Boiler lingers at intermediate temperature

Discussion in 'Plumbers' Talk' started by pilky01, Jan 12, 2021.

  1. pilky01

    pilky01 New Member


    before I begin I’ll list my setup to help diagnosis.

    Vaillant Home regular 15 boiler
    2 zone heating (upstairs(6)/downstairs(5)) with zone control valves in airing cupboard
    Gledhill stainless lite unvented cylinder 180l, indirectly heated zone 3 of CH loop
    grundfos ups2 15/60 pump
    Honeywell ST9400C controller
    Mistral ii TRV’s

    I live in a new build 3 bed detach which is insulated up to the 9’s. I did a bit of decorating and took rads off walls without checking balance settings on lock shield valves so started from scratch. Unsurprisingly new build installer left everything on default settings, ABV, pump, lockshields, everything. Realised pump and boiler are far too big and rads heat house in no time so I’ve experimented with turning pump down and balancing to get good temp spread at boiler. Even with pump on lowest setting the rads get evenly warm throughout house so left lock shield valves on half turn open with TRV’s set to 3.

    When heating kicks in at 1600ish the system operates brilliantly, takes about 15 mins for the boiler to get to set point of 70c where it then starts to modulate itself, presumably by return temp as it still shows 70c flow temp. House gets toasty, radiators red hot, after about 30 mins thermostats “click” and turn off heating demand.

    What then happens is really weird. Subsequent demand for heating causes the boiler to switch on, momentary blast of heat (which the manual says is to purge flue), the flow temp reaches 70 in about 60 seconds and then the boiler harshly moderates itself until the flow temp stabilises at around 40c for anything between 5 and 10 minutes. All the while the flame symbol does not disappear from the indicator on the front of the boiler. I’ve checked the D codes and return temp is usually about 9c lower at this time. It’s as if it knows that by targeting 40ish it can finely tune the temp differential.

    What I’d expect is that after a time the TRV’s would begin to close as the rooms get up to temp, meaning all boiler utility would be diverted around rad in room with thermostat as those ones don’t have TRV’s. Then thermostat would turn off heating demand quicker. What actually happens is all rads on the loop seem to get evenly warm to about 40C.

    I know this isn’t “cycling” because I made it do that when I was messing around with the pump and put it on proportional control. That time the boiler overshot 70, went to 75, the flame signal disappeared while the radiator symbol kept flashing to indicate demand and the flow temperature went down to about 30c, whereby the boiler kicked in at Chuck Norris power and the cycle started again.

    This is different and I’m wondering how the boiler could know what flow temp to target as I don’t think my thermostats are “smart”. They are similar to these fellas:

    The manual says max efficiency is achieved at lower (sub 40c) temps because of condensing so if it can target this temp intelligently then great, but I want to make sure what I’m experiencing isn’t actually a safety feature of the boiler and I’m not going to end up with a hefty repair bill.

    Answers on a postcard
    Thanks in advance
  2. pilky01

    pilky01 New Member

    I think I may have made some progress. I put the heating on during the day today and as described the house heated well. The second time the boiler came on it behaved as described so I set it to show boiler status and got S.53 which is:

    “Product in waiting period/operation block function due to low water pressure(flow/return spread too large)”

    and I also found this forum feedback:

    I also remembered that the boiler did this once when I turned one of the loops on by increasing the thermostat to 25, which must have caused the return temp to momentarily drop as the water was added in to the mix. The system pressure is a steady 1.5 bar which I set it to last time I bled the rads.

    So I think what is happening is the house heats up nicely from cold because the TRV’s are all open. Then when the house is up to temp the TRV’s begin to close so when the boiler kicks back in there is very little flow around the loop and the flow temp races away. Because of the low flow the boiler receives a very low return temp and falls in to this waiting mode, even though it has modulated itself down and the flame doesn’t go out.

    I’d set the pump to its lowest setting which gives 4m head, and the min flow rate for the boiler at min output of 4.5kW with deltaT 20 is 192l/h. Looking in the docs for the du144 ABV gives a setting of ~0.37 bar which is what I’d tried to set it at but I’m thinking if I set it slightly lower than this then it will constantly be bypassing some flow water and the boiler shouldn’t sense too large a flow/return spread.

    I’ll experiment and report back.
  3. Teki

    Teki Screwfix Select

    Try reducing your flow temperature to 65°C.
  4. I Love Boilers

    I Love Boilers New Member

    Too bigger differential between flow and return temps is common on a Vaillant and causes it to go to limp mode.

    Egg timer on the top right of the display?

    I would have the lock-shields more that 1/2 a turn open as that’s going to put a lot of resistance on the pump. Only the ones that get hot quickly near the boiler should be closed that much. The index circuit should be fully open and the rest adjusted depending on their proximity to the pump.
  5. pilky01

    pilky01 New Member

    Thanks for replying. I tried that a while back and the boiler kept cycling while the hot water demand is on. The set point for the tank is the advised 60C so I’ve a feeling the two were too close together at that set point.
  6. pilky01

    pilky01 New Member

    Thanks for your input. I’ve read that the lockshields are likely giving full flow at the half a turn open setting so I’ve been playing with having them around there but perhaps I should open them all up and start again. I’d been mucking about with a digital thermometer and tightening them up to give a specific temp spread in vs. Out, but that was insane I can only imagine to achieve 20C you have to have them bollocked up so tight so I gave up and just set them all around half a turn and they’ve been heating up evenly so happy days.

    Re: my previous post about slacking off the ABV didn’t work, the boiler just ended up cycling as the flow was immediately returned so I’ve set it back to about 0.35. I’ll tinker with the lockshields some more and report back.
  7. kiaora

    kiaora Screwfix Select


    have you been on the valiant/glow-worm course ?

    you can do zoom courses now

    its exceptionally good, I would recommend it,

    unless you are particularly good at computer/electronics systems, it is easy to make a mistake when playing with the d code settings.

    maybe check the parameters again, pressing the mode button at the wrong time can upset the boiler !

    good luck


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