New Viessmann Install + Weather Compensation, Tado Confusion

Hi All,

Forgive me for asking potentially a stupid question....

I just had my old combi boiler (with Tado) swapped out for a new Viessmann 100-W 35kw with the weather compensation fitted.

The boiler installer disconnected the Tado from the boiler, and only left power going to the Tado extension kit (so that we can actually control the Tado).

He advised to just set Tado for all rooms as always on and permanently at 1 degree higher than the boiler temp on the Viessmann in their app.

I understand that using the weather compensation, that the boiler is basically always on and it adjusts the water temperature.

Right now, all of my rooms are at 20 degrees, except the 3 biggest rooms in the house - which are coincidentally the rooms we spend most of our time in (living room, kitchen etc).

But if I turn the overall temperature up, then our bedrooms will get very hot...

I think I need Tado installed to at least do the geofencing for when we leave etc, but I'm not sure exactly what I should be doing as it seems that the Tado is basically useless now??


  • GrilledCheese2
    GrilledCheese2 ✭✭✭
    edited April 14

    It's unclear what Tado devices you had installed and how the new boiler is now controlled. Perhaps a lack of knowledge about Tado is why your heating engineer has disconnected it. Other Viessmann owners on this forum report using Tado in relay mode to switch the heating on/off, and not using Tado's digital interface when the boiler has weather compensation. This will allow the boiler to adjust the radiator/flow temperature and Tado to schedule the heating times.

  • Montage
    Montage ✭✭✭

    Sounds like you need to adjust your system. Assuming you have TRVs upstairs, then set them low enough that the bedrooms don’t overheat.

    As @GrilledCheese2 says, you just want Tado to have a switched live to the boiler, so it can interrupt that when you are all away. From a Viessmann perspective, it’s just tuning your weather comp curves to suit the property and system.

  • Ah yep sorry about forgetting to include that!

    I have the boiler extension kit wired directly into my boiler.

    Then I have the TRV's in every room in the house, plus a couple of wireless temperature sensors in rooms where the TRV's weren't giving an accurate room temp reading.

    The new boiler is a Viessmann which has an app where you can set the room temperature, but there is no internal thermostat - so I'm confused as to how the boiler knows that the house has reached 20 Degrees?

    I've also seen people talking about having the Tado in OpenTherm mode, and wired in accordingly? But the Tado instructions don't seem to mention this, or that it's even an option.

    This makes sense thank you - So with the Viessmann set to 20 degrees (still confused over what the point of this is if there's no thermostat anywhere??) I then set all downstairs TRV's to 21 degrees, and the upstairs to 20 degrees so they shut off when it gets too warm.

    I guess adjusting the curves is just a case of trial and error?

    The one issue I seem to have is downstairs in the larger rooms (Living room etc) these rooms are 17 degrees, when the other downstairs rooms are 20/21 degrees.

    In one room, the radiator doesn't even seem like it's on?? It's very cold to the touch, whereas other downstairs rads are at least warm. I've bled everything so there's no air anywhere.

  • Montage
    Montage ✭✭✭

    Sounds like you have a lot of functionality that you no longer require. I don’t use Tado SRTs, but I presume you’d set them to independent if you haven’t already, so they essentially work like my dumb TRVs.

    I’m going to assume your boiler installer has really kept to swapping the boiler over and is not taking responsibility for the system as a whole.

    There are two distinct approaches to WC. A common one is to use some kind of on/off control and have the WC control the flow temps. You may have read of that. There is the ‘all in’ approach where you solely control the heating using WC, via the curves. This latter one seems to be what you have.

    It’s a bit beyond the scope of what I like to get into on the Tado forum, so I’ll keep it brief. Imagine you have a home and know the heat loss, having made calculations. The heat loss will be fairly predictable and the inside and outside temperatures (the delta) will be the factors that change it. If those are the 3 factors in the equation, you really just need 2 and can derive the third one.

    I’d expect the WC curves will need tweaking as the heat loss will not be exact. Using those and maybe some suitable overnight set back temp and you should be comfy.

    Hope this helps.

  • GazzaH
    edited April 16

    Hello, the new Vitodens wifi boilers have optional wifi for their app and controls but don't use it at all if you're using other controls.

    Phone Viessman technical and say that you just wish to control the boiler via 230V relay control via another make of controls and get them to tell you how to reset it back to basics or get your installer to do all this.

    Then set the Viessmann weather comp on the front of the boiler to say 21C if that's the highest you'll call for on the Tado as a room thermostat.

    I've fitted a Viessmann Vitodens 050-w wifi to a house with an old lady who didn't want to lose her old basic roomstat so to comply with boilerplus I fitted a weather sensor. She has never had the app and it works absolutely fine.

    I had another customer the same but a Vitodens 100-W wifi who wanted to keep his old control but have weather comp but he also wanted to see the temperatures in the app too so I got him access to the app. This may have been a mistake though.

    I need to go back there as there is now some kind of conflict or he's done something in the app.

    Another customer, I fitted Viessmann's own Vicare controller and it unlocks all the Viessmann app and the front of the boiler then shows the actual flow temp and not a weather comp setting.

    In short, forget all the Viessmann app and delete it, set the weather comp to 21C on the boiler and get Viessmann to make sure the app is not involved at all like my old lady customer's set-up. Maybe wait until Viessman give you the go ahead before deleting the app. Then let Tado switch it via 230V relay and the Viessman weather comp modulate the flow temperature as it wishes after Tado gives the command to switch on.

    Tado with Viessmann's own weather comp should work well, those boilers modulate down quite low, lower than most. I think they can go down to 3KW and stay lit. Most others are 5 or 7KW minimum. You'll find it's so quiet sometimes you'll have to look at the flue to see if it's on!

  • GazzaH
    edited April 16

    Also, as Montage says, it may just be simple tweaks on TRV's and to do with that rather than conflicting electronic controls.

  • Cheers @GazzaH !

    I read that the Tado only has 2-3 modulation settings compared to the Viessmann's multiple modes (so it can modulate lower than when using the Tado).

    I have the boiler connected to WiFi and using the ViCare app (though we can't get ViCare+ here in the UK apparently, so not all the features)

    I guess you also need the ViCare app to set the heating curve too?

    Think I have some air stuck in the system somewhere as one of my rads isn't heating up since the new boiler's been installed. Need to switch it all off tomorrow and try to push it through somehow. Bled the rads and no air so far!

  • Ah, I'm not sure about the modulation, I think what you mean is an indication of low demand - one wavy line on Tado, medium demand - two, and high demand - three wavy lines.

    I think they are just an indicator and the boiler will modulate the way it needs to.

    I'm pretty sure the boiler can be commissioned manually without any involvement from the app. The weather comp curve and an offset can be set via buttons on the front of the boiler in a particular menu. The instructions for this might have been in the user manual and not the installation instructions.

    The other thing with these Viessmann's is that the pump modulates too so if a radiator has poor circulation, the boiler may not reach it when the pump pumps less hard. I had this on one - from the day the house was built the guy always had problems with one radiator, the other 7 needed to be almost closed at the lockshields and the problem one fully open for it to work (with a normal pump). With the new Viessmann, the rad sometimes doesn't heat as the pump is doing all sorts of clever things monitoring flow and return tempos and reacting to them.

    I'm going to go back and re-pipe that one rad or search for a blockage or kink in the pipe.