Intermittently no hot water from boiler

[MarkSmith]

Hi,

There seems to be a problem with my boiler or a related component: it often fails to give hot water.

Description of the system
Cold water and header tank in loft.
Potterton "Profile" boiler.
External pump, and mid-position valve.
Hot water cylinder and central heating system.

Description of the problem
Start from everything switched off, room thermostat up to full.
On the controller, select "hot water advance" or "central heating advance", or both - result is the same.
The mid-position valve moves to the appropriate position.
Some whirring starts (hard to tell what's making it - the boiler? the pump?)
Click click, the boiler pilot light ignites.
But the hot water pipe coming out of the boiler does not get hot.

If I pull the fuse from the boiler, then replace, it fires up and water gets hot. Looks like it's working. Then some time later (e.g. next morning), rads are cold or tepid, apparently no (or only leftover) hotwater. Unclear whether it came on at 4am (as per programme) and subsequently stopped, or didn't come on.

Any ideas please?

Possibly related: we first noticed this immediately after having 4-5 electricity brown-outs a few days ago.

Thanks,
Mark

 

[Agile]

It sounds like a boiler fault which will need a gas registered engineer to diagnose and repair.

Tony