Central Heating Control ..... ?

"Wall thermostats, when fitted, are in series with the CH signal from the time-switch. Some have a built in resistor ("accelerator heater") to reduce the hysteresis, i.e. the "backlash" in temperature between switching on and switching off. When the thermostat is on, the resistor supplies a minute amount of heat, which makes the thermostat think that the room is slightly warmer than it is, and therefore makes it switch off again earlier. The room temperature will thus fluctuate less. Such thermostats require 3 cores and earth: live, switched output and neutral for the resistor. If live and switched output are interchanged, the resistor will be powered constantly and the advantage will be lost. Simpler/older thermostats only need 2 cores and earth, so if you're replacing a thermostat you might want to check the wiring beforehand to see if you can use this feature."

Found by a Google search on "room thermostat accelerator heater"

Don't be spoilsport Sue. If you give hints like "Found by a search on Google" then there would be no posts here at all :)

It is worth mentioning that this thermostat may not be suitable for a boiler that uses 12 or 24 volts on it control output.

Adam