Vaillant 831 Intermittently does not reach set flow temp

Several times in a week, system flow water temp settles at mid 50s (return: high 40s) when the set flow temp is 75.
Room stats calling for heat, burner on, pump on, pressure OK, no new codes. Just doesn't rise to the set flow temp. This continues until boiler is switched off, cools down (to mid 30s), and when it restarts sometimes it's all fine and runs at flow 75 (return: high 60s) and sometimes settles back to flow mid 50s again. Can't predict.

5-bed house, HW cyl, DHW to kitchen, underfloor heating in extension - and on a good day even when v cold outside runs fine all day at 75 with everything calling for heat - house comfy. On a bad day, mid 50s again - need extra jumpers.

Any ideas...?
It's as if it's modulating too far down and then not noticing the gap between flow temp and set temp....sometimes.

I've tried....
- Turn set temps (CH & DHW) down and back up again; no effect.
- Switch boiler off and on again; no effect.
- Boiler reset; no effect
- Switch each heat demand off and on again; no effect.
- Set thermostats higher; no effect
- Turn set temp down, waited for water to cool significantly, set temp up again. Sometimes this fixes it, sometimes not.

Have you got Vailant ebus controls ie a VRC?

Controls are Danfoss TP5000 RF. Set to 25C all day at the moment (usual winter setup). Effectively I'm using the thermostat as an on/off time switch and the standalone TRVs on the rads regulate the temp in the rooms.
Underfloor heating has its own Danfoss and I'm using that as a thermostat, but again it is simply on or of.
No weather compensation or wireless TRVs, nothing sophisticated!

It sounds like under certain conditions the boiler output is too high, if the difference between flow & return is over 30C the boiler will go into a reduced output mode.
It may be worth turning the heating power output down in D0 in the engineers menu.

Check the status code the boiler is running at when flow temp is not high, it could be s53 (i think) too much differencr in flow temp, then it does not go to full power.

Hi wannnocks and rbranco,

Thanks guys. Your quick responses much appreciated.
That you both came up with the same possible cause must say something! I haven't finished investigating that yet but have put initial comments below.

But first:- I discovered something new. System had switched off for the night 30 mins before. I switched it on with the room CH thermostat. System quickly rose to 50C/40C (Flow/Return) and settled at that. It was showing code S.24 ("burner ignited" in combo/warm-start mode) and the warm-start symbol was flashing => stored water not up to target temperature (65). Obviously it was never going reach 65 with flow temp only 50, so I decreased the DHW target to 37, heard the diverter valve move and burner noise increase, and the system temp then rose quite quickly to 75/65, now showing code S.04.

While DHW or warm-start is demanding heat, it's not surprising that the boiler is controlled by that, but still puzzling/problem that flow temp did not rise further. This might be the cause of the overall problem. As an experiment, I have switched off warm-start to see if the main problem still happens. It hasn't yet, but 6-7 hours is not a long enough test (some days I don't get the problem anyway).

Coming back to your point about big flow/return difference and reducing output power (d.00)....

• With the system stable at 75/67, I reduced d.00 from 24kw (max, factory default) to 9kw. Unsurprisingly temps began to fall rapidly. I had to raise it to 20kw to maintain the temps. But that is on CH demand only (i.e. no demand from underfloor heating or HW cylinder), so I'm sure it needs to be at 24kw to cope with maximum demand and I've reset it to 24 for the moment.
  
• I will test again at reduced output as you suggested, by leaving the whole thing switched off for a long time so I get the biggest flow/return difference. TBH I have not seen a flow/return difference of much more than 10C, though I guess it must get a bigger difference briefly when it starts from stone cold. I have seen difference of more than 30 between target flow (d.05) and return (d.41), but at the same time the actual flow/return (d.40-d.41) difference is about 10 as normal.
  
• I haven't seen code S.53 (Appliance is within waiting period of the operation block function due to water shortage (VL-RL spread too big)) nor S.54 (as S.53 but for temperature gradient), nor S.76, 96, 97, 98.

Thanks again for your advice. I'll keep you posted, and any other comments in the meantime would be welcome.

The warm start (comfort mode) is there to pre heat the HW heat exchanger. This should not take more that a few minutes. If the boiler is sticking in comfort mode you may have a problem with the diverter valve. (Check for heat in the heating flow pipe with the system cold and a HW tap running. The flow temperature in HW mode is set by the boiler software and is not adjustable. The HW control on the boiler sets the HW demand (outlet) temperature, the heating control sets the flow temp in HTG mode only.

You normally get S53 if the circulation round the heating system is inadequate. If the power output is too high for the size of the system or too many radiators are closed (manually or by a TRV) the differential temp will be too big.

Hi wannocks,

I hadn't realised flow temp is s/w controlled in DHW mode. It's odd that when CH is operating and flow=75C, it can drop to the low 50s when there's DHW demand. But when there's no CH and the system is cool, DHW demand results in flow=75C. So there's something funny about flow temp control in DHW mode.

I think this is probably the root cause of the CH issue I've been seeing, because when flow temp is less than DHW set temp it gets stuck trying to heat the warm-start reservoir and takes forever to go back to CH mode. The easy answer is to reduce the DHW set temp (I've done that and it's been OK for 2 days now) or better still switch off warm-start (it's a minor feature, so I'll probably do that in the end).

As you suggested, I checked diverter valve operation - seems OK. With system cold and no other demand, there was no heat in the CH flow after several minutes of DHW demand.

I tested again starting from cold looking for the large flow/return difference you talked about. It started at return=32C and maintained a flow/return difference of 10-12C all the way as it heated up. I had set d.0 to 15kw and it settled at flow=55C. Increased d.0 to 18kw=>65C, 21kw=>72C. So I have returned d.0 to 24kw (max).

Only time will tell if this DHW/Warm-start work-around really has resolved the original issue, but I think all I can do now is wait (and life's too short to try to get a s/w problem fixed!).

Thanks again for your ideas and advice.