I have an old Satchwell/Sunvic TLX2203 thermostat.

Satchwell TLX2203

It has 3 wires connected to it which look to be:

  1. switched live (green)
  2. live (red)
  3. neutral (black)

(at least I think that's the case)

I would like to connect this to a more modern line voltage thermostat like the Sinope TH1123ZB. The recommended wiring looks like this:

enter image description here

My understanding is a line voltage thermostat should sit in series with the live line (basically sit in the middle of the live line between the heating unit).

How can I wire this up given the existing wires I have?

Edit: I may not have the correct terminology as I'm new to this. Perhaps line voltage is not what I need here? Either way, I'm looking to understand what my options are to replace the current thermostat with something more modern (ideally zigbee compatible) -- what I have is the wiring pictured above and it is a 240v line.

1 Answer 1


Step one would be to confirm the function of the wires -- that might involve looking into the circuit breaker panel and the wiring compartment on the heaters. You might use a volt meter and/or a non-contact voltage detector to confirm which is the hot/live wire and which is the switched live/load wire going to the heater.

Based on the diagram you showed for the Sinope this device does not require a neutral connection. The neutral could be left capped so that it's safe but available for future use if needed. Then the live/supply wire would connect to one wire of the Sinope device as shown by the L1 connection; the switched live/load wire would connect to the other thermostat terminal. It appears this thermostat doesn't care which of its wires is the live and which is the switched output.

It looks as if the cable directly enters the old thermostat without any wiring box inside the wall. The Sinope appears to use pigtails for its connection -- it might not have a built-in junction box as the old thermostat did. In this case you may need to cut in an "old work" junction box to provide wiring space inside the wall. Pictured below is an old work box made by Carlon (photo from Gordon Electric Supply).

old work box

  • Thanks for the great answer! One thing I'm unclear on, is that my current Satchwell thermostat simply clicks the switch on/off based on temperature. But the Sinope and other thermostats are considered "line voltage" thermostats. Will they be able to click the power on/off? Or instead is it more of a gradual management of the voltage? (like a dimmer).
    – AdamK
    Oct 27, 2020 at 1:36
  • @AdamK I haven't done any survey of line voltage thermostats.. but from an engineering perspective, I know that dimmer-like functionality costs more than on/off functionality does (price of components, dealing with more self-heating, installation complexity). I think you'll be unlikely to find a consumer-grade modulating thermostat.
    – Greg Hill
    Oct 27, 2020 at 4:13
  • Reading more online, it seems "line voltage thermostats" are more for heaters or fans that can take any intermediary state (en.wikipedia.org/wiki/Thermostat#Line_voltage_thermostats) - eg. a fan can go slower or faster or faster, etc. My Satchwell thermostat is 2 state only - "on" or "off" so I think (?) a line voltage thermostat would not be a direct replacement for that. I'm not sure what a thermostat is called where it is just a simple on/off switch that changes based on temperature. I will have to keep hunting for that.
    – AdamK
    Oct 27, 2020 at 6:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.