...or am I being spun a yarn?

Long story short, a few months ago I had my old heating system 'modernised'. Had the unpressurised heating converted to a sealed system and an unvented hot water cylinder fitted. Kept the 20+ year old boiler which was working fine. I also had a system flush and boiler service at the same time. Had a problem where I noticed that the pump run on was not working and sometimes the boiler safety shut off button would pop so called them back. They replaced the run on thermostat and boiler stat. It now runs on how it should when the system is hot. Boiler button is still popping occasionally and I've narrowed it down to doing this ONLY when the hot water part of the system is in operation.

Phoned them up and they reckon the heat exchanger is scaled up and it cannot get rid of the heat quick enough but it can when the heating is on because I have three radiators that are fully on at all times.

The return valve is fully open and it doesent starve any of the radiators of heat. I have run the hot water system and observed it and the run-on kept going for about 8-10 mins when I turned the h/w temp down but it literally is just some 22mm pipes going from the boiler in the loft to the h/w cylinder and back so I can't see where it can lose the heat.

Had a shower last night before bed and heard the pump running (actually got out of bed to check - getting obsessed now) and it was just the hot water warming up (I have it on continuous) but again, it must have not been able to get rid of heat and 'popped' the boiler safety as just the pump was running this morning.

I know with cars, pressurising the system raises the boiling point of the water so could this be contributing to the problem which I didn't have with the old unsealed system? Didn't even have a bypass/return circuit on the old system (the heating company said I should have!).

No problems at all with the heating side of the system.

Now, I don't mind changing the boiler if I have to but with almost nowhere for the hot water only circuit to lose heat when the pump is on overrun, is the same thing going to happen or is it because my boiler is scaled up?

It's a Potterton profile 60e and I've drained it a few times over the years when moving/adding radiators and never bothered with corrosion inhibitor. Never ever had it serviced either. I bloody said in jest to the wife we'll probably have trouble now we've had it serviced and it looks like I was right!

Never had this problem with the old setup although I was permanently bleeding air from one particular radiator.

If it is the boiler, could I add a small radiator or extra piping into the hot water circuit to help with heat loss when boiler shuts off but pump keeps running to bring temp down before fully shutting down or just change the damn thing?

New boiler, modify the system or get them back to have another look? Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.

I would suggest it is more to do with the H/W setting on your pressurised H/W tank thermostat, as you say it's only a couple of 22mm pipes to the tank indirect heating, therefore the heat is dissipated in the H/W tank. Should the boiler hot water setting be too high it cannot dissipate the heat into the H/W tank 50deg C is plenty hot enough for hot water though many are set at 60deg, really a waste of gas. Now that you are on a pressurised system it may well be that the new pressurised H/W tank does not have the same surface area or route that the old indirect tank had and therefore not be able to dissipate the heat at a fast enough rate, is the safety trip adjustable or knackered and is set low? the settings would be different for a atmospheric and pressurised system, the object being not to boil the water in the heat exchanger so will cut out in the low 90's to account for latent heat continuing to heat the water after shutting off the flame. my PC is slightly to the right of Genghis
2012 HSE SD4 In Orkney Grey now gone, best car ever.

I fitted a rad for almost exactly that reason for my folks - works a treat!

Just a thought - I'm assuming there is a relief valve which allows water flow once the HW demand is satisfied, to allow the over-run to actually lose heat if there is a zone valve on the water? 2000 Xei 1.8 gone
2004 HSE Td4 Auto, MT's, sump guard, sliderz - gone
2014 GS Man - AT's, Mantec Sump guard and tank guard, split charger in boot, towbar, work in progress...

I'll try knocking down the immersion tank and boiler temperatures a little to see if that makes any difference. There is a relief valve and that is now fully open. The plumbing co. said I could try that but it might starve some or all of the furthest radiators from getting hot when the heating is on but they all still get hot. Don't know if the safety trip is adjustable but they have already replaced that (is that the boiler 'stat?) at the same time as they changed the overrun stat. I know they changed two stats in the boiler but at work now so cant see invoice to confirm.

If knocking the temp down doesn't cure it, it looks like I might need to fit some kind of heat soak in the H/W circuit as simong-uk has done. Can't see a new boiler being any different if you cannot remove heat from the heating circuit.

Will the speed of the pump make any difference? It's set on speed 2 of 3? Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.

The bypass needs to always return a small amount of water therefore ensuring an open circuit and yes the pump speed may be too slow the pump needs to dissipate the heat from the boiler fairly quickly