Vaillant 242/1 E - Hot water only lasts for about 10 minutes

[UserNameIsTaken]

Vaillant 242/1 E - Hot water only lasts for about 10 minutes

Any ideas what is wrong with it? Getting frustrated in the shower, when the hot water cuts out after 10 minutes or so (sometimes after 5 minutes, sometimes not at all). To get it hot again I have to turn it off, wait a few minutes, then turn it on again and usually it will work again.

It appears to stop if the pressure fluctuates a bit. I can recreate it by running hot tap (hot water ok at this point), turning it off, then on again straight away. The boiler just does not kick in again.

Other info:
Boiler at least 12 years old (I think)
June 2012 - pump replaced (2nd hand)
April 2013 - Diverter valve unit replaced (new)
There is a caleffi pressure reducing valve on the inlet pipe too. Been there for about three years. (this one has no guage - 3 bar printed on top)

Thanks in advance for the help!

 

[UserNameIsTaken]

Also the pump is hot to touch... is this normal?

 

[RoryD]

Does the boiler actually cut out or does it continue running whilst the water goes cold??
A GSR Engineer should check pressures max/min and check resistances on sensor(s) - unfortunately I haven't them to hand.

 

[Dotty]

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