I have a nice whizzy CM907 that I would like to replace my old thermostat.
The old stat has three wires in red to one, blue to two and yellow to three. The old stat is an apprently boring honeywell.
The system is pumped and has the valve for heating and hot water. Looking at things, and making assumptions - red would be rasonable as live, and go to A, so is it safe to assume 2=B and 3=C?
Ta, Paul
'Zackly why I was asking! Ta Muchly.
How do I confirm which is which?
Just spotted a diagram inside the cover of the old 'stat. 1 (red) does indeed look live. Image at
Looking at the diag, and at
No!!! One of the three wires to the old stat - blue, hopefully - is a neutral so needs to be made safe.
Sounds about right. The neutral was used in the old stat to operate a small heater which helps to reduce hysteresis (the temperature difference between switch on and switch off) in the stat.
It must *not* be connected to anything in the new stat, but must be taped off and made safe. If you do connect it, you'll have a condition - either when the stat switches on or off, depending on which way you've wired it - when the stat switch will directly connect live and neutral - resulting in a loud, and potentially expensive, bang!
Put a choc block on the end. Seems to work OK, no smoke. Heating on when on at the timer and temp < thermostat temp, off when temp > thermostat temp.
Not the ideal time of year to test a fancy thermostat I know!
Thanks for the help, P.
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