Replacing old mechanical room stat with Honeywell CM907

[rj1211]

It said 'will replace any room thermostat' and I can see that it is a simple switch. However, the stat I am replacing (an old mechanical one - marked DELTA) has three live and one neutral. The wiring diagram on the old one shows live and neutral coming in and wired to terminals 1 and 2 respectively. There are then two switch terminals 3 and 4 which are marked 20(4)A and 6(4)A on the diagram but don't show where the connections end up. I assume they are off to the boiler or to a control valve but don't know whcih I should be using for the CM907.

I have a standard pumped CH gravity HW with a timer control for each and want to leave the CH on all the time and control it with the CM907.

Can anyone help?

 

[jackthom]

Not sure about this but if you have a multimeter or mains tester screwdriver you'll probably find the Delta has a Common Live terminal plus a pair of changeover terminals so it could be used for cooling as well as heating.

The CM907 also has a Common and 2-way operation so fairly sure it could be wired the same way if that's what your control system needs.

 

[twgas]

The 2 power out wires will be calling and satisfied, once you sus which is which (multi-meter or mains tester) you only have to look at new MIs to see what goes where.
Write down what is what before you disconnect anything, at least if it all goes wrong you can replace the old and have a think about it

 

[D_Hailsham]

the stat I am replacing (an old mechanical one - marked DELTA) has three live and one neutral. The wiring diagram on the old one shows live and neutral coming in and wired to terminals 1 and 2 respectively. There are then two switch terminals 3 and 4 which are marked 20(4)A and 6(4)A on the diagram but don't show where the connections end up.

20(4)A and 6(4)A refer to the current which each terminal can carry; it has nothing to do with heating and cooling. Have you checked which terminals have wires connected to them? I doubt if there is one connected to the 20(4)A terminal.

I have a standard pumped CH gravity HW with a timer control for each and want to leave the CH on all the time and control it with the CM907.

I assume you mean that the hot water cylinder is fed from a cold water tank in the loft; not that the circulation of water from the boiler to the HW cylinder is by convection aka gravity, i.e not pumped.

Leaving the CH on constant is OK - until someone "accidentally" changes it. I think it's safer to feed the new thermostat from a permanent source which can be done simply by moving the wire connected to the timer CH on terminal to the L terminal.

Wiring of the CM907 is easy. Live (old T1, from timer) to terminal A; switched live (old T4, to boiler) to terminal B. The neutral is not used and should be made safe at both ends.