The old stat probably only switched one line of the 240 volts and the new one switches both lines. In addition to the one line switching the old one had a neutral connection for the timer. The new one probably only operates at 240 volts and does need the neutral.

I have never seen a white wired used for anything but a neutral.

On a SPDT (heat only or cool only) line voltage stat with wire leads, white is used as the common.

hi! just to let you guys know that this is not a furnace thermostat, just a 240V electric baseboard heater thermostat. The house does not have central A/C. All I want to know is how should I transfer my connections from my old 3-wire thermostat to my new 4-wire thermostat. Thanks.

I think we are all aware it is a line volt stat, and in this case used for electric B/B heat. Many stats such as the H/W T651 can do both (Heat or cool) because it is SPDT, which only breaks one leg.

Without being there myself with a meter, I wouldn't begin to tell you which color wire goes to what, sorry

240V thermostats with 2 sets of blacks and reds are wired for switching both hot legs and are called double pole thermostats. You wire them with one set of colors to the incoming hots, and the other set to the line out for the elements. It doesn't matter which color set goes where. With a double pole thermostat, you can turn the elements "off". With a single pole 'stat, you still have one leg energized and there is no "off", only a minimum temperature setting.
I have no idea why your previous 'stat had 3 wires with purple, yellow, and white insulation.

w0lley32, your patience is requested, and I am sure we will figure this out, probably very easily.

The box in the wall where the old thermostat was. REMOVE the old thermostat, and tell us how many and what color wires are coming into the box.

Hi! I removed my thermostat and here's what I found: I have 3 cables entering the box; the purple leg of the thermostat goes to the three red wires with a wire nut; the yellow leg of the thermostat goes to two blacks marretted together, and finally the white leg of the thermostat goes to the third black wire. At the baseboard heater I have only one 12/2 cable. The thermostat is electronic, but it is not programmable, if that can help you.

Thanks for your patience!

OK

Here's what I'd do. I'll make a 90% guarantee this'll work.

Existing wiring:

New thermostat wiring:

BE AWARE that the colors of the wires drawn to the new thermostat may not be exactly correct - it's been a while since I've put one in. You'll note that I've labeled them (in yellow) as "LINE 1", "LOAD 1", "LINE 2", "LOAD 2", etc. The thermostat should be labeled similarly, and you should follow the labels rather than my colors.

Forgot to post this part.

The existing thermostat, per my guesswork (and your description) is an SPST switch on yellow/white, which, for some reason, wants a connection to the other pole (the purple wire). Frankly, the yellow/purple/white colors make ZERO sense here, hence the 10% uncertainty.

I'm assuming that one of the wires is the power source, the second goes off to another thermostat somewhere, and the third one winds up at the heater.

It makes sense if you connect the purple wire to the black source and the white to neutral instead of the other pole. The red does not connect to the stat. The stat is 120 volt.

I agree, joed, that the change you suggest would make sense. A lot more sense, actually. However, the change is not what the original poster described.

Given the existing wiring that w0lley32 described, the coloring does not make sense.

That gives us three options:

1) the colors truly do not make sense

2) the original thermostat is miswired
   2a) it worked anyway
   2b) it never worked right, but seemed to do "something"

3) the original poster got confused and swapped a couple wires around in his description.

Hi! Actually, what Pharkus told me to do in the first place is probably right, as it is what I did and everything works perfectly. The thermostat could not have been hooked up to the neutral since there isn't any, neither at the thermostat nor at the heater. The old thermostat did not have any brand stamped on it but it read "250 VAC 16 A" molded on the plastic at the rear.

Thank you all guys for your help, problem solved! I can now replace my other thermostats.

Just one last question: since nobody seems familiar with these colors, could it be that my thermostats and baseboards heaters are from another country? I would doubt that something was not working properly since I had the house built in 1991 and everything has been working since. I just wanted to replace my thermostats with something programmable and less ugly than the ones I had. Thanks again!

I had the same issue tonight, and read this thread with interest, but I have 2 problems. First, the diagrams that pharkus posted have invalid links and they would be very helpful. Secondly, I did get the new thermostat working after some trial and error and a few bad words) but the problem is the same as the old one. That is, the heat stays on regardless of the setting or the temperature. The room was so hot I turned off the circuit breaker for now.

Any ideas would be appreciated, and if anyone can email the diagrams to me I would appreciate it.

klenoble, sorry about that - I reformatted my server recently and hadn't bothered to restore my web directory.

The diagrams should be working now.

If you have changed the wiring around in attempt to get the new ones working, and don't have accurate diagrams of how it was before you changed it, then some investigation will be in order, but it's nothing too difficult.

"With a single pole 'stat, you still have one leg energized and there is no "off", only a minimum temperature setting."

You only need to open one leg of a 240 V heater to produce zero output.

The danger is that the heating element is still hot with 120 V to ground.

A double pole thermostat removes all voltage from the heater when it is off.

I have seen a number of Ruud A/C condensers that only open one side to the compressor.
There is no real reason to open both, since a disconnect is required nearby that opens both legs for repair.
The single contact was also a real relay contact instead of just the old cheap copper to copper type contactor.

Thanks to you all for your replies, and to pharkus specifically for resetting the link. I'm going to try again tonight. Two more questions though. Should I buy a double pole thermostat? Also, is it possible that I fried the new 'stat when I tripped the breaker, despite the fact that it "looks" OK? What I mean is that the room temp displays and I get a "heating" graphic when I should.

I know it's been awhile since this thread was started but it would be super helpful for me if I could access the diagrams that pharkus posted before. I am replacing a line voltage thermostat, box in the wall has two blacks, two whites and a blue (I've been told this is the leg). Tstat I am hoping to install is HW T651A which has one black, one red and one blue wire.

A contractor had removed the old tstat and tied the whites together and the blacks together to make the convector a manually operated setup.

I have same problem and I have yet to make sense of the answers. I have line, load and common. is common neutral or hot? the new stat is L1, L2 T1, T2

My wall thermostats for electric heater has 3 wires how do I connect my 4 wire

Where do the existing 3 wires come from? Are you counting a ground (bare or green) and/or neutral (white) in your count?

3 wires in the wall. White red n black

When you say "in the wall", do you mean at the thermostat or at the baseboard? In either case, are these three wires in both locations?

In the wall. Yes same 3 in baseboard

So, as others have said, red and black are probably hot, 120 volts from ground, 240 between. In the thermostat box, I assume the red or black continues through and the other one goes to the thermostat. Is that the case? Which one goes to the thermostat and which one goes straight through?

Yes 3 wires in the wall

baseboard. Has black to black.

black to white

n red is into the wall. Nothing screwed onto it

3 wires in wall to thermostat

Still trying to figure out your terminology. I'm guessing that the black in the wall at the box where the thermostat is cut and the resulting two blacks go to the thermostat, right? And the red enters the same box but goes through without connecting to anything. If that's the case, you can cut the red, giving you 4 wires (2 black and 2 red) and connect your 4-wire thermostat to them. This is safer than the 3-wire connection you had, because the 3-wire leaves one hot on all the time (the red wire).

In the thermostat in the walk now there are 3 wires. White red n black. In the badeboard heater there are 2 blacks connected and another connection has a white n black together. The red one is fixed in the baseboard

There should never be a black and white connected together. Could you make a drawing with all the wires labeled and take a picture of the drawing AND the two boxes (one in the wall and one at the baseboard) and post them? How are the three wires in the wall connected?

Actually, there can be. 240V can use 2 wire, with ground. Two hots and the ground. It should be marked that the white is hot but it is very common to have that configuration.