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Old 07-10-2011, 04:09 PM   #1
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Radiant heater thermostat switch wiring question


Hi folks, this is a photo of a friend's thermostat switch that controls electric radiant heating in a bedroom ceiling. Can someone explain this wiring to me? It doesn't make sense. The switch appears to be single pole. The switch has just two terminals. The circuit is reportedly 240v. Why are there two black wires going to one side of the switch and two black wires going to the other? I would expect to see one hot wire going to the switch and one wire going to the heater, and then the other hot wire (red?) returning from the heater to the panel. The condo was built in the mid 1970's. Sorry, but I haven't had a chance to verify if there is 240v between the two switch terminals and also if the red wire in the box is the return from the heater. I apologize if I haven't given enough info for an answer, but I can get the info in a day or two when I go over to the house. Just wondering what is going on in this switch box. Thanks.
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Old 07-10-2011, 04:19 PM   #2
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Radiant heater thermostat switch wiring question


What does it look like at the breaker side? Post a close up of the label on the thermostat.

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Old 07-10-2011, 04:32 PM   #3
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Radiant heater thermostat switch wiring question


The label with the red letters says "DO NOT PUSH ON KNOB. CALIBRATION CHANGE COULD RESULT."

155-0104

On the switch body it says "CAUTION. HIGH VOLTAGE. DISCONNECT POWER SUPPLY BEFORE SERVICING"

It's a White-Rodgers thermostat switch.

I will go take a look tomorrow and see what the wiring at the panel looks like, but right now I don't know.

Thanks.
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Old 07-10-2011, 05:19 PM   #4
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Radiant heater thermostat switch wiring question


Are there any other electric heating elements in the condo? There are red, black, and white wires in the box. Could this be a MWBC?
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I haven't had a chance to verify if there is 240v between the two switch terminals and also if the red wire in the box is the return from the heater.
Reading from terminal to terminal on the T-stat should show 240V with the T-stat off or turned way down if it is 240V circuit.
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Old 07-10-2011, 06:49 PM   #5
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Radiant heater thermostat switch wiring question


I assume it is a MWBC using the white wire as the balancing neutral. But I don't get the wiring going to and from the switch. I'm going to look at the panel, test for voltage between the two sets of wires.

Yes there are two other bedrooms and they too have radiant heating. I was thinking that I will find each bedroom heating element has its own dedicated branch circuit, but you seem to be suggesting that they may be wired in parallel. I thought that they would not be wired in parallel as this would have too high a total amp draw.

Thanks
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Old 07-10-2011, 06:53 PM   #6
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Radiant heater thermostat switch wiring question


Go look at the connection point on the heater and see if there are two blacks up there.
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Old 07-10-2011, 07:04 PM   #7
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Radiant heater thermostat switch wiring question


Will it be possible to view the connections at the heater element if it is in the ceiling and there is no crawl space above (they are on the first floor of a 2-story building)? Sorry to seem so difficult. Thanks.
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Old 07-10-2011, 07:25 PM   #8
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Radiant heater thermostat switch wiring question


There should be an accessible panel at each heater. That would be the best way to start. The circuit breaker panel should have a two pole breaker, if 240 volts, identified for the heater circuit.
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Old 07-10-2011, 07:42 PM   #9
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Radiant heater thermostat switch wiring question


Quote:
Originally Posted by tumblinman View Post
I assume it is a MWBC using the white wire as the balancing neutral. But I don't get the wiring going to and from the switch. I'm going to look at the panel, test for voltage between the two sets of wires.

Yes there are two other bedrooms and they too have radiant heating. I was thinking that I will find each bedroom heating element has its own dedicated branch circuit, but you seem to be suggesting that they may be wired in parallel. I thought that they would not be wired in parallel as this would have too high a total amp draw.

Thanks
You can have 3840 watts on a 240 volt 20 amp circuit. If each heater was on an individual circuit you would have a breaker in the panel for each circuit.
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Old 07-10-2011, 07:43 PM   #10
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Radiant heater thermostat switch wiring question


If there's an accessible panel for the heater or heating element that will be great. This is interesting to me. Thanks.

Do you think it's possible that the heating element may have two halves (hence two switch legs?
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Old 07-11-2011, 09:49 PM   #11
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Radiant heater thermostat switch wiring question


I think I figured this out. There are no access panels so I was unable to see the wiring at the heater elements.

There are two switched heaters on one double breaker, which explains the two wires on one side of the 2-terminal tstat switch. There must also be more than one element per room and each has a switch leg. I know this because when the two black switch leg leads were separated, one had 240 volts to the hot on the other side of the switch, and one did not. The one that did is the hot going through the element to the switch leg. The one that did not is the hot going to a broken element (this heater does not work) so there was not 240 volts between it and the hot.

One other thing I noticed; at the panel, of the two breakers tied together to produce the 240 volts, a black wire and a white wire were used. I didn't expect to see a white wire inserted into a breaker. That white wire, which is the 2nd hot, goes to the red wires, which must go to the heater elements, and then from the heater elements, black wires go to the switch.

Thanks again.
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Old 07-11-2011, 09:53 PM   #12
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Radiant heater thermostat switch wiring question


That white wire needs to either be colored Red or Black (use a Sharpie).
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Old 07-11-2011, 10:05 PM   #13
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Radiant heater thermostat switch wiring question


Quote:
Originally Posted by tumblinman View Post
One other thing I noticed; at the panel, of the two breakers tied together to produce the 240 volts, a black wire and a white wire were used. I didn't expect to see a white wire inserted into a breaker. That white wire, which is the 2nd hot, goes to the red wires, which must go to the heater elements, and then from the heater elements, black wires go to the switch.

Thanks again.
FYI - this is perfectly acceptable but as posted, the white wire is to be re-identified at both ends as black using tape or a marker.

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