Fitting a Honeywell CM927 to a Worcester Bosch 280 combi

[PhilStebbo]

Hi, I've just bought a Honeywell CM927 wireless programmable thermostat and need some advice on how to wire it up to my boiler.

Connection to power seems straight forward, but I could use some advice for the thermostat connection to the boiler.

The instructions tell me to connect the following:

Terminal A (Honeywell CM927 End) to T1 on my boiler
Terminal B (Honeywell CM927 End) to T2 on my boiler.

The Worcester Bosch 280 has 5 terminals decending vertically, the top 2 are for the Frost Stat (I'll call them 1 & 2) - so not bothered about those. The next two down would have had a bridge across them originally (I'll call them 3 & 4) and the botton one is labelled 'N' (I'll call that number 5).

Number 3 has a BROWN wire coming out that goes in to the existing (to be replaced) programmer and Number 4 has a BLUE wire that connects to the existing programmer - I'm pretty sure that they connect to the existing programmer because the original bridge in the boiler was removed - hence the existing programmer is acting as the bridge, which is great for me as the wiring is already in place. Number 5 has no wire coming out of it.

The Question is: Out of 3 and 4, which is T1, and which is T2 - I'm guessing 3 = T1 and 4 = T2, but would like to know for sure

Cheers
Phil

 

[MJN]

It doesn't matter either way. The CM927 has volt-free contacts i.e. in this scenario it is just a relay that closes/opens hence, as per your existing stat, it acts as the bridge across terminals 3 and 4. It will therefore work the same regardless which way round it is wired.

The blue wire should have a brown sleeve (e.g. a piece of brown insulation) to identify it as a switched live.

Mathew

 

[PhilStebbo]

Thanks Mathew - that makes perfect sense, cheers
Phil

 

[PhilStebbo]

One more question...

There's no mention of 'earth' in the installation instructions from Honeywell - my current programmer has an earth with two wires connected - one from the main power supply and the other from the wiring that connects programmer to the boiler's circuit board - what should I do with these two earths, are they now suplus to requirements?

Phil

 

[D_Hailsham]

my current programmer has an earth with two wires connected - one from the main power supply and the other from the wiring that connects programmer to the boiler's circuit board.

That could be the main earth for the system.

Connect them together in a connection block and hide it behind your new thermostat.

 

[PhilStebbo]

Done - all working - cheers fellas

Phil