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Thread: Vaillant flow temperature control for HW

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  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 07:22 PM.

  7. #7
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    The way I use the VR65 is by using the microswitch (grey and orange) wires in my DHW to close the CYL inputs on the VR65.
    My DHW zone valve is switched on by Evohome, but it doesn't need to be. Because 230V is going round that CYL loop, you can effectively use that as a SL for your DHW reheat cycle. You will just need to work out which of the two CYL terminals is Live 230 and which is expecting the loop back from the cylinder safety Thermostat.
    The triggering of your boiler can stay the same.

  8. #8
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    Thanks Bruce. I'm actually thinking of using the grey/orange (230V mains voltage) directly on the VR65 CYL terminals. Even though the orange will backfeed from the other zone valves, only the DHW zone valve will bridge the CYL terminal.

    My working assumption is the CYL terminal won't blow up with the mains voltage! I can't make out from your post if you believe this to be safe or not?

    The only alternative is for me to use a relay to activate the CYL terminal volt free. But I like to keep things as simple as possible.

  9. #9
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    You can test this quite easily. I actually think one of the CYL terminal is actually supplying a 230V live and the other terminal is simply there to accept that 230V voltage via the cylinder thermostat. So no I do not think you will blow this CYL terminals with mains voltage.
    I'm going back up to my loft tomorrow to check a few things. I can check this for you. The bit I don't know is which of CYL terminal 1 or terminal 2 is 230V Live and which is expecting the 230V return loop.
    The reason this is important in your case, is because you may actually need to only connect your DHW SL output to one of the two CYL terminals.

  10. #10
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    That's great info. Thanks very much.

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