Cooling with a Robur K18 ASHP

[kdmnx]

30 Year old Potterton eh, would not be one the Netaheat or Profile ranges would it.
A lot of them had positive pressure combustion chamber design which made room sealing a safety issue. Positive pressure boilers were on our radar to do extensive case sealing checks on. A few Worcester 9.24s out their too with similar issues.
Get it replaced with a modern condensing boiler with load and compensating controls and and benefit from improved efficiency and better reliability. Win Win.

Are modern boilers considered reliable? I've had a few fail within the first few years with faults caused by horrible design choices. Eg a Main combi failed after 2 years and 2 months (2 months after the warrantee ended) because the valve that switched between hot water and heating leaked onto its own power supply causing a short. As the plumber was showing me the fault I couldn't help but think it was a terrible piece of design. The plumber reassured me that Main boilers are "better than most" but said that the standard wasn't particularly high.  

[Philip R]

Slightly off topic from OP.

I have dealt with a similar failure of a same type of divertor valve fault in a Baxi Platinum combi. Wet the kitchen and trashed the actuator motor too.

The trouble with combi s is the divertor valve and plate heat exchanger. Your Main boiler is basically a budget baxi duotec/ platinum. Baxi just use a different warranty, price point and a cheaper aluminium main heat exchanger in the Main brand ( Quite a reasonable boiler).

Your plumber is probably right. The problem is that we all have our favourite brands of boiler. A lot of the big installers ( Those who advertise on the TV) like to fit market brand leader but won't mend them later.

Personally, I fit the Glow-worm, some people don't like them, but I think they are good all rounders. Great controls too. I fit ideals too sometimes ( same divertor valve as the Baxi) .

The best divertor valve (Inherently Leak proof) is that fitted in the Viessman and Worcester Bosch combi s (That is the only good bit of a WB imo.)

Seams that corrosion inhibitor additive can lead to premature divertor seal failure, but so does corrosion and sludge deposits too. Therefore must have a leek free system, good expansion vessel and non drip PRV along with a good magnetic system filter to keep to particulate levels down.Other things that dont help matters are very hot water and high system pressures.