Replacing room thermostat

[jonnybegood]

I have an old sunvic mechanical room thermostat that was disconnected when the new combi boiler was installed in the loft.
I now want to connect a new honeywell digital CM901 programmable thermostat in the hallway to replace the old disconnected sunvic.

There are 3 wires going to the existing thermostat
The other end where the wires used to connect to the old boiler has been terminated.

Looking at the manual for the new thermostat it only needs 2 wires from the stat to the combi boiler, a voltage free switch.

Can I use the same cable to connect the new stat to the boiler in the loft as I am only looking to create a switch, well reading the manual this seems to be the case unless somebody knows otherwise.

The stat is powered by 2X Batteries, plus I would need to extend the existing cable that has now been terminated in the old boiler room so that it can reach the loft, is this a simple case of a junction box to connect the existing 1mm cable with around 10m of heat flex as the cable will be connecting to a terminal block inside the combi boiler and is this cable ok for the voltage free connection.

Thanks for any advice, I can provide screenshots of the manual if required

 

[Taylortwocities]

You'll be fine with the old cable sssuming it has been been completely disconnected from the old boiler and circuits.
Terminal box must be accessible. You only need two wires for the combi.

 

[jonnybegood]

I have removed the old thermostat and can see 3 wires, 1 yellow , 1 red and 1 blue and then the earth attached to the backplate.

Is there an easy way for me to test that all 3 wires have been completely disconnected using the multimeter, the terminated cable I found directly above in the old boiler room seems obvious but would like to make sure that for some reason the old thermostat has not remained connected elsewhere.

 

[Taylortwocities]

Start with the earth and test to make sure there is no voltage from earth to any of the coloured wires.
Then test between each of the coloured wires in turn so all combinations have been checked.

Then short out, say the red and the earth wireas and check you can see the short at the other end. Do the same for each of the other wires. then you will have checked continuity and that there's no voltages straying onto your cable from some hidden junction box..

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