When I removed the old thermostat there are two sets of wires inside the box, one pair reads 240 volts and the others read 120 volts. How do I hook my new digital thermostat up to these wires? The schematic only shows 240 volt power supply and the load wires.

how was the old one wired? what is it wired to? how did you measure the voltage? whats the catalog number of the T-Stat?
...the more you tell us, the more accurate and useful our answers will be...

The old Honeywell thermostat was wired with both red wires connecting to both white wires and the blacks going to the black, these are wired to two 8 foot electric baseboard heaters. When I disconnected the old thermostat I used a multi meter to verify the power leads and when I checked the voltage one set of wires (black and white) they read 240 volts and the other two (black and white) leads read 120 volts. I purchased a new digital programmable thermostat and tried wiring it as to the directions reds to whites and black to blacks with no success. Everyone I asked about the one set of wires having 120 volts is stumped.

The colors of the wires are almost meaningless. Many thermostats have the two reds as the line. Lately I have had several where one red and one black are the line and the other red/black are the load.

There will not be both 120V and 240V feeds in the box. You are reading 120V off one leg through a load. Are there any other splices in the box?

How many breakers control these heaters? You shouldn't be getting 120v anywhere unless there is a neutral in that junction box. Pull the covers off the heaters and see which wires go to each heater. The heaters should be wired in parallel, just like every outlet in the home. Where are the heaters connected together... at the thermostat location? If you have 4 wires in the box, 2 come in hot and 2 go to the heater(s)... that's it. The new thermostat should be wired the same way as the old one.

I checked the voltage of the two sets of romex wires coming into my thermostat box, one set has 120 volts on both the black and the white and the other set has 120 volts just on the black and the white is neutral. There are two breakers that supply power to these wires. I'm not sure where the heaters are connected together.

lake, when you say that the black wire has 120 volts on it, is this a measurement of the black wire to a ground wire? or from the black wire to another wire? Also please describe the kind of meter you are using, is it a digital multimeter? or a wiggy?

At the beginning, were all of these wires connected to the old T-stat?

The black wire to ground is reading 120 volts. I'm using a digital Fluke multi-meter. Yes all of these wires were connected to the old thermostat.

You have (4) wires (two romex cables) in the box...is this correct?

romex #1 black = 120V to grnd.
romex #1 white = 120V to grnd.
romex #2 black = ?
romex #2 white = ?

What was the power rating of the old T-stat? Was it 30 amp, 240 volt?
Does the new one match the same rating as the old.

Sorry about all the questions, but based on what you've told us it sounds like you are seeing a phantom (or induced) voltage on one of the load side (heater) wires. If you know how the old one was connected and the new T-stat is equivalent to the old why not make the connections to the new one the same way and try it?
If of course you are not comfortable with this maybe you should call an electrician...

An analog meter would be a better device for you, even a Wiggy. I'm a little curious as to how the heaters are wired. Do you have the old thermostat? Are there any wiring diagrams on it? When you say two breakers... do you mean two single pole, one double pole, two double pole... which is it? Usually, a heating circuit consists of a double pole breaker with no neutral. Unless you can verify where each wire lands every answer you get here will be an educated guess. Why did you change the thermostat in the first place? Did something stop functioning?

The old thermostat doesn't have any diagrams on it. There are two double pole breakers controlling the voltage to these two sets of wires. The old thermostat has a dead spot in it so I was going to replace it.

Ok. So there are two breakers controlling two heaters. How was a single thermostat controlling both of them? I'm having problems understanding how this was wired originally. Until you can figure out where each circuit goes you won't be able to wire the t-stat. It may be time for you to call someone who can wring out this mess. I can think of a few ways it could work... one is with a contactor (relay) and the other is if the t-stat was just breaking one leg of each 240v circuit. Can you determine which wires (cables) go where? I'm trying to draw this out by your description and I think the latter of my scenarios just could be the working one. Once you trace the wires you will know. If you can do that, fine. Your Fluke set to ohms will do the trick if you disconnect the power and use some jumpers to isolate the wire sets.

Thanks for the help time to call an electrician

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Originally Posted by lakehome

Thanks for the help time to call an electrician

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Everyone need a little help sometime. It's the smart ones who realize when that time has come. :)

oooh! ooooh! I need help now too!