A classic with a twist - no HW, CH OK, conventional boiler

I'd be looking for a boiler engineer that understand and is happy to work on your system. It may be a simple fix, but I suspect a 1995 boiler is unlikely to have a permanent pilot light. From your description I would say it is overheating, and causing the overheat protection to kick in. Overheating is usually down to poor circulation, (and kettling indicates the same problem), so the problem lies within the system rather than the boiler. I would be reluctant to change the boiler unless it was completely knackered, a new boiler will take a long time to pay for itself in gas savings, if at all during its lifetime.

Sludge and corrosion are possible issues, as is a weak pump. Pump head has been changed at some point, can you get to it and see what speed its set to? The Magnaclean would also be worth checking, if it contains a lot of crud it, (apart from needing cleaning out!), would indicate a dirty system. (Check pump speed first though.)
 
I think that's the biggest problem - finding someone. I'm willing, now, to think about investing in a newer boiler to save the issues we have seen since moving in... and tbh we plan on being in this house for a good 10-15yrs min, so we would eventually recoup the boiler costs. I may investigate Green Deal options to see if I'd be eligible. There is a huge lack of trust by all of us in the boiler and I think we're at the point where we are resigned to changing things to improve the house and settle our nerves about the system.

Anyway - the pump is quite possibly an issue. It's a Grundfos (blue, but that's all I can tell you at the moment) and has been set to 3 (out of 3) speed wise. It was on 2 until a fortnight ago when we (sorry, bathroom fitter) attempted to get the new tall rad working and balanced the heating system. I moved it back to 2 but that caused most rads to stop heating up, so it's back on 3. I'd be happier with the pump on 2 from an energy saving perspective. Presumably that needs me to re-balance?

The Magnaclean is next on the list to unscrew/check. To be honest though, rather than me now faffing around with it I think my best bet is to call a couple of local plumbers and see if I can find someone willing to take on the task.

You're right - the Potterton F60 has one of the pilot lights that kicks in only when needed (or so the literature would tell you), so often cycles on and off.

Ironic that kettling is down to poor circulation but there's so much air that gets into the system, something must be circulating perfectly in that respect!
 
Looking at that pump, it was a Grundfos, but someone has changed the head, possibly British Gas looking at the colour. Please bear in mind, the boiler only heats the water, it still needs to be got round the system. Fitting a new boiler to a poorly performing system is like trying to inflate a tyre with a puncture, it wont achieve very much, and could shorted the lifespan of any new boiler. You need to be treating the cause, not the symptoms.

All boilers will heat the water until either the external controls tell them to switch off, or the boiler's internal stat is satisfied. If the system cannot pull the heated water away from the boiler quickly enough then the boiler will switch off on by its internal stat. If air continues to be an issue then its the pipework that needs looking at, if poorly designed it can allow 'pumping over' which will cause air to be drawn into the system.
 
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