Unvented Cylinder - Heating Problem

[slid_away]

We recently changed our hot water/heating to an unvented cylinder system.
I noticed about a month ago that the radiators were getting warm when the hot water was on but hearing off.
I called the fitter who came out and said the only possible way it would do it if he had piped it incorrectly but was convinced it was correct.. He changed it over but it was even worse the other way around.
He then changed a valve (not sure what that is called but you can turn it and it has numbers on) and was certain it was that would sort it but after fitting a new one within 40 minutes the radiators were getting warm again.
The fitting has said he had done countless unvented cylinders before and never had this issue and is stumped
It's worth noting there are 2 motorized valves and not a 3 port valve
Any ideas would be hugely welcome.

[Etch]

It sounds to me like the motorised valve which controls flow to the central heating circuit is not shutting off completely. It has not failed 'fully open' because if this were the case it would not take 40 minutes for the radiators to feel warm. I suspect that some debris left over from the installation, probably a lump of solder, is stopping the valve from fully closing.