Viessman 200-W and Tado wireless V3+ starter kit connection via KM-BUS

Good morning,

Yesterday the plumber was trying to connect the Viessman 200-W and the new Tado Wireless V3+ starter kit.

All Tado parts worked perfectly (bridge, wireless receiver, and the wireless sensor) however he was not able to connect the Viessman 200-W and the Tado via the Km-Bus (digital bus). He called Tado assistance and connected remotely to my wireless receiver to confirm that all was working well in the Tado). Viessman assistance was closed at the time he called the technical assistance, hence he could not talk to Viessman.

He connected the Tado system as a relay and it worked well to turn it on and off.

QUESTION1 : Do you know if it is worthy to ask them to come back again and connect via KM-BUS? I do not know what extra I will get in comparison with being a relay.

QUESTION 2: DO you know why it did not work out well? Is there any link or video to watch and learn about it?

Thanks in advance for your help.

For reference: I am located in Belgium

Best Answers

  • Montage
    Montage ✭✭
    Answer ✓

    Q1. I thought you guys were heavily into the boiler modulation stuff, like your neighbours in the Netherlands. What you gain is a demand for heat of, say, 40%. On the relay it's 100% on or 0% on. If you have a condensing boiler you will use less fuel by keeping in condensing mode far more of the time.

    Q2. It's a pair of control wires from the Tado wireless receiver to the boiler. He might already have put the cable in place. Then you 'tell' the Tado to use digital control and specify the protocol (D37 in Tado terms.

    There are documents here in a variety of languages. You want the Digital low voltage docs.

    Armchair guess - he didn't loop the SL connection when trying digital. But I wasn't there :-)

    Can you look inside the wireless receiver?

  • Montage
    Montage ✭✭
    Answer ✓

    As a general rule, the digital connections don’t have to observe a polarity. Tado can work regardless of which way round they are

    The old way of doing it is the relay method. This is just a switched live. Often called SL but the terminal on the boiler might use a different naming convention. Applying 230V to the boiler SL terminal is a demand for heat.

    If the boiler doesn’t modulate with the pair of wires connected to KM Bus on the boiler and also the two digital connectors on the wireless receiver (assuming Tado also reconfigured to right protocol) then it might be expecting to also see switched live. The solution would be to loop from a 230V inside the boiler to the terminal that receives the old fashioned switched live signal.

    Hope that is clearer.


  • Many thanks!

    what do you mean by ‘loop the SL connection when trying digital”?

    when trying digital:

    • first he put one cable on + and one cable on - in the wireless remote. But he did not know which cable has to go to the + or the -.
    • second he put one cable in pin ‘1’ and another in the pin ‘2’ in the Viessman. Again he had no clue which cable had to go where.

    As it did not work with the first try, then he interchanged the cables in the Viessman part for a second try. It did not work in any of the two cases.

    is it possible that by putting the ‘coloured’ cables in the wrong pins in the boiler he had damage the system?

  • I should add, don’t assume the boiler uses SL to switch. The installation instructions for it may say otherwise and should be followed.

  • I should add, don’t assume the boiler uses SL to switch. The installation instructions for it may say otherwise and should be followed.