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When removing and tagging the wires from the box to the old Thermostat, the two Neutral (White) were wired together with a wire nut, and a single White wire (pigged-tailed) was going into the Thermostat. One Black (Gray) into LOAD and One Black (Gray) into LINE were connected into the old thermostat.

The New Thermostat is a Honeywell CT410B that has 4 wires - L1 (Black/LINE)to connect Power Source/Breaker Circuit. Then L2 (Red/LINE) to connect Power Source/Breaker, The T1 (Black/LOAD), and T2 (Red/LINE) to connect to Radiator.

Question? Do I separate the two White to connect to new Thermostat (L2) separate (T2),

OR, Where can I connect the connected single White to what connection on the new Thermostat?

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  • You're dealing with a 240V heat circuit, right? – ThreePhaseEel Jan 17 at 19:46
  • If it's 240V, they aren't neutral -- they're hot wires that somebody didn't bother to put red phase tape on. – Harper - Reinstate Monica Jan 17 at 22:15
  • Figures. I knew something was not right. I will call an Electrician to test the wires to see which one is coming from the Circuit Breaker panel and other to the Radiators. THANK YOU! – Kevin Gatta Jan 18 at 20:25
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This is a common configuration, the thermostat shows terminal 2 as the line.

With 240v split phase heating it is very common to only open one of the hot conductors this makes for a cheaper thermostat.
(This leaves 120v to ground live even when the thermostat is in the off position.

I would see where the line connected to terminal 2 goes where it joins up with the white that should be the other hot.

The white should be reidentified with any color other than white, gray or green. Most commonly black is used if done at all. I use a black sharpie / Permanent marker often.

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  • Great advice. Yes a Sharpie does the trick. I'll have an electrician make the test for the line and the proper connection. Thanks a lot! – Kevin Gatta Jan 20 at 3:43

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