Some advice please on DHW overheating problem.

[bulltaco]

Thank you picasso.

oilhead. Thanks, I'd rather guessed that the PRV was blowing off , so it's an inside the house job right enough. Thanks again.

 

[ianmcd]

The system is an oil fired Grant Vortex Eco boiler, pressurised system, Horstmann CentaurPlus C27 programmer and an immersion heater.
Just after lockdown began, the boiler tripped and I reset it.

so was it the boiler or the unvented HW cylinder that tripped ??????? they are different things you know

 

[Madrab]

... and there are 3 different pressure relief valves (PRV's) in your system ... 1 in the boiler, one on the UVHW cylinder (T)PRV and one as part of the combi valve supplying the UVHW.

2 of them nor anything attached to the unvented should be DIY'd.