Help wiring honeywell CM907 heating thermostat

Broadsword

I am replacing a Horstmann centaursat wall thermostat in my fully pumped conventional boiler system. Timer controlled, Gravity fed, no zone valves. There are three wires to the centaurstat wired to terminals 1,2 and 4 . Diagram here

http://www.tlc-direct.co.uk/Technical/DataSheets/Horstmann/CS1_7_I.PDF

I am replacing it with a CM907. Both of these thermostats are battery powered. I have transferred the existing 1 to the CM907 terminal A, 2 to B and 4 to C (there are only three CM907 terminals). I have also tried disconnecting C so only A&B are connected but it does not appear to work wired either way e.g it does not fire up the boiler when temperature is reached I have now reconnected the centaurstat with the three wires and it works fine.

How should I transfer the wires across to the CM907 (wiring diagram here)

http://europe.hbc.honeywell.com/products/pdf/en0h8547uk07r0906.pdf


Many thanks!

EliteHeat

Terminal 4 serves no purpose.

Terminal 1 to A 2 to B should have been the right way to do it.

If you have a multimeter then I would be checking the relay is switching OK if I were you.

mikey72

Terminal 4 was the "neutral park" on the Horstmann.
Connecting it on the new CM907 will directly short out the live to neutral. If a fuse was fitted it should have blown. I'm surprised the old Horstmann worked when it was put back in.
The new CM907 doesn't need the neutral either. It may have been damaged by the short circuit. The neutral should be disconnected, and safely put out of the way, in a terminal block and insulated, or similar.

EliteHeat

mikey72 wrote: »

Terminal 4 was the "neutral park" on the Horstmann.
Connecting it on the new CM907 will directly short out the live to neutral. If a fuse was fitted it should have blown. I'm surprised the old Horstmann worked when it was put back in.
The new CM907 doesn't need the neutral either. It may have been damaged by the short circuit. The neutral should be disconnected, and safely put out of the way, in a terminal block and insulated, or similar.

It wouldn't bother the Horstmann as it isn't connected to anything.

I reckon you're right about frying the new one though!

mikey72

EliteHeat wrote: »

It wouldn't bother the Horstmann as it isn't connected to anything.

I reckon you're right about frying the new one though!

It should have bothered the timer as well, or whatever was feeding the Horstmann, at least it should have blown the fuse, if there was one fitted. If not the trip in the consumer unit should have gone.