I am trying to be a friendly neighbour by trouble-shooting someone's vented heating system. This system has operated fine for many years.

The problem: The boiler cuts-out after a few minutes after trying to heat the radiators.

The problem started when trying to get the heating going after the summer. During the summer, the boiler had heated the hot water cylinder without a problem. The hot water is still working fine (the boiler flow is hot and the boiler return is warm when the hot water is 'on').

The boiler feeds two Honeywell valves (one for radiators, one for the cylinder) via a heating pump. The pump is gently vibrating when 'on' and the valves have hot water on their outlet sides when operating. I have also tried manually opening the heating valve to eliminate it as the source of the problem.

I have bled all the radiators. I checked the small central heating 'top-up' tank in the roofspace (the ball valve was fine although the water was a bit dirty).

When the central heating starts up, the boiler flow gets hot quickly but the return remains cold. This happens even when I close off every radiator valve. Where is the hot water flow going in this instance?

When the radiator valves are open, the corner of the radiator near the valve warms up but the water doesn't seem to flow through the radiator.

Any diagnostic tests that I could perform, please?