Connecting room thermostat

[Hugo Nebula]

I'm replacing my room therrmostat with a programmable one, but the wiring doesn't seem to correspond. Below is an (Arthur) ASCII sketch of the existing wiring, with the colours alongside. _____ _____ -|_____|-|_____|--¬ | |_ 3 N (Blue) |_______________ \|____ 2 H (Yellow) . |______ 1 TL (Red)

The new thermostat has connections for Common, Heating Satisfied and Call For Heat. Which is which? Am I right in thinking H (yellow) is Call for heat, TL (red) is Common, leaving N (Blue) as Heating Satisfied?

If it's any help, I've traced the wiring back to the main wiring block, and the red of the existing thermostat connects to the live from the programmer, the yellow connects to the white of the 3-way valve, and the blue to blue wires from the pump, 3-way valve and boiler.

[Dave Plowman (News)]

The blue is likely neutral and not needed on a battery operated stat. Insulate and tape up out of the way. (It was needed for a heating coil in the old stat to improve the accuracy and response of a bi-metallic strip; modern units use a solid state sensor which is vastly more accurate and reacts near instantly.)

To simply replace the existing one, red to com and yellow to call for heat. The heating satisified - the normally closed contact - would be for 'special use'. To use it would likely need a total re-jig of your wiring and IMHO isn't needed to replace a 'standard' thermostat.

[Set Square]

You haven't been listening, have you? [This is a fairly frequent question in this NG!]

Your old stat had a little heater in it to warm up the innards when the heating was on, and to make it more responsive - reducing overshoot etc. The blue (neutral) wire is the return for this heater.

Your new electronic stat doesn't have/need an internal heater, and only uses

2 wires. The blue wire *must not* be connected - but must be insulated and made safe.

The other 2 wires connect to COM and Call for heat - it doesn't really matter which is which since it's just a switch, but convention has it to connect Live (R) to COM and Switched live (Y) to Call for heat. [The Heating Satisfied terminal is not used in this application - but could be in some - or could be used if you were controlling a cooler rather than a heater].

HTH.