ugly. I want to replace them with electronic thermostats. I pulled one of the old thermostats off the wall to check the wiring. A single pole should have 2 wires, while a double pole has four. My thermostats have 3 wires plus a ground. One red, one black and one white. On the electrical panel, there are 2 breakers for the heat, so I'm assuming it's 240 volts to the heaters. I've looked for hours on the web, and I can't find an example wiring diagram that matches what I have. Any ideas or suggestions? The mini wiring diagram on the old thermostat shows it to be a single pole with one wire to line and one to load. No mention of the third wire at all, making it rather useless.
Thanks in advance!
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Possibly a neutral for some function of the thermostat? In any event, figure out which two wires complete the circuit to the heaters and use them on the new thermostat

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This is a complete WAG but ....
Your thermostat may have some kind of "heat antitipater" circuit built in.
The white wire may provide the return path to power going to an internal heater. If so, the thermostat should have a definite voltage rating (120 or 240) since the resistor would get 4 times the power at the higher voltage.
The easy way to check this is to label the wires, remove the thermostat and do some checking with a VOM. If my WAG is correct then at a very high setting the Red and Black should be connected (zero ohms) and the white wire would have something on the order of 40 k ohms to either red or black. At a very low setting the red & black will be disconnected from each other but there will will be the 40k reading between white and ONE of the two colored wires. The colored wire that's always connected to the resistor is the LOAD side.

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Interesting concept, but a moot point, if he's replacing it

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snipped-for-privacy@gmail.com wrote:

The extra wire could be a line for a relay... just a guess.
Do your hear a click near the breaker box when turn the thermostat on?
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snipped-for-privacy@gmail.com wrote:

You mentioned that there are two breakers for the heat, I assume that you mean there is one double pole breaker, and the poles are tied together so that they both shut off together. One pole of the breaker is connected to one bus of the breaker box, the other pole is connected to the other bus.
Typical wiring for 240 volt circuit would be: Black = HOT (first bus) Red = HOT (second bus) (other bus or leg from the BLACK) White = Neutral
There are 120 volts between Black and White There are 120 volts between Red and White There are 240 volts between Black and Red.
For strict 240 volt applications, the neutral is not needed. Not sure why your thermostat would need a neutral, unless it uses 120 volts for some reason. Perhaps you can find out the thermostat model number and work back from there. As somebody else suggested, it is possible that it uses 120 volts for a heat anticipator. If you know how to use a volt meter SAFELY, perhaps you can try that as well to verify.
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The third wire has to do with the way the overload is wired into the circuit.
Does this diagram help. http://www.smarthome.com/manuals/300604.pdf
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