After the Cylinder Came Out: Faults on Converted Systems
Plenty of Liversedge houses swapped a hot water cylinder and a tank in the loft for a combi at some point, and that change alters how the whole heating system behaves. An older open-vented system ran at low pressure, fed from the loft. A combi seals the system and holds it under pressure, which means radiators, valves and joints that were perfectly happy for decades are suddenly being asked to hold more than they ever did.
That is why a converted terrace can start losing pressure months or years after the work was done. The boiler itself is often fine. Somewhere a radiator valve or an old joint under the boards has started to weep, or the expansion vessel has lost its charge and the relief valve is letting water go every time the heating warms up. Topping the system up keeps it going for a while, but it brings fresh oxygen in with the water, and in West Yorkshire's soft water that feeds the corrosion which ends up as magnetite sludge.
The repair is finding which it is. We check the vessel charge, looks at where the relief valve discharges and works back through the system before naming a part. If it is the vessel or a pressure sensor, both normally come off the van. If it is a leak on the pipework, you are told where it is and what putting it right involves, and the price is agreed before anything is opened up.