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Old 03-20-2010, 05:15 PM   #1
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Problem w/ two 240v baseboard circuits on single double-pole switch.


I have an outbuilding with three small baseboard heaters on two 240v 20amp circuits, one on one circuit, two on the other. They're wired to a single thermostat. This used to work, but the thermostat was sticky (would sometimes stay on) and the temp would of course skyrocket, not so great on power bill also.

Last year when I was out of town, my father in law 'helped me' by replacing the thermostat. When he was done, one of the heater circuits (with the two heaters) was always on, the other was controlled by the tstat, so I threw the breaker, and just have the other heater. I had thought that the problem might be a bad/wrong tstat, so I bought a new double-pole switch (Rite-Temp manual from Home Depot, but it's AA battery powered and has an on/off switch).

Installed it today, same problem, with a bit of a twist. Unlike the previous all-manual thermostat, this one has both a temp control and an on-off switch on the side. When this switch is OFF, all heaters are off. When it's ON, then the two on one circuit are always on, but the third (on the other circuit) is temp controlled by the thermostat.

Here's how the wiring works - I think as this was existing and I've never seen the walls uncovered, this is all going into a one-gang deep plastic box. Two lines (10/2 romex), and two load (10/2 romex), with the whites (which I realize is not neutral for 240) connected together in pairs with Marrette connectors, and all the blacks used for the thermostat. I run the two line inputs into the red line wires for the tstat (L1 and L2), and the two outputs to the two black load wires on the tstat (H1 and H2).

Of course something is goofy. I suspect that my assumptions about which wires are which might be off.

Can I assume that with a double-pole thermostat, each red and black pair (line 1 and heat 1, and line 2 and heat 2) are switched independently? So that it basically acts like two fully independent switches, that just happen to be controlled by the same thermostat?

Clearly both circuits have to somehow be going through the tstat, or else turning it 'off' would not shut them both off. But I'm confused to what a double-pole switch would do that would allow this. I'd think if tstat setting was low (no heat ask), then both switches would be open, no current flowing from Line1 to Heat1, and from Line2 to Heat2. And if tstat is high (heat ask) then both switches would be closed. I'd also assume that the on/off would just make the switches fully open always, ignoring the dial setting for heat. But clearly this isn't the case, as even with tstat set full low (room temp is around 70 today so clearly no heat ask with tstat at 55), the one circuit will turn on when I set the switch 'on'.
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Old 03-20-2010, 05:32 PM   #2
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Problem w/ two 240v baseboard circuits on single double-pole switch.


What you are attempting to do can be done with what you have but I wouldn't recommend it. You are attempting to control two 240 volt circuits, one each with one pole of a 240 volt switch(ts).

If you insist:

Circuit 1 white from breaker and whites from baseboard all connected together
black from breaker connected to red on TS, black from baseboard connected to corresponding black on TS.

Circuit 2 repeat above

This will give you a single break on each 240 volt circuit, which will leave a constantly hot wire at the baseboard thus making servicing more dangerous.

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Old 03-20-2010, 06:11 PM   #3
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Problem w/ two 240v baseboard circuits on single double-pole switch.


I suspect you may have a thermostat that does not switch both lines. It only switches one line. That is why only one set of heaters is controlled.
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Old 03-20-2010, 06:14 PM   #4
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Problem w/ two 240v baseboard circuits on single double-pole switch.


Thanks! That verifies that it should work as I thought. I figure that's how it must have been originally wired (or something very similar), otherwise I don't see how it could have worked, unless there was a quad-pole thermostat there before, which I'd never heard of.

I know I'd never service anything without throwing the breaker, I also always triple check with both NCV detection plus voltmeter stuffed into the marrette connectors to ensure no voltage.

I'll need to pull the box apart and verify all the leads, since I'm not confident I have them all correct, and I of course need to ensure the two circuits are fully independent. I'm now not 100% sure there were only four and not five romex leads into the box.

I can easily figure out how to detect the two incoming circuits (check for 0/+240 as breakers flip, all the load circuits should have 0v with breakers either on or off).

Is there a straightforward way to A) verify if something is a load (I assume look for resistance, first ensure zero voltage across), and B) tell which load is which? Not a super big deal if I get them confused, all three are in the same (large) room, identical heaters, and both breakers are the same (20amp 240v paired), so as long as two end up on one circuit, and one on the other, it'll work fine.

If no easier way to tell directly, I can always just tie one of the line ins to each load pair, throw the breaker and see which heater turns on, repeat.

Not 100% sure if they're all wired parallel to thermostat junction box or if two are chained. I guess resistance diff should tell me this? (If chained in parallel, resistance should be half of the since unit)? Also I guess 4 vs. 5 romex pairs in the box will be a big clue :-)

Old thermostat had ground, new one has no ground terminal. Just tie all grounds in the box together I assume?
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Old 03-20-2010, 06:16 PM   #5
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Problem w/ two 240v baseboard circuits on single double-pole switch.


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I suspect you may have a thermostat that does not switch both lines. It only switches one line. That is why only one set of heaters is controlled.
Just saw this post. If it only switches one line, why is it explicitly labeled a double-pole thermostat, and have two pairs of wires, only a single input and single load would be needed if it worked as you suggested.

Note that this replaces another double-pole thermostat, that also had the same problem (but I suspect may have been wired incorrectly).
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Old 03-20-2010, 06:18 PM   #6
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Problem w/ two 240v baseboard circuits on single double-pole switch.


Also see link to the thermostat in my original post (inline where I first mention the RiteTemp), it has full specs etc. and very clearly says 4-wire double pole.
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Old 03-20-2010, 06:39 PM   #7
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Problem w/ two 240v baseboard circuits on single double-pole switch.


I don't know if it makes any difference but according the online manual I found you have the line and load wires reversed. The line should go to the black wires.
Yes double pole should switch both lines and it seems to if you use the switch. It should work the way you want it, if the wiring is as you suspect.
You could try swapping the two line and load connections and see if the other unit comes on. I'm still thinking the stat only switches one line, but the switch switches both.
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Last edited by joed; 03-20-2010 at 06:42 PM.
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Old 03-20-2010, 06:48 PM   #8
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Problem w/ two 240v baseboard circuits on single double-pole switch.


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I don't know if it makes any difference but according the online manual I found you have the line and load wires reversed. The line should go to the black wires.
Yes double pole should switch both lines and it seems to if you use the switch. It should work the way you want it, if the wiring is as you suspect.
You could try swapping the two line and load connections and see if the other unit comes on. I'm still thinking the stat only switches one line, but the switch switches both.
Hmm, the previous switch was wired like that (black to line, red to load), but the printed manual that came with this switch very explicitly said red to line, black to load. Do you have a link to the manual? I couldn't find one online, I don't see it at the RiteTemp site, and HomeDepot doesn't seem to carry it any more (I've had it a while, just got around to installing).

If it's a true DPDT I don't see how it would actually matter, and since it's AA-powered I assume it doesn't have any line-powered electronics.

But I'll check, you could be right. Actually I'll pull out the thermostat and fully test the operation with my meter, to see exactly what gets switched when. Wife on her way home with a new list of chores, I think this might have to wait until next weekend :-)
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Old 03-20-2010, 07:32 PM   #9
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Problem w/ two 240v baseboard circuits on single double-pole switch.


The two poles of a double pole line voltage thermostat probably don't turn on and off at exactly the same temperature, and the thermostat is is not well calibrated, then the temperatures where the circuits turn on and off may be somewhat different. Then if two circuits are controlled, one circuit will "dominate" and the other will be on almost all the time or off almost all the time.
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Old 03-20-2010, 07:39 PM   #10
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Problem w/ two 240v baseboard circuits on single double-pole switch.


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The two poles of a double pole line voltage thermostat probably don't turn on and off at exactly the same temperature, and the thermostat is is not well calibrated, then the temperatures where the circuits turn on and off may be somewhat different. Then if two circuits are controlled, one circuit will "dominate" and the other will be on almost all the time or off almost all the time.
As shown by what data?
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Old 03-20-2010, 09:45 PM   #11
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Problem w/ two 240v baseboard circuits on single double-pole switch.


Double-pole thermostats use one pole to control the loads, thus maintaining room temperature.

They are marked with an "off" position to act as the required disconnecting means, and only disconnect both poles when turned to "OFF"

Single pole line voltage thermostats do not have an off position.

Using a double pole thermostat to directly control more than one 240 volt circuit probably violates the UL listing of the product, since you have not fully disconnected both lines from each circuits when placing the thermostat in the "off" position.

If you want to control more than one circuit with a single thermostat, then you will need to utilize a contactor, and have the thermostat control that.
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Old 03-20-2010, 09:46 PM   #12
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Problem w/ two 240v baseboard circuits on single double-pole switch.


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As shown by what data?
One I bought.

I also noticed that turning the knob to the off positione put a lot of stress on the thermostatic element so as to suggest to me that frequent use of the off position could throw the unit out of calibration.
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Old 03-21-2010, 12:13 AM   #13
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Problem w/ two 240v baseboard circuits on single double-pole switch.


The rite temp page for that stat.

http://www.ritetemp-thermostats.com/80XX/8046.html
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Old 03-21-2010, 05:48 AM   #14
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Problem w/ two 240v baseboard circuits on single double-pole switch.


A. You have it wired wrong. Your using the disconnect pole as a thermostatically controlled pole.

B. That thermostat is NOT rated at a high enough amperage to handle more then 3,840 watts. So how many watts are your 3 heaters?
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Old 03-21-2010, 10:42 AM   #15
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Problem w/ two 240v baseboard circuits on single double-pole switch.


According to the OP, the wiring in the box (whites marretted aka wire nutted), the circuits were originally wired for one double pole thermostat with both poles thermostatically controlled (like mine).

Do they make tandem (dual) double pole thermostats i.e two DP thermostats to fit in a single gang box? I have not seen one.

Otherwise the only way to upgrade to meet code and break both black and white sides of the line is to rip out the box and put in a two gang box for two thermostats.

If the installation is grandfathered then one double pole, both poles thermostatically controlled unit as was there orignally will work fine so long as each feed -- OP stated that one feed serves two heaters -- does not exceed the wattage for a pole.

(The two feeds may not be combined into the same pole e.g. thermostatic controlled pole for both circuits' black wires and switched pole for both circuits' white wires since two branch circuits may not be commingled.)

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