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Thread: Vaillant flow temperature control for HW

  1. #21
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    Quote Originally Posted by Symbolics View Post
    Yes, I have D.078 set at 80 degrees. Regardless, what happens when there is heating demand is that the boiler goes to about 65 degrees on the flow temp, and then goes into pump overrun mode. So even when there is no NTC connected allowing the boiler to figure out what the cylinder temperature is, the output DHW is modulated down to 65, and the actual temperature varies between 60 and 65.
    I recently replaced my timeclock plus mechanical stats with a VR65 VRC470F etc. on my existing Vaillant 418. For four days I struggled to get my system to properly respond to a water heating request, despite working perfectly for room heating. I tried lots of settings and nothing worked, but after four days I decided to try resetting the boiler to factory defaults via the d.?? menu, then set it up from scratch. It then heated water absolutely fine. What was wrong with the original settings, I never found out, other than it is fixed now.
    Last edited by harrym1byt; 4th February 2019 at 07:57 PM.

  2. #22
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    Quote Originally Posted by mtmcgavock View Post
    To me it sounds like you've got some form of circulation issue on the cylinder circuit, at a guess it sounds like it is returning too quickly or your pump is on it's way out. It's not a problem with the VR66 or the boiler setting at 65oc.
    Thanks all. It turns out that the boiler pump needed to be set at a fixed high speed. With the automatic pump speed, the pump was going slowly which then made the boiler modulate its output and it never got past 60 degrees. A fixed high speed pumping scheme from the boiler is ok in my setup because I have a low loss header into which the boiler feeds, which hydraulically decouples the boiler from the Megaflo.

  3. #23
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    I was wondering if there was a further improvement that could be made to solving this problem.

    I have a very similar situation. My Vaillant has Y-plan wiring (i.e. CH and HW heating fed with mid-position diverter valve) and I've implemented my own thermostat with software to control the room heating using TPI (with a relay to switch the CH demand signal). I am planning to move this to Opentherm to regulate the circulation temperature instead of TPI. However, when the hot water tank needs to be heated I would need to switch to a high enough circulation temperature.

    This might conflict with the room heating requirement so I was wondering if I could have two modes:
    a) HW demand OFF - CH is regulated using the boiler temperature OT control.
    b) HW demand ON - set OT temperature to max and control CH circulation using the TPI signal (i.e. modulating the CH demand signal into the Y-plan wiring as before)

    Would this work?

  4. #24
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    Evohome now comes with HW priority, so it will switch off the CH circuit when it switches to a HW heating cycle. OT jumps to a 90C set point when HW heating is required.

  5. #25
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    I don't have Evohome as I'm using my own control system. I was hoping that I could have CH heating at the same time as HW but not have the radiator feed running at full temp (which is currently 65C on my boiler setting). I was thinking that reverting to TPI would then avoid the rooms overheating. The diverter valve would be operating the TPI on/off switching in the same way as it does in my existing (non-OT) configuration.

  6. #26
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    Native OT has the ability to distinguish between CH mode and DHW mode. You could use that to alter the behaviour of your two circuits.

  7. #27
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    I'm not sure if that would help me - but apologies if I've missed something. My boiler has only one circuit with one demand signal connected to it (terminal 4 on the circuit board) and is driving a single pump. There is a 3-position valve in the airing cupboard that can divert the feed to CH or to HW or to both. That is controlled simply by the demand signals from the two thermostats (CH and HW) i.e. the Y-plan wiring setup. I've intercepted those two signals and and am able to drive them directly.

  8. #28
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    Quote Originally Posted by Vikings View Post
    I was wondering if there was a further improvement that could be made to solving this problem.

    I have a very similar situation. My Vaillant has Y-plan wiring (i.e. CH and HW heating fed with mid-position diverter valve) and I've implemented my own thermostat with software to control the room heating using TPI (with a relay to switch the CH demand signal). I am planning to move this to Opentherm to regulate the circulation temperature instead of TPI. However, when the hot water tank needs to be heated I would need to switch to a high enough circulation temperature.

    This might conflict with the room heating requirement so I was wondering if I could have two modes:
    a) HW demand OFF - CH is regulated using the boiler temperature OT control.
    b) HW demand ON - set OT temperature to max and control CH circulation using the TPI signal (i.e. modulating the CH demand signal into the Y-plan wiring as before)

    Would this work?
    Yes, this should work and would at least limit CH overshoot on DHW heating

  9. #29
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    You've just given me an idea to try something else. My Vaillant 438 doesn't have a HW sensor input, but it can have a HW setpoint separate from CH. The HW set point dial is useless unless used in conjunction with a Vaillant wiring centre and Vaillant controls. I'm going to have a play with a Vaillant wiring centre and see if I can get the HW demand to register and make the HW dial operational. It will allow me to remove a lot of custom HA that I use to detect a HW heating cycle and then performing various actions, one of them being raising the max Flow temperature setpoint.

  10. #30
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    Quote Originally Posted by kevinsmart View Post
    Yes, this should work and would at least limit CH overshoot on DHW heating
    That was certainly my thinking.

    I'm assuming that the demand signal into the boiler will override what the OT control might be doing so - in HW mode - if HW thermostat has reached temperature and CH TPI control say OFF then switching off the demand signal to the boiler (terminal 4 on my Vaillant board) will stop the circulation.

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