Potterton Suprima room thermostat wiring

[barry19]

I have an old BG RS2 room thermostat controlling a Potterton Suprima boiler. Want to upgrade this old mechanical room stat, which still works, but has no digital display of temp or LED indicator to show heating is on. The stat wiring only has two wires, live and switched live, plus tied earth. The 5 core cable into stat from boiler also has two black leads which have been cut back. I need a neutral for the digital thermostat.
My question is can a Potterton Suprima have both 2 and 3 wire thermostats fitted? Are the black wires likely to be unused neutral which I need?

 

[stem]

The basic answer is yes. A room thermostat is simply an on/off switch. When the thermostat senses heat is required the thermostat will electrically connect the two wires you refer to together, when heat isn't required it will separate them, and that's all that they do. The three main wired types of thermostat are:

Mechanical Thermostat (eg RS2)
As the name suggests is purely mechanical, an internal bimetallic strip physically moves the switch on/off as the temperature changes.

Battery Operated
Some digital thermostats are battery operated. These only need the same two wires to the on/off switch, making them an easy exchange for a mechanical type.

Mains Operated
These use the same two wires for the on/off switch exactly the same as the other two types do, but in addition a permanent L & N is also required to provide it with operating power.

You will also see Wireless, or RF thermostats. A receiver part has the same connections as the mains operated thermostat. Then a battery operated thermostat which can be located remotely communicates with the receiver wirelessly. This type is generally used with new installations as it avoids having to run a cable out to where the thermostat is to be located.

As regards the unused wires you have, that would be down to what the original installer decided to do with them, but most likely they will have been cut back at both ends.