Still struggling with boiler cycling

[Bargain Bucket]

Hi folks. I'm still struggling with a problem on my Glow-worm Ultra-com 24Hxi. System is indirect vented cylinder with 3 port valve. The continuing issue is that whenever we are heating DHW, the boiler continually cycles (don't get this issue when heating CH only). With the tank cold, boiler fires at full capacity and heats the water up ok. But as the tank temperature gets within about 10 degrees of target (65c), the boiler goes above its high temperature limit (modulates down as low as it can go but still overshoots) and shuts down. Pump continues to run. After a few minutes the boiler fires up again on full pelt, overshoots within a minute and shuts down again. The cycle keeps going on and on like this until the timer says HW is no longer scheduled.

We had the cylinder moved from an upstairs cupboard to a downstairs cupboard (I.e. through the wall from the boiler) about 3 years ago, fitting a new cylinder in the process. The problem has been happening ever since then. This issue doesn't happen when we run the CH without the HW being scheduled so I don't think it's a boiler issue. Totally flummoxed and frustrated that we can't schedule CH and HW at the same time as this issues means the CH gets cold with the boiler constantly firing up and shutting down.

All ideas and suggestions gratefully received.

 

[iateyoubutler]

I have a Vaillant 400 series which I believe is the same boiler badged as something else. Mine does this too, never been able to stop it. It`s days are numbered..........

Getting it replaced next year

 

[DP]

Have you tried running the boiler at higher temperature or lower the cylinder to 55 degrees. See what happens then

 

[Bargain Bucket]

Yep. Tried the boiler at its highest temp (82) and it still overshoots. I'll try the cylinder stat lower and see what happens but I'm not sure what that will tell me really.