First I have to tell you that around 10-15 years ago I went the other direction. I had only a couple of electronic line voltage thermostats, but both failed, and I replaced them with mechanical thermostats. Both are still working fine today.

Now to your problem. It sounds like power was run to the heater, and then the thermostat was connected as if it were a switch loop.

Before I say anything else, tell me, is the heater 120 volts or 240 volts?

And, is the thermostat rated for the same voltage as the heater?

Eh? Are you sure your new thermostat only has two terminals? What is the exact model. The Honeywell line voltage thermostat I'm looking at has two red wires and two black wries.

Thanks for the quick replies! Good questions. I am assuming it's 240 because there are two 20w circuits tied together at the box for the electric heat. Although, those two circuits control the electric baseboards in both the bedroom in question and another bedroom.

The thermostat says it runs 120-240. I bought the only digital line voltage thermostat at Home Depot. It's this one.

https://www.honeywellstore.com/store/images/pdf/rlv4305a1000-e-5-2-day-programmable-triac-line-volt-thermostat.pdf

Thanks!

The picture on page 5 is a big hint. Hopefully, you remembered which wire went to where on the old thermostat. One wire going to the load and one wire to the line need to be connected together. The other two wires go to the terminals on the new thermostat. I suspect the old thermostat had a metal housing which is why it had a ground. You don't need that for this plastic cased thermostat.

The problem here is that you have only 3 wires in the thermostat box, not 4.

My best guess is that this is a "switch loop" setup., with an extra wire to supply line voltage in case the thermostat needs it (which apparently your new one doesn't, to my surprise).

Two of the 3 wires in your wall box probably carry 240 volts from the breaker, and the remaining one is a return to the heater. So probably you'll have to cap the common wire with a wirenut, and connect the other 2 to the thermostat.

The problem is to determine which wire goes where, and which to cap. Get this wrong, and either it won't work, or you'll destroy the thermostat, possibly with a bang.

Maybe someone else can think of something, but I don't see any shortcuts here. You'll have to trace out the circuit from the breaker to the heater, including any other junction boxes along the way.

Most likely 2 wires in a cable go from the breaker to the heater, providing the power. If I'm right, one of these will be connected to one side of the heating element, and also to one wire in the cable going to the thermostat..

The other wire from the breaker cable will be wirenutted to the second wire in the thermostat cable.

The remaining thermostat cable wire will be connected to the remaining side of the thermostat element.

Your assignment is to figure out which wire colors go where.

The two wires you need to connect to the new thermostat are the wire that comes from the power source cable that DOES NOT also connect to the heating element, and the wire that goes to the heating element. You'll cap the one that's connected to both the other source cable wire, and the heating element.

If this description doesn't make sense, maybe someone else here can describe it better, or you may have to call in an electrician to get this done safely.

I don't understand how the original thermostat worked with only one line connection.

Do you know the model number of the old thermostat? That would help figure out how it is configured.

If the thermostat was connect to two heaters, it might make more sense.

20w? Should be 20a or 20 amps. Two breakers tied together to make 240v (and 20 amps). I might take the cover off the panel and look at the color of the wires on the breaker. I know what they SHOULD be, but there is no telling.

Is it possible the the thermostat is working like a junction box? The line comes of the breaker into this thermostat, but then it passes on to the next room to the other line voltage thermostat? Like I said before, I have two bedrooms that have electric baseboards, although there is a dedicated thermostat for each room.

I have a DMM. I'm comfortable taking some readings if that will help me diagnose this problem.

Unfortunately I don't have a model # on the old thermostat. All I know is that it is a Federal Pacific brand. It's the exact one pictured in this post.

https://www.doityourself.com/forum/electrical-ac-dc/588950-federal-pacific-thermostat-panel-replacements.html

Also, my mistake. I should have said that there are two 20 AMP circuits tied together for the baseboard heat. Sorry! Is it safe for me to take the cover of the breaker with the power to the breaker turned off?

Thanks!

The thermostat in the photo you provided is wired how I would expect it to be. There are two load wires and two line wires. The line wires carry the two 120V phases from the circuit breaker box to make the 240V. The two load wires connect to the two baseboard heaters. The heaters would have to be wired in a parallel configuration.

What you described seems to be an open circuit. I don't see how this could work unless it supposed be wired for 120V.

mike_home said:

• The load wires carry the two 120V phases from the circuit breaker box to make the 240V. The two line wires connect to the two baseboard heaters.

Uh - the other way around!

dennsigli,

You are correct. I edited my prior post.

So the thermostat is the one in the photo, but you said there are only 3 wires in the junction box, and the photo shows 4 wires. This is not an actual photo of your situation, correct? Can you post a few photos of your actual situation?

Using a DMM or test light to deduce what wire is which can be tricky, unless you disconnect the heater itself. The low resistance of the heating element can fool you.

If I were doing this job, I'd take photos, label and disconnect everything, and then use an ohmmeter or continuity tester with a long clip lead to identify the wires.

That should be pretty easy for you if you understand the basics of electricity and how simple circuits work. If not, you should probably call in a pro at this point. I don't recommend learning electricity fundamentals by experimenting with 240 volt circuits. :)

Ha! Ok, thanks for the honesty DavidR! What about if I replace the thermostat with a different one that has 2 load wires and 2 line wires. Something like this?

https://www.supplyhouse.com/Honeywell-TL7235A1003-Digital-Non-Programmable-Double-Pole-Line-Voltage-Thermostat-for-electric-baseboards-7855000-p?gclid=Cj0KCQiAkMDiBRDNARIsACKP1FHZHG4FdJlCOJTHqBT-4sp_1-ZuaxlsV5TW7yJ26yS2FeUQglFWrlYaAq_DEALw_wcB

Something I am unsure about is that it seems like this stat is spec'ed for 15 amps at 240 volts. Am I looking for something that runs 20 amps? 40 amps because the two circuits are tied together?

Thanks for all the great advice everyone.

AM

Please don't take this the wrong way, but the more I think about it, the more I think you may be in over your head. Your 3-wire setup there is unusual, and I'm concerned about your ability to trace out what's really going on.

At this point I'm going to suggest that you either put the original thermostat back in and live with it, or call in an experienced electrician to help with this project.

DavidR said,"Your 3-wire setup there is unusual, and I'm concerned about your ability to trace out what's really going on. "

You think so? That sound's similar to something said 2 weeks ago when this first began. "People who do not have a multimeter or have one and don't understand how to use it shouldn't monkey around with electricity."

"People who do not have a multimeter or have one and don't understand how to use it shouldn't monkey around with electricity."

The only way you learn to trace circuits and use test gear is by doing it. It's nice to have expert guidance, but you can also get help from books, which is how I learned to use a VOM as an adolescent. (No DMMs in those days. Also, no internet. We had libraries, though. :)

But here we are more than 2 weeks after the original post, and if the OP has done any circuit tracing, I haven't seen reference to it. My guess (and I hope he'll correct me if I'm wrong) is that he doesn't want to get into the project quite that deeply, and is hoping he can just buy a solution.

His proposed purchase is a thermostat which has a similar configuration to the old one. That seems like it ought to work, but a digital thermostat still works on a completely different principle from a mechanical one.

So the NEW new thermostat may work when connected like the old one was. Or it may not work at all. Or it may work only partly. Or it may go pop or BANG. Or it may just quietly release a little puff of smoke and stink.

There's even a slight chance he could get hurt or damage something else, the worst case scenario.

I just think that if he doesn't want to do the hard technical work of tracing things out (and there's nothing wrong with having different priorities), it's probably best to turn this over to a pro, and get it over and done with and WORKING.

Thank you for your candor DavidR and for all the replies to this thread. You are correct that I am not willing or able to dig into the project as deeply as it requires. I think I will take your advice and hire an electrician.

Thanks again and take care!

Andrew