Best way to wire Extension kit for weather adaptation/modulation?

Hi, Got a new Worcester Bosch 8000 Life boiler and the electrician/heating engineer don't have tado experience.

I have the extension kit and smart thermostat, smart TRV's etc. gravity fed hot water cylinder circuit with 2 port valves etc

They are wiring the tado extension kit in the relay/switched live config in the extension kit, not using the Digital bus.

Am I losing something in weather adaptation/modulation on the heating side by having it wired like this.

They couldn't work out if they used the digital bus, how to control the 2 port valves on the water cylinder side, that why they used the "normal" way.

Make sense?


  • Pete
    Pete ✭✭✭
    I have a WB 30CDi modulating it's a feature worth having. I had to fit the WB hot water diverter valve with tank sensor. Mine fitted inside the boiler and then is controlled by the digital bus.
  • I guess the question is, how does it modulate differently through digital bus vs the standard relay? The modulation happens at the boiler based on return flow temperatures as I understand. I also read the tado weather compensation adds no modulation smarts, just adjusted the temp of the TRV’s accordingly. If this is the case, there is no additional benefit of the digital bus vs the standard switched live setup?

    This is what I read, not saying this is the case. Anyone can confirm this?

  • Pete
    Pete ✭✭✭


    Every gas/oil boiler modulates its output based on return flow and demand. This stops it from getting dangerously hot and also enables it to condense more (hence improving the efficiency). Before the modulation control, my boiler was set at 65 degree flow and it would achieve that whether one radiator or all radiators are open. It does that by modulating it's output internally.

    This is fundamentally different to the target temp modulation via the digital bus.

    From my personal experience, the boiler reduces its target temp to sometimes 50 degrees, other times 70 degrees. My understanding is that it does this based on the demand from the TRVs (how far from room target temp) but also based on the temperature outside (based on internet local weather report).

    The more time the boiler spends at a low flow temperature, the more it will condense and the greater the system efficiency will be. Additionally the variable flow temperature improves comfort by balancing the heat input with the heat loss so as not to massively overshoot the target temp when weather is mild outside.

    Hope that helps.

  • JTT74
    edited February 2021

    OK, so I’m starting to understand better now.

    My current switched live S-Plan setup will get the standard boiler modulation based on return flow temp.

    But, unless I connect via the digital bus, It won’t get input / target temperature modulation from tado.

    My guess is that will give me some additional benefits, but not a game changer ( a few percent improvement in efficiency max, as I have smart TRV’s everywhere)

    Anyone know if the Digital Bus on WorcesterBosch 8000 lifestyle works with tado extension kit?

    If its an easy change, I will do it, if it involves cost, prob not worth it

    Sound sensible and logical?

    thx all,

  • @JTT74 the advice from @Pete is correct, all modern boilers can modulate regardless of how they are wired up. The terminology you are looking for is load compensation, which is when the boiler flow temperature is adjusted relative to the heating load of the room. This can only be achieved with a digital bus.

    Weather compensation can be a big money saver in autumn and spring when you don’t need the radiators to be extra hot to heat a room to the target temperature. Running them at a cooler 50°C, or less, will allow your boiler to maximise the amount of heat it recovers from the flue gases.

  • Thanks, I guess you can also decrease the max temp in the boiler manually also, the old fashioned way.
  • Manual adjustment will give you a similar result. It doesn’t have to be changed often. Set the flow temperature too low and you’ll soon notice that some rooms are struggling to reach the required temperature.

  • It’s a shame Tado won’t do the digital bus and the relay signals at the same time. It makes it a lot more tricky for S plan

    if someone can make a simple knob turner with a stepper motor I could turn the thing up for HW then down a bit for CH the rest of the day.

  • Pete
    Pete ✭✭✭
    I agree @Montage . Relays along side the bus wiring makes a lot of sense and gives much great flexibility. I've got 2 hot water cylinders and it is presently fudged. No real way of working properly with bus and tado as far as i can see.
  • Got the same response when I contacted Google about Nest.

    A breakout box that understands OpenTherm (say) and presents relays as well as passing the OT signals would do it. Bit like the old Vaillant VR65 control centre.

    Neither Tado nor Google seem to understand it’s a problem they could easily solve.

  • I think it’s also worth checking the minimum output for the central heating circuit on the boiler. On my Vaillant Ecotec 938 it’s minimum is around 6.7kwh at 70oC and about 7.1kwh at 50oC so no matter what I do it always ends up being the boiler cycling Olympics.

    To mitigate this somewhat I don’t let every TRV call for heat, but I do set a schedule for each TRV. This means less important rooms will get heat if another zone is on, but if not it’s tough. For example in bedrooms the rads might only be 800 watts at best, thus no matter what Tado or the boiler does it can still not maintain such a low output, so efficiency is lost stop starting all the time.

    I limit the heating water temperature at the boiler too (as you are suggesting) however am using eBus so do get tado modulating. However it’s not great as if I don’t set an artificial limit, tado is happy to push 75oC to lift house by just 1.5 degrees which seems potty to me.

    IMO the UKs use of these big combis means we will never get anywhere near the rated efficiency no matter what we do as the boilers simply cannot rate down enough.

    I notice heating more rooms up as a group at similar times is more efficient than letting Tado call whenever it feels like it. It’s also got to be better for the boiler, as 6-8 starts an hour can’t be good for it.
  • Hi Gary333. My name is Vlad and I live in Romania. I currently use a Vaillant green IQ boiler manufactured in 05.2019 and a Danfoss link system with 5 electronic thermostat heads. The boiler control is On / Off. The minimum power is 4 kW and the maximum is 26 kw. I thought I'd buy a Tado kit with wire and 5 heads. I would like to communicate via Ebus. Is it worth making this change? It will work ? I put a question on the forum but I didn't get any answer Thanks in advance, Vlad.

  • Hi Vlad, I would say it's certainly worth connecting via eBUS if you've got the Tado equipment already that can do it (or are looking to replace non-smart controls). I don't think I would buy extra equipment to make it work if not though, as I am not sure it saves a massive amount of money in energy usage (well at least not for me). It can make the rooms a bit more comfortable though as the heating stays on longer at lower temps.

    If your boiler is a combi (with no seperate water tank / valves etc). then it should be a real simple connection. If you have have a more complicated system then the setup could be more difficult as you might have the newer wiring centre that is not supported by Tado.

  • Hi Gary333. Thanks for the reply. I haven't bought any Tado systems yet. I think the Tado wired kit would be right for me. My plant is of the latest generation and is very economical. The Danfoss system is also very efficient. I would have bought a Tado if it provided modulation of the boiler. Thanks for the reply. Respectfully, Vlad.