Oso 20 RI problem

[RachaelB]

We have a Oso 20 RI cylinder which is supplied by a Vaillant Turbo Max boiler. The system (CH and HW) has been fine for 8 years with annual maintenance. Just recently we noticed that the HW "fades" after a couple of minutes at every tap in the house. The water then runs stone cold. If the taps are turned off for a while and then turned on, the cycle is repeated. The CH is fine. The boiler fires up. The inlet pipe from boiler to cylinder is hot and the outlet pipe from the top of the boiler is also hot so we know the cylinder is full of hot water. Any ideas what could be wrong? Our regular service engineer thinks there may be an unnecessary (and indeed hidden) blending valve somewhere but is otherwise mystified. All ideas/solutions welcomed please.

 

[Dan Robinson]

Are we talking temperature or flow?

 

[RachaelB]

Sorry I should have made it clear that it's temperature. The flow doesn't seem to be affected significantly.

 

[Dan Robinson]

Well it is just a vessel. It either has a full load or it doesn't.

The tank is heated from the bottom up. Had a recent visit to customer who'd had three or so visits from other engineers.

None had looked at the timer - or twigged that the settings were conflicting with each other.

Start with the simple things.

 

[RachaelB]

The cylinder is full of hot water but something is stopping it getting to the taps as a constant flow. It's not the motorised valve and the HW is on constant so it's not the timer. I'm aware that the water is drawn from the top of the cylinder and that pipe is red hot.