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Thread: Vaillant flow temperature control for HW

  1. #1
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    Default Vaillant flow temperature control for HW

    I'd like to be able to run my CH at lower flow temperatures during the summer, but am restrained by the Unvented Cylinder (not Vaillant) requiring 70 degrees flow in order to satisfy the cylinder stat at ~60 degrees (to prevent legionella).

    I've looked at the various Vaillant controls available and whilst the VR65 seems to be able to achieve this, it doesnt cater for multiple heating zones.

    So the plan is to use an ebus adapter which takes the SL from the HW zone valve and adjusts the flow temperature on the boiler to MAX, reverting back to the set flow temperature once the HW zone valve closes.

    Has anyone done something similar? And any pros / cons compared to using the V65 and the supplied NTC?

  2. #2
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    Quote Originally Posted by fezster View Post
    I'd like to be able to run my CH at lower flow temperatures during the summer, but am restrained by the Unvented Cylinder (not Vaillant) requiring 70 degrees flow in order to satisfy the cylinder stat at ~60 degrees (to prevent legionella).

    I've looked at the various Vaillant controls available and whilst the VR65 seems to be able to achieve this, it doesnt cater for multiple heating zones.

    So the plan is to use an ebus adapter which takes the SL from the HW zone valve and adjusts the flow temperature on the boiler to MAX, reverting back to the set flow temperature once the HW zone valve closes.

    Has anyone done something similar? And any pros / cons compared to using the V65 and the supplied NTC?
    This is exactly how my boiler works with Evohome/Opentherm. Most system boilers will revert to a max flow temperature when hot water heating is required. That said, I would have thought that Evohome with the hot water kit and weather compensation would give you what you need. Other more knowledgeable contributors will have a view.

  3. #3
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    I don't have Evohome or the Opentherm bridge. Just a standard S-Plan plus setup, with upstairs, downstairs and HW zones. I've been reading up on a Raspberry PI setup with an ebus/usb adapter and controlled by ebusd: https://github.com/john30

    My thinking is to use the SL from the HW zone valve as an input into the pi and have the software send the relevant ebus command to increase the flow temp on the boiler. It seems like a common use case so I'd be surprised if someone hasn't already done this.

  4. #4
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    bruce_miranda has built something similar. He uses Domoticz to listen to what's going on in his Evohome and when the hot water is switched on he uses ebusd to alter the max flow temperature on his boiler. Provided you're comfortable with the electrical side of things (in particular, using a 240V AC input to an RPi) then this should be reasonably easy to build.

  5. #5
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    I think the VR65 or VR66 will achieve what I want, but not certain it can be used without the corresponding Vaillant ebus programmer / thermostat (i.e. VRT 360 or VRC 400).

    Would it be possible to link the HW zone valve SL to the CYL terminals, and will the VR65 then instruct the boiler via ebus to boost the flow temp whenever the CYL terminals are closed? Could anyone who has a VR65 without the vaillant controls confirm? Thanks.

  6. #6
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    A VR65 without a controller will kick the boiler into an 80C HW flow situation and keeps going because there is no controller to tell it to stop! I've tested this....

    You can do this without using a VR65 but you can't do this without a controller.
    Last edited by bruce_miranda; 10th December 2017 at 08:22 PM.

  7. #7
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    Ah ok. Seems a bit nonsensical. I'd have thought once the CYL stat was satisfied, the VR65 would revert to the flow temp set on the boiler. Oh well - that would have been an easier solution to my issue.

  8. #8
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    The way Vaillant have designed their boilers is that the DHW knob and function only works from a Vaillant controller and only when a VR10 NTC sensor is used. That sensor comes with the VR65 but has to be bought separately with the VR66. I worked out the eBUS command to get the HW knob to work but the problem is that I have the VR33 connected which shortly overrides anything I send on the bus. Also the OT Bridge doesn't use the two channels anyway, so it was a bit pointless.

  9. #9
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    The NTC is only required if you want to control the cylinder temperature (using the HW rotary dial on the boiler). If you want your cylinder to be heated to the temperature on the cylinder stat, you only need to ensure the flow temp of the boiler exceeds this.

    What's unclear is whether the VR65/VR66 will boost the flow temp to 80 degrees on HW demand *even if the CH rotary dial on the front of the boiler is set to less than 80*. Different people say different things, and I wonder if it differs based on firmware's. If the ebus can override the flow temp regardless of what's set on the boiler, then this would be a cheap and easy way to achieve independent HW flow temp control on vaillants.

  10. #10
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    if you give me till the New Year I can test anything you want. I accumulated quite a lot of test gear and we have the same boiler with the new PCBs.

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