Open Term vs Relay Connection to Boiler


I am considering purchasing a Tado "Starter Kit – Wireless Smart Thermostat V3+" and lots of "Smart Radiator Thermostat V3+s".

My boiler is a "Remeha Avanta 30s", S-Plan with an external Megaflow hot water tank.

Before I purchase I have 3 questions I hope someone can answer:

1) Does the Wireless receiver in the starter kit allow for Open Them connectivity to the boiler?

2) If not would I be better getting an Extension kit?

3) What are the advantages to using a Open Therm connection to the boiler versus relay control? I have heard that OT might offer boiler output modulation and so be more efficient?

In other-words is the OT interface more than just a low-voltage on-off or does it depend on the boiler and or the version of OT it support?



  • sdmaino
    sdmaino ✭✭✭
    edited February 2021

    I can answer only to question #3

    Yes, OT can give you power modulation , with great advantages in terms of boiler efficiency, while relay control can only turn your boiler on/off.

    In orde to use OT control your boiler must have this protocol and being pre-set to manage different modulation steps.

    Higher number of modulation steps -> better modulation -> higher power save as the boiler will be nearer to the required heat.

  • Tado can’t do OpenTherm and S Plan.

  • Hey Montage - please could you expand on that? Why can't you have S-Plan plumbing but use an OT boiler control?

  • It’s a technical limitation of Tado (and Nest). They will only do ebus OR relay switching.

    If you have a means of solely receiving ebus and that device can itself sort out your valves then you are fine.

  • >They will only do ebus OR relay switching.

    Oh that's a shame, thanks for the info!

  • Tado's website seems to lack detail on what the differences are between the various Wireless Receivers. Based on other user comments in these forums it looks like the following applies.

    1. The UK Wireless Receiver appears to be only able to do relay switching
    2. The European Wireless Receiver appears to be only able to do OpenTherm/eBus and not relay switching
    3. The original Extension Controller as sold in both the UK and EU can do either OpenTherm/eBus or relay switching but not both at the same time

    As far as I am aware myself if you have a Tado product that supports OpenTherm it also supports eBus in that both OpenTherm and eBus use the same digital wiring.

    Contrary to @Montage stated Nest v3 Thermostat only supports OpenTherm and not eBus, it can also do relay switching. Tado is as far I know unique in supporting both OpenTherm and eBus. In general eBus is proprietary and again as far as I am aware only Tado have bothered/managed to reverse engineer the two different eBus systems used by Vaillant and Worcester Bosch.

    Note: In The Netherlands I believe regulations require boilers use OpenTherm so in the Netherlands Vaillant and Worcester Bosch come with OpenTherm instead of eBus, this is achieved in both cases by their including as standard translator modules to convert their proprietary eBus to standard OpenTherm. Sadly in the UK despite the fact these translator modules are made/sold by Vaillant and Worcester Bosch they will invalidate any warranty if you fit them although they will work fine.

    It also appears to be the case that with Tado if you use OpenTherm or eBus you cannot independently control hot water. Apparently Tado say you can only do this if you have a 'combi' boiler. Of course if you have a combi boiler you don't have a hot water tank and then separate hot water control becomes meaningless.

    My visit to these forums today was to research what the differences between the Wireless Receiver and the older Extension Kit are. The fact that apparently the UK Wireless Receiver does not do OpenTherm/eBus is a shock. It used to be that a number of boiler suppliers in the UK used to bundle Tado as a way to do smart 'modulating' control for energy saving over relay switching.

  • " The fact that apparently the UK Wireless Receiver does not do OpenTherm/eBus is a shock"

    Does that mean you cant use modulation/OPenTherm with Tadio if you want to use the Tado Wireless Thermostat?

  • Has anyone received a definitive answer on OpenTherm compatibility with the UK Wireless Receiver?

    I have just received one, I've looked through the provided installation guides and walked through the installation steps on the app for a Baxi 800 Combi and there is no mention of OpenTherm.

  • I'm pretty sure the UK one wont. It doesnt have the digital connections.

    If you compare the UK to English International manuals here, one shows "Parking" (UK) the other "Digital" (EU).

    It looks like the EU one can be bought though

  • Would there be anything special that would to be taken into consideration using the EU version in the UK? I assume any power leads would be 2 pin instead of 3 pin.

    Looking at the English International instructions there is a 2 pin plug for the wireless receiver. Is the UK version powered from the combi boiler or does this also require its own power supply?

  • Yes, they ship a EU socket, but they can ship a UK one if you ask.

    Or you can get an electrician to sort it. Here is what you get:

    I'm not too sure how the UK version comes.

  • The UK version comes without the plug. I don't see any reason why either version can't be powered from a UK boiler, just take the plug wire off and wire it into L and N on the boiler, or just plug into a nearby socket if you prefer. Totally bonkers of Tado not to sell the Opentherm version in the UK, claiming that it "confuses installers". Surely the significant number of combi boilers here justifies it. If they want to lose business to Drayton or Nest that's their affair I suppose.