Vokera Linea 28 Combi - water heating problem

[Sarah2]

Hi, having problems with hot water supply to upstairs taps/shower.
Around a month ago we noticed that the hot water temp was tepid and that the radiators upstairs were warming up even when the shower/bath were running when the CH was switched off.
Vokera engineer came out recently and replaced part of the diverter valve in the boiler as he said that he believed that the problem was with the Aqualisa shower and that the non return valves were failing and allowing cold water into the hot water system thus affecting the water temp and the valve in the boiler. He said that we should get the shower looked at and repaired and that this would resolve ongoing problems.
A plumber has been out today and isolated the cold water supply to the bath and shower and run the bath water but the problem remains.
Hot water to the bath is only hot when running very low or if the cold tap in the sink is running. Shower will run hot if cold tap has been on downstairs for a while but then revert to being just warm.
The boiler is 8 yrs old, well serviced and has given no problems like this before. Any ideas?? Thanks Sarah

 

[glasgowgas]

you say the hot water is only hot when its on low.

if you put your boiler to standby and run bath tap fully what flow rate do you get? you are looking to find a figure in litres per minute.

ie if it fills a 1 litre jug in 10 seconds you have 6 litres per minute.

non return valves failing not only puts cold water into hot pipework,
it also creates a back pressure of sorts causing cold water flow switch to close/open/close/open

post back with flow rates