I'm replacing a old Eaton M402 mechanical 2-pole 240v thermostat with a Cadet TH114A-240D-B double pole, 240v DIGITAL tstat.

In the junction box, I only have 2 conductors plus ground. I was expecting 4 conductors. See attached picture for how the M402 is currently wired, note that the black and red wires on the M402 are connected to each other:

enter image description here

Here is my double pole breaker (off): enter image description here

The new Cadet TH114A-240D-B looks like this: enter image description here

So the wiring for the new T-stat looks much different than the original. Can you help me understand what I need to do? I saw somewhere that in order to get a digital double pole tstat working I need 4 wires?...

The voltage at the electric baseboard is reading 240v.



1 Answer 1


You can't get there from here

That thermostat simply will not work in that location. The wiring to that thermostat is wired as a switch loop.

This is the 240V version of "no neutral wire in the switch box, so no smart switch".

Back to the store it goes. They probably get 30% of them back this way.

Go 24V system

Fuel furnace thermostats run on 24 volt DC low voltage. Your best option is to install the ~$25 worth of parts needed to convert this to a 24V thermostat system. Go back to the heater (there's usually plenty of room in there) and fit a 240V-24V transformer and 24V-240V contactor. Have the contactor switch current to the heater (preferably both legs).

On the 24V transformer, you define the two terminals as R and C. The ideal color for C is blue, so get a multi-color pack of electrical tape to make this clear to yourself and the next guy. R is ideally red.

On the contactor coil, one terminal is C and the other one is W (ideally white).

On the cable to the thermostat, those are R and W. Don't use the ground wire.

So now you just connect them up: R to R, W to W etc. You'll need a jumper wire from thermostat C to contactor C.

At this point, it should work with the old thermostat, because the old thermostat is dumb, and doesn't care if it's switching 12A@240V or 0.05A@24V.

Now you can change it to a "smart thermostat" of the 24V persuasion, which is a huge variety including the Nest. I recommend one that is capable of functioning without a C-wire. If the contactor doesn't flow enough current for that to work, the factory can instruct you how to connect a special resistor module in the heater between W and C.

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