Relay and Opentherm together

The Tado extension kit can control the boiler through a relay (simple on-off switch) or through the Opentherm bus. The bus allows for better temperature stability and higher efficiency as it tells the boiler to turn the heating up or down as necessary.

If a heating system contains other elements, like a shut-off valve or external pump they would require the relay to switch them on or off. Unfortunately there's no way to use both control mechanism together: the Opentherm bus for boiler heat output and relay to turn on other elements of the system.

It would be really nice to have a hybrid mode where the relay and Opentherm bus work together, possibly with the relay switching the auxiliary devices off after a short grace period.

This seems to be rather simple thing to implement in the firmware, but there's no option in the settings menu to enable both.

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  • Would this work with an extension kit? Two wired thermostats each connected to the zone valve, plus an (EU) wireless receiver talking to the boiler on opentherm or similar?

  • Rob
    Rob | Admin


    That would work. Or, alternatively, two wired thermostats each connected to the zone valve, plus a third wired thermostat talking to the boiler on opentherm or similar.

    It will only work properly when the thermostats wired to a zone valve are configured as a relay thermostat, and their rooms use the tado device wired to the boiler as the zone controller.


    I see your point, but this isn't just a simple change in firmware. It would change a lot of other factors in how our system is built as well. I guess it would be possible, in theory, but the amount of work on our end would be large. My expectation is this will only be considered when there is a strong demand from the community. Which is something I am not seeing at the moment, but I'm very open to be proven wrong.

  • Montage
    Montage ✭✭✭

    Having gone through this and drawn a blank with Tado nearly two years back, I can appreciate the frustration. Despite there being plenty of people posting on here about it, we remain a tiny minority.

    I think the S plan/Y Plan world has moved on and hot water priority and X Plan with a high recovery cylinder is the next thing. One of the smart thermostat big players seems to support this and can do OT whilst energising the relay.

  • Hi Rob,

    Thanks for your comment. What you're saying is that for 2 zones it would take 3 Tado devices, say, extension kit controlling the to the boiler through the Openthem bus and 2 wired thermostats controlling their zone valves through the respective relays, correct?

    How does the Extension kit (or 3rd wired thermostat) know how much heat to request from the boiler through the Opentherm for all zones? Could you point me to some example how to set-up such system, please?

  • This functionality is badly needed and you should add it.

    For YEARS this has been outstanding and it's the reason I install competitors controls that do support both a relay output and opentherm at the same time.
  • @Rob

    When you say "It will only work properly when the thermostats wired to a zone valve are configured as a relay thermostat, and their rooms use the tado device wired to the boiler as the zone controller."

    Can you please confirm that a relay stat will still engage aswell as the OT zone controller? How would you do this with say, 2 relay stats and a 3rd OT'ed to boiler?

  • Really? No response to how we can set tado OT with two zones?
  • Hi, I’d also like to know how this can be configured. I would like to run multiple zones using Opentherm from the extension kit and wired thermostats to control an external pump and zone valves.
    My concern is that extension kit could call for heat without the thermostat relay operating, or vice versa, the thermostat relay operates without the system boiler being fired.
    My question is whether there is synchronisation between opentherm firing the boiler and the thermostat relay operating?
  • Could anyone from Tado explain the setup, please?

  • I would also like to know if there is any combination of Tado devices that can currently control 2 Zones while still maintaining OpenTherm control?
  • Klaus_Ludwig
    Klaus_Ludwig ✭✭✭
    edited October 2022

    Some systems seem to manage to integrate S Plan with Opentherm, so I guess it must be technically achievable:

    Is it viable for Tado to develop a system that supports Opentherm on Y-Plan and/or S-Plan? I hope so and I definitely support the OP @Tomek on this upgrade suggestion.

  • I would like to see a Tado “supported” multi zone setup using wired thermostats (or wireless as the measuring device) and an (EU) extension kit.
    It does work with my Remeha Avanta Plus boiler. I ensure the zone valves & external pump can’t operate unless the boiler is fired by using a normally open relay contact that closed when the boiler is running. That way the external pump isn’t trying to pull water through the boiler if it isn’t operating.
    When I change the boiler I will probably have an issue unless I can replicate the boiler run relay contact.
  • Open therm coming back to the UK. See the heat geek video below.
  • swwils
    edited November 2022
    Where is @Rob where is opentherm WITH relay
  • Bump.

    @Rob please respond.

    Add this badly needed configuration.

    Opentherm + RELAY on the extension kit!!!!

  • I have been using the thermostat relay for my underfloor heating zone for a few weeks now. I have observed similar behaviour to using relay switching with the extension kit for heating control, but with one additional variable. I can see when the heating demand is low signified by a single “~” the relay will be switched off for a percentage of time. This is particularly prevalent when opentherm has set the flow temperature high to satisfy other (radiator) zones that are demanding three bars ~.
    When opentherm has set the flow temperature relatively low e.g. 40c, with multiple zones at one ~ of demand, including the thermostat relay zone, then the rely remains active for the zone whilst there is demand.
    In conclusion, there seems to be a relationship between the relay operating in the thermostat, demand level and flow temperature to satisfy the required total energy requirement.
    Ideally a technical explanation from Tado would be very helpful, rather than trying to understand this behaviour through observation.
    @Rob your thoughts please
  • This is an absolute must for the UK market, where so many central heating systems have zone valves.
    This could be done via firmware, just give an option to allow the relay to continue to operate in sync with the demand signal. Simple
  • Never going to happen though. I’d look for other solutions.

  • I’m considering switching to Honeywell to make this work and only use Tado as TRVs perhaps. I’ve invested way too much into the ecosystem to make a sensible switch. 6 years with Tado and 5 years since requesting this feature

    Would love to see this finally implemented!
  • This is frustrating, I'm in the UK using a Nest V3 and Opentherm on a Baxi 800 system boiler but I want better control of individual room temperatures as I use the office during the day but not the rest of the house and the opposite is true during the night. I've the added problem that some of the house has UFH and the rest has standard radiators and we use a thermal store to buffer hot water to the UFH and the DHW. The thermal store is heat pump ready.

    The Tado system looks like it has the capacity to do much of what I need but it 'must' combine control of the valves required to direct water through the hydraulic system (S Plan) and set the correct boiler temperature appropriate for the demand through Opentherm. Currently, I don't think the UK product does.

    The UK community have been asking for these features for months and if Tado can't see that this is the way the UK has to go then some-one in marketing needs to look for a new job. And others will pick up the gap. Tado has the capability to do this, they need to respond to customer demands and market changes. And when they do, I'll install Tado.

  • Likewise, this is a showstopper for me. I have an OpenTherm boiler and very large detached property (6000 sq ft.). Currently I only use a few Tado radiator valves and cheap OpenTherm thermostat for controlling boiler, but obviously they don't work together.

    My house is too big to run off a single zone (if I did that then I end up with a much bigger boiler than I need), so OpenTherm with 2 or 3 zones is needed.

    HoneyWell do an OpenTherm bridge which allows multiple HoneyWell thermostats to work together on OpenTherm I believe? ( but I really like the Tado system.

  • Bimi
    edited March 2023

    Was hoping to find a solution here but disappointed.

    I got a new system boiler Ideal Vogue Max and 16yrs old Cylinder Tank. Got myelf the TADO EU model which supports OpenTherm as was aiming to get Opentherm and PDHW.

    Problem is: how the hell is boiler supposed to control HW / CH via the Relay switch?

    Ideally would expect the OT to control CH and them the relay to switch on/off the HotWater!!

    Am I hitting the dead end here?
    Ultimately maybe I have no choice but to get NEST GEN3 who is supports HW / CH and Opentherm.

  • Montage
    Montage ✭✭✭

    Time to buy something else. Tado have been clear they don’t see the market for OT combined with relay to be big enough for them.

    I’d look at Nest, as Urban Plumbers on YouTube did PDHW with it and a Vogue.

  • andyblac
    andyblac ✭✭✭
    edited March 2023

    IMO OpenTherm is not necessary and is just as good as Weather Compensation if setup correctly, as both do the same job really, (OT is just Demand Compensation). I am in the process of looking for a better alternative to Tado, as tado support is none existent imo. From the dealing I have had with Drayton about their Wiser system they seem way more supportive about there customers needs..

    I would just sick to double relay, and add a weather comp sensor to your boiler.

  • Cheers guys.

    I was under impression that weather comp is not easy to setup & configure correctly also may give you false reading impression from outside.

    Whereas OT is more clever and can overule it but wth weathercomp ur inside temp is dictated by whats given by the tool.

    May I ask if S Plan is setup and weather comp added, how will that modulate the temp flow to CH even tho in systemboiler thats not possible unless u switch to X plan?
  • stevieg1
    edited March 2023
    Come on Tado are you not listening you have potentially hundreds of new customers and existing customers who looking to leave due to your failure to respond.
    Why does Tado not interact with the community October 2022 was last interaction from @Rob
  • johnnyp78
    johnnyp78 ✭✭✭
    Tado appears to have left the building. Probably not the best sign of a healthy company. If the next generation of matter compatible Tado hardware ever arrives it will be a lot easier to do things like this, until then I would look at a different heating system.
  • @johnnyp78

    Very interesting turn of phrase......." Tado appears to have left the building "

    I think your comment has hit the nail on the head.......exactly.

  • Well… sadly it leaves me no option but to go with NEST GEN3 and possibly add the Tado SartTRVs as nice feature.

    Shame! What a shame of TADO! Seems to be OK for combi boilers but not good for system boilers!

    Now need to figure out how NEST does work with Opentherm and PDHW. Any ideas let me know
  • I think someone else may have mentioned this video.