thermostat wiring

[Al M]

My house was built 2002 with an oil fired boiler and has two heating zones, one upstairs and one downstairs with a wall mounted thermostat (in bedroom and livingroom) for each zone . There are no TRVs anywhere. (I'm using a digital 3 channel programmer to time the 2 zones and the hot water.) Problem: The thermostats work if I manually turn them up and down but they don't seem sensitive enough to adjust with the rise and fall in temperature. What I mean is if the temp is high and the heating on I turn the dial down until it just clicks off. (say 21deg) Next morning when its freezing cold the heating hasn't come on. My thoughts are that it's because the stats only have two wires going to them so the stat is only switching the live feed to the motorised valve. There is no neutral wire to actually make the thermostat itself work "properly". (not sure how to explain this technically) Would a digital thermostat with a battery power source solve my problem or am I missing the point? I don't want to have to start tracking wall and rewiring. Any advice? thanks

AL

[Graham]

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.

there is no neutral to complete the circuit it doesn't work.

My advice is dump it and go digital.

[Graham]

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

[The Natural Philosopher]

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 :-)

[Roger Mills]

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.

[Al M]

Thanks for advice guys. Al