Let me start by saying that central heating systems are a black art to
me. Ours is a conventional oil fired boiler supplying DHW and three
traditional wet radiator zones. Two zones are working, but the third is
not, and has not been used for two or possibly three years.

The zone valves are Honeywell, with a silver box on top, and having read
that the protruding lever should have resistance when the heating is
off, and no resistance when on, I have found the offending valve. There
are three valves in total, presumably one for each zone. Just one pump,
too.

I've taken the top off the silver box (one screw) and can see the
actuator, but beyond that? Will tapping the actuator or the valve or
anything else release it? I guess it is stuck because it hasn't been
used. Can the actuator be changed without water pouring out?
--
Graeme