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Saunier Duval ISOTWIN F 24 E Installation to Smart Thermostat - 1046842


Further to my previous post https://community.tado.com/en-gb/discussion/comment/4031#Comment_4031 ,

I have taken photos everywhere to determine the exact cable routing and boiler type.

Removing the boiler covers reveals the Saunier Duval ISOTWIN F 24 E identification label along with the relay switch cabling and an "open relay" 0° diagram to the left of the relay switch cabling. The brown cable is on the right and the blue cable is on the left of the relay switch connections.

The cable from the relay switch connections goes into the following junction box.

So the:

  • brown cable of the boiler relay switch is connected to the black cable going to the thermostat
  • blue cable of the boiler relay switch is connected to the orange cable going to the thermostat

The other red, blue, and green/yellow cables are the mains AC cables that go to the thermostat (the boiler mains cables are on a plug, which is plugged into a socket).

At the thermostat, as stated previously in the other post :

The AC mains cables are as follows:

  • green/yellow earth cable never connected or plastic covering stripped back
  • the blue (neutral cable) is on terminal 1 of the original thermostat baseplate
  • the red (live phase) is cable on terminal 2 of the original thermostat baseplate
  • nothing connected to the terminal 2 of the original thermostat baseplate

The switch relay cables are as follows:

  • the black cable (which is connected to the brown cable at the right of the boiler relay) is on terminal 4
  • the orange cable (which is connected to the blue cable at the left of the boiler relay) is on terminal 5

So here's a diagram of the current progress so far :

The unnecessary (red and blue) AC mains cables are in the parked (unused terminal positions).

For now the orange and black boiler relay cables are not yet connected to the tado° Smart Thermostat baseplate.

The boiler is now powered back on, and obviously produces hot water on demand from my taps (as it's a combination boiler).

Also, since the relay is "open" (i.e. the orange and black cables are not touching each other), the boiler is at 0°C configuration for the radiators, and thus not supplying hot water into the radiators for the moment.

I had a look at the service manual quickly for this boiler, and came across this in French, which basically says connect the cables from the thermostat as I already mentioned above. The manual however does not seem to imply whether this boiler is in Normally Open (NO) or Normally Closed (NC).

I assume the boiler is a Normally Open (NO) type, where the normally open relay means that the boiler is not heating water for the radiators, and that when the relay is closed it will heat the water for the radiators. However, boilers are not my domain, so at this point I will wait for support to answer my ticket 1046842 so that I can complete the installation.

From all of the above information, I would assume I can connect the orange cable to the COM terminal on the tado° Smart Thermostat, and the black cable on the NO terminal on the tado° Smart Thermostat.

However, I am "standing by" on tado° support to confirm the next steps.

P.S. Frank (from tado°), please fear not. I will not do anything to void the warranty (I could apply a LOTO procedure in the house, but there's no risk of anyone electrocuting themselves :-) )


Rob Wisbey (https://robertwisbey.com)


  • Hi again,

    Just a bit more information.

    I think my configuration with the old thermostat was like this :

    And with the battery operated tado° Smart Thermostat will be like this :

    I came across a spare parts manual (in French), which shows the thermostat connection (TA) :

    But still not 100% sure if the boiler is of the type: call for heat (normally open NO) and satisfied (normally closed NC), and thus whether one of the relay cables needs to be connected to NO or NC.

  • Well, the connection is open now, and its not producing heat, so its a normally open switch. Which I think is true for most boilers. Why dont you try tricking it, by shorting the two wires, and see if the boiler fires? That is assuming you double checked there is no main voltage on them (and there is 220v on the ones you parked).

    If that works as expected, you can triple check your wiring by measuring resistance on the tado side as you increase/decrease the temperature until you hear the relay latch, and measure if the right ones "close".

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