I want to replace my thermostat with raspberry pi board. I have already connected solid state relay to rasberry pi and it works just fine. The only part left to do is to connect solid state relay to the thermostat. I have an idea how to do that but before actually connecting anything I want to ask your advice if I am doing it right. My solid state relay looks like this

solid state relay
(source: asia.ru)

And the diagram on the thermostat looks like this


I have 4 cables

Blue - N
Red - L
Call - yellow
and ground yellow/red

So I think I should connect Red(L - Red) to SSR contact 1(ac in) and CALL (yellow) to the SSR contact 2 (Load). Is that the right way to do ?

P.S. Its in UK so voltage is 220V therefore I want to be extremely careful

  • 1
    \$\begingroup\$ If you were being really careful, you'd replace your thermostat with a thermostat! \$\endgroup\$ Nov 13, 2013 at 22:28
  • \$\begingroup\$ Does anyone know what "CALL" means in this context? Is it "Call for heating" (or cooling for an air-con)? \$\endgroup\$
    – Transistor
    Aug 21, 2019 at 6:24

1 Answer 1


connect Red (L - Red) to SSR contact 1 (ac in) and CALL (yellow) to the SSR contact 2 (Load)?

Yes, that seems correct. You should make safe the neutral wire and prevent it from any possibility of coming into contact with any other wiring, metal parts or the outside.

It's in the UK, so voltage is 220V

The EU standard is 230 V ± 10% but in the UK you'll typically measure 240 V which is within this range and is what substations for UK domestic supplies were previously standardised on. Other EU countries had previously standardised on 220 V, not the UK.

I want to be extremely careful

That means going to the consumer-unit (fuse-box) and switching off the circuit marked "boiler" or "heating", then checking your thermostat connections with a safe voltage tester (preferably non-contact) before rewiring them. keep the Pi well away from 240 V wires, ideally in a separate, well-insulated box.


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