One of our zone valves has failed in that the spring on the little lever
has broken, or possibly come adrift. The lever moves across without any
resistance in either direction with the result that, when the boiler is
fired by any zone, the radiators in the 'unfired' zone heat presumably
because the motorised valve is permanently open.

I'll need to dismantle the valve a little, just to see whether it is a
broken spring, but what I'm not sure is how much dismantling can be done
without water coming out unexpectedly! Any clues? Thanks.
--
Graeme