thermostat wiring

Yes that's right. When a bi-metal strip type room thermostat closes it applies mains voltage across a small resister in close proximity to the strip. Its called the pre-heater. The idea is to make the thermostat switch off a little earlier than it would otherwise do. In technical terms it reduces the hysteresis of the feedback system.

My advice is dump it and go digital.

When I say a small resistor I mean a physically small high resistence. About 220K-ohms IIRC

Having fitted a digitstat, and having loads of mechanical stats as well, I have to agree whole heartedly. A program of replacMEnt as time and money allows will happen, honest :-)

It sounds as if your stats have a *lot* of hysteresis - assuming that the timer for that zone is actually on, and that the volt-free contacts on the zone valves are wired correctly.

A mechanical stat with an accelerator heater built in - requiring a neutral connection - would probably improve matters. But as others have said, a digital stat would be far better and would provide much closer temperature control without requiring any additional wires. If you go down that route, you can automatically programme different temperatures at different times of day if you wish, as well as getting a free frost stat. You'll need, of course, to set the 3-channel programmer to 'constant' for any zones controlled by digital programmable stats.

I would be inclined to fit digital programmable stats on both heating zones, keeping the existing cylinder stat which will continue to be controlled by the appropriate channel of the 3-channel timer.

Thanks for advice guys. Al