4-wire thermostat but only 2 wires

gjhDecember 4, 2011

All HD had for a 240 V thermostat was a 4-wire Honeywell CT410B. My baseboard heater is apparently wired with power to the heater and a switching wire to the thermostat. I hooked up black to L1 on the tstat and the white to T1. Now the heater is on all the time. What SHOULD I have done. For those of you with protective instincts, yes, I always turn off the breaker when working on wiring.


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You SHOULD have gotten the correct thermostat for the application. Just 'cause HD doesn't have it does not mean it doesn't exist.
Get the proper thermostat from an electrical or plumbing supply house, industrial supply (Grainger, McMaster-Carr, etc), online, etc.

    Bookmark   December 4, 2011 at 10:41PM
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That thermostat should work fine for your situation. One advantage of that one over the single pole is that the CT 410B has a real OFF position. Perhaps you connected it wrong. Why the need to replace the thermostat? Was it heating all the time previously?

    Bookmark   December 5, 2011 at 7:46AM
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Ron Natalie

I disagree with Petey (and with Bus). There is nothing wrong with the thermostat in this application. 410A would have been a better fit but all you're doing is leaving out the second pole on the 410B.

You've wired it wrong, though for the life of me I can't think of how you did it. If you'd hooked up the wrong terminals it would be OFF all the time. I suspect you've somehow shorted the wires together separate from the thermostat itself.

The color of the wiring isn't particularly obvious. I got it wrong myself (my dumb fault for not reading the instructions).

There's not much to this device, the operation is purely mechanical which closes the two switches.

    Bookmark   December 5, 2011 at 8:38AM
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There is no reason to switch both legs of a 240 V load exepct at a disconnect.

The hazard for something like a heater that ay have exposed elements (even if they are out of touch range) is that only switching one side leaves the heating element with 120 V on it.

If anything touches the heating element is is electrically hot, even though it is not producing any heat.

    Bookmark   December 5, 2011 at 9:41AM
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I think you may have connected the wiring to the L2/T2 side.

According to the Honeywell datasheet, L1/T1 is thermostatically switched AND also breaks when turned to the off position. L2/T2 is NOT thermostatically switched and ONLY breaks when turned to the off position.

If wired on the L2/T2 side only, the heat would always be on unless the control was turned to the off position.

    Bookmark   December 5, 2011 at 12:12PM
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Ron Natalie

Thanks Bill, there ya go.

    Bookmark   December 5, 2011 at 4:42PM
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yosemitebill is right on. That is the reason I said what I said. Both poles of a 2-pole t-stat are not the same.

Rather than have a DIY try and figure out which is which I simply suggested using the correct one.

    Bookmark   December 5, 2011 at 5:03PM
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