Ebus Max Flow Temperature setting (like the one just released for Opentherm).


Would it be possible to do the same for Ebus controlled boilers via the wired Tado, +- settings?

My Vaillant can ramp up to 80C if it wants even if I set the max temp lower on the boiler. The ebus will ignore these settings and do what it feels is best. I might only want it to go to 66 or 70C max for example.

It's just been done for Opentherm which is very similar.


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  • eBUS should not be overriding the boiler set temperature. What boiler are you talking about here?

  • GazzaH

    Hello, Vaillant Technical told me on the phone that Ebus will ignore the maximum set target flow temperatures and go to 80C if it wants to for load or weather compensation which is the maximum allowable flow temperature. It's an Ecotec Plus 418, GC: 41-694-15.

    Having said that, I have twice walked in and looked at the current boiler flow temperature when ramping up the heating from cold and it happens to be 66C which is the maximum I set it to...

    It is possible that I have been told the wrong thing by Vaillant or they have misunderstood what I was telling them. I may just have walked in twice at 66C by coincidence too.

    With the VR65 wiring centre and hot water priority, it may also do something different than the heating mode, it may ramp up to 80C and 25KW for fast cylinder recovery. 18KW max on heating, 25KW max on HW.

    I explained to Vaillant I was using Tado ebus but he may have had to explain it in terms of what Vaillant controls would do, such as Vsmart.

  • GazzaH

    I've received a VR10 sensor in the post today and connected it instead of the cylinder stat.

    What seems to be happening is this: If I set the maximum CH flow temperature on the boiler fascia controls to 66C then, even if Tado wants more than that, the maximum the ebus us allowed to ask for is 66C. If I set the HW cylinder target temperature to 55C at the boiler controls then the maximum the Tado is allowed to ask for is 55C, even if I set it to 60C on the Tado. BUT, when in HW mode, the maximum central heating flow temperature is being ignored and the boiler goes up to 80C if it wants to for a period of time them ramps back a bit. It seems to be more concerned with reaching the target HW temperature as quickly as possible rather than limiting the flow temperature leaving the boiler.

    So, Tado controlled ebus on Vaillants does have user settable limits on ch flow temp and hw cylinder temp but not ch flow when in hw mode, that defaults to anything up to 80C.

  • Sounds like expected behaviour for PDHW. If you could change to set up to close off CH valve when it’s doing it you’d be sorted.

  • "If I set the HW cylinder target temperature to 55C at the boiler controls then the maximum the Tado is allowed to ask for is 55C"

    This statement is not correct. Due to heat loss in the pipework the cylinder thermostat will never be satisfied and the boiler will be permanently running. The HW flow temperature needs to be sufficiently hotter for the cylinder coil to heat the water inside. As @Montage wrote it's expected behaviour for the boiler to heat the HW as quickly as possible. The VR65 should turn off the CH valve when the cylinder needs heating.

  • GazzaH

    Hello, the hot water cylinder target temperature is not the same as the boiler's flow temperature. The boiler's flow temperature indirectly heats the hot water in the cylinder via a coil of 22mm pipe filled with radiator water until the hot water cylinder's clean water that you bathe in reaches the desired temperature. The dirty chemical radiator water never mixes with the clean cylinder water and just exchanges heat as it passes by. Like an element but the heat source is not electricity.

    The boiler flow temperature in Vaillant's hot water only priority mode with ebus will keep heating the central heating flow pipe (radiator water) for the cylinder coil anything up to 80C until the cylinder's clean water surrounding the hot coil reaches the setpoint. As 80C is higher than 55C or 65C there will be no problem in the cylinder reaching temperature.

    Ebus also allows the boiler to modulate to full output up to 25KW in hot water mode even though my boiler is an 18KW heat only boiler. Ecotec plus 418. The boiler data sheets confirm this and the indicator on the boiler goes to the top in hot water mode and never in heating.

    In heating mode, it is not allowed to go above the (chosen) flow setpoint of 66C in this case. In hot water mode there is an upper limit on the boiler's flow temperature in any case of 80C. The heating is also not allowed to go any higher than 18KW. The 25KW in hot water mode was unlockable and unavailable until I used ebus controls.

    In 'hot water priority' with a VR65, it is either hot water only or heating only when a Vaillant 400 or 600 series boiler is in 'normal' mode for diverter valve operation in the boiler's parameters. This allows a very high cylinder charging temperature and KW output to get the cylinder heated ASAP before allowing heating to run at lower temperatures for load compensation or weather compensation if using Vaillant controls. If it was left in 'parallel' operation (allowing both y-plan ports to open at the same time) in the boiler's parameters, then the hot water's higher flow temperature could pass 80C water through to the radiators making them much hotter than required.

    So, with Ebus, Tado, a VR65 and a Vaillant boiler with hot water cylinder and a VR10 sensor:

    Hot water cylinder set-point is controllable but the boiler flow temperature modulates anything up to 80C in hot water only mode and anything up to the boiler's true output in KW. The KW can be lowered at the boiler. If the hot water is hot (satisfied) or OFF then the heating is allowed to operate on its own and fire to its own, lower, temperatures.

    The central heating flow temperature in heating only mode can be set at the boiler and will stop Tado getting the flow temperature higher than allowed. In this case 66C.

    Maximum HW cylinder temperature - user settable at the Vaillant boiler.

    Maximum CH mode flow temperature - user settable at the Vaillant boiler.

    Maximum HW flow temperature - 80C and the ebus decides how high it wants/needs to go.

    Maximum boiler temperature allowed in any mode - 80C.

    Maximum HW cylinder temperature allowed by Vaillant - 60C and settable at the boiler by the user.

    Maximum HW cylinder temperature allowed by Tado - 65C but the Vaillant ebus changes it to 60C at the boiler if set above 60C.

    Maximum HW KW - engineer settable.

    Maximum CH KW - engineer settable.

    Minimum boiler KW for heating and hot water modes - engineer settable.

    This is as I understand it after testing for a few days in my own house. I'm a heating engineer and fitted the boiler and Tado myself but not the rest of the system. I've had the boiler two years with a conventional y-plan but fitted the VR65 and Tado at the weekend.

    Vaillant technical had told me some slightly incorrect info the other day so I've had to test it all myself and look at the boiler codes and settings when different demands were requested by Tado.

    I had typed in a detailed explanation in another thread last night about exactly how this is all working but went to correct a typo and it wiped the whole thing and I left it at that!

  • If you are using weather compensation and Tado, it's the weather compensation that is over-riding the built in variables. If you disconnect the weather compensation you should see the maximum flow temp for CH is respected.

  • GazzaH
    edited March 2023

    gary333, the Vaillant weather comp only works with entirely Vaillant controls I believe. I don't think you can mix and match. I do know that Ideal boilers, Viessmann and recent range of Baxi's among others allow their own weather sensors while using 230V switching from any controls. I also think some Worcester Bosch are allowed to use their weather comp while using any 230V switching.

    I'm still working my set-up out. It is interesting though. Sometimes if very near the setpoint, Tado ebus runs the pump only with no flame which is quite clever as that will be drawing heat from other rads and equalising temperatures around the property.

  • Whilst it might be nice to credit Tado with running the pump, this does happen in OT according to some other customers. It coincides with small calls for heat. OT has no command for ‘just run the pump’ and I suspect it’s a boiler strategy.

  • andyblac
    andyblac ✭✭✭
    edited March 2023

    as far as I understand OT, the thermostat just sends the Target Temp and the Current Temp (highest for TT, and lowest for CT for the rooms calling for heat), and the boiler decides what to do regarding the pump / flow temps. This is why you have to set the max flow temp for the OT protocol on the boiler via tado's webIF, it nothing to do with tado stats.

    it works pretty much like Weather comp, just on demand. The grater the offset the more demand on boiler.

  • GazzaH

    Good comments, even if it's not Tado and it's Vaillant's strategy, it's still working well.

    The main thing I wanted was to be able to modulate the Vaillant boiler to its capabilities, control the hot water via a thermistor and have a system with decent SRV's that can do all of the above.

    Currently, the only options are Vaillant's own controls or Tado and I don't fancy Vaillants SRV's.

    I haven't added SRV's yet but so far, Tado is looking good. I'm waiting to see if there are internet outages as that will be the big issue I think.

  • GazzaH

    I think it is actually the Tado which is deciding to make the 'pump only' run at times (with no burner) because on the app, when there is a low heat demand and only one wavy line and light grey shading on the graph, I'm finding the boiler is either on a low flame and low flow temperature or the boiler saying that heating mode is active, the radiator symbol showing on the boiler display but only the pump is running. It's not a pump overrun I'm seeing as the Vaillant Status codes would tell me.

    Surely this is Tado using PID and then telling the boiler through ebus commands what to do? Maybe when very close to the setpoint, the Tado tapers off and runs the pump with no flame due to a PID calculation and historical data so that it doesn't overshoot the setpoint? The boiler may have inbuilt operations like pump overrun, anti cycle delays, and low fire for a few minutes to prove no blockage before going to a higher flame but it still requires actual commands from the ebus controller (Tado in this case) which is constantly learning and logging heat up times and other data as well as taking outside temperatures and how far away from the setpoint it is into account.

  • GazzaH

    It's doing it now actually as I speak. The pump is running, there is a radiator symbol on the boiler's display but the S code says S00 no heating demand. It is 0.1C over the setpoint due to extra heat from hot kitchen's cooking earlier. It was 0.2C above, I think the Tado may be running the pump with no flame to try to disperse heat or even the whole house temperature out to as close to setpoint as possible?

  • GazzaH

    The pump has now stopped but there is still a radiator symbol on the boiler but it says S00 no heat demand. No wavy lines on Tado. I'm not quite sure what it's doing, it could be that the radiator symbol means that the last position the 3 port valve was in was heating through the VR65.

  • ooghes

    Hi all, I would like to revive this feature request. I believe allowing the configuration of the max flow temperature via eBUS would be a real value-add for many Tado customers.

    I currently have a Vaillant ecoTEC plus VC 306 central heating boiler, connected to Tado via eBUS and working fine (and working in modulation mode). I have also integrated Tado as part of my Home Assistant installation, allowing me to add several additional automations and collect/visualise statistics (but that's irrelevant for the feature request).

    My Vaillant boiler allows configuring the max flow temperature (Vaillant setting D.071) and I have it currently set at 60 degrees Celsius. It's a fixed value.

    Via Home Assistant I am monitoring the flow temperatures by using the Tado API explained in https://community.tado.com/en-gb/discussion/8/tado-api/p2

    What I would like to be able to do, is to dynamically change the max flow temperature on the boiler, depending on the seasonal outside temperature. So rather than having to manually change the value on the boiler every few days, have it controlled through the Tado API.

    I have been playing around with setting the "boilerMaxOutputTemperature" by sending a HTTP PUT as explained here: https://community.tado.com/en-gb/discussion/8611/release-opentherm-max-flow-temperature-setting/p3

    I'm able to set the value and I confirm it is set, I get following output of https://my.tado.com/api/v2/homeByBridge/IBxxxxxxxxxx/boilerMaxOutputTemperature?authKey=xxxx


    However, this value is NOT sent to the Vaillant boiler via eBUS. When I check the flow temperatures, I see they regularly exceed the 50 degrees value (and sometimes very briefly exceed 60 degrees Celsius, but I assume that is due to the boiler behavior):

    So, the idea is to use the Tado API to set the boilerMaxOutputTemperatureInCelsius to the desired value (e.g. 50, 55, 60 degrees Celisus). Tado, in turn, then uses eBUS to communicate with the Vaillant heater.

    According to the unofficial eBUS specification (http://ebus-wiki.org/lib/exe/fetch.php/ebus/spec_prot_7_v1_6_1_e.pdf), this can be done using command B5h 05h 0Fh "SetMaxFlowTemp"

    Given Tado supports Vaillant eBUS today and the Tado API is already available to configure the max flow temperature for OpenTherm, I would assume that extending this to Vaillant eBUS interface would be fairly straightforward.

    This extension would greatly improve the value to Tado, as it would allow all Vaillant eBUS systems - and possibly also other vendors - to control the max flow temperature and increase the heater efficiency.

    Thanks in advance for taking this feature request into consideration.