hot water not staying hot in an unvented system

[selondonman]

hello we have an unvented system. we have an issue where the hot water is hot for 10 seconds and then luke warm, turn off taps / wait and turn back and it repeats itself. had boiler checked and told it was the expansion vessel. Had the expansion vessel drained and recharged. plumber told me that the pressure was holding, but still having the issue. The radiators work fine. Should i replace the diaphragm ? is it worth replacing diaphragm or just putting whole new expansion vessel in ? I turned on immersion heater and makes no difference. system 8 years old, had been working ok . thoughts ?

 

[Chuck]

Is your boiler a 'combi' that fires up whenever you draw hot water, or do you have a hot-water cylinder, e.g. in an airing cupboard?

 

[selondonman]

system boiler vailant 637 with centre store unvented cylinder . external expansion vessel

 

[ShaunCorbs]

Might have a nrv that’s failed and cold overpowering the hot

Showers are the first port of call

 

[selondonman]

Might have a nrv that’s failed and cold overpowering the hot

Showers are the first port of call

it is happening for all taps, not just for the shower, if that helps to find the issue

 

[SimonG]

As Shaun said cold backfeeding through mixer or shower.