That’s a power supply for the smart thermostat, required in systems lacking suitable power at the existing thermostat. My Ecobee required a gizmo installed at the furnace for the same reason. I don’t think the power adapter came from Bosch but was likely added by the installer.

 

Since it is not clear what that wall wart voltage is, nor is it clear if the thermostat needs AC or DC, it is not clear if it could even use the 24 volts from the furnace transformer.
I am guessing it is a furnace because the TS has not responded with that information.
The thermostat needs the external power because of the fancy LED illumination. NONE of the smart thermostats I have seen that are not illuminated need external power, although many can use it. Instead, it needs a battery change yearly.
Once again: I suggest running the added extension cable from the thermostat terminals. I can not imagine a thermostat with a barrel connector for power.
ALSO, once again I suggest replacing the cable from the furnace with one having the correct number of wires. Then it will be done right!! Usually not that complicated, unless the installer has no clue as to how to pull wires.

 

I think The thermostat could be a "Bosch EasyControl CT200" If it is then here is a link to posssible information on it.
I am only going by the picture that the TS has posted without providing a model number. so I may have the wrong model number. It would be helful if the TS could confirm this guess.
I have not yet read through it.
Les.

 

Personally, I would look into finding a replacement wall wort (AC or DC proper voltage /current etc.) with screw connections.

Some crimp terminals.

A roll of white lamp cord.

Plug the supply into the proper outlet, connect and run the wire from the supply to the thermostat along the baseboard and trim using insulated staples or some better kind of attachment system. (wire mold?)

That would give a connection between the supply and thermostat without any splices and look much better.

 

Personally, I would look into finding a replacement wall wort (AC or DC proper voltage /current etc.) with screw connections.

Some crimp terminals.

A roll of white lamp cord.

Plug the supply into the proper outlet, connect and run the wire from the supply to the thermostat along the baseboard and trim using insulated staples or some better kind of attachment system. (wire mold?)

That would give a connection between the supply and thermostat without any splices and look much better.

We already know that the wire from the wall wart is not nearly long enough. and already the suitable extension wire has been described as white speaker cable, the 2 conductor #24 wire size. AND I have already pointed out that a better alternative is to replace that short run of five wire cable with a length of six conductor cable made for exactly that application.
And now it occurs to me that perhaps the white wire was already in the cable but not connected on the other end.

 

everything is installed and working however as you can see it’s running off an extension lead temporarily which I don’t like I would rather it be to the mains and hidden so just want to extend it
Thank you

Hi,
This is all the TS asked for, IE a simple extension to his 12V Wall wart output cable

Why are we suggesting rewiring the complete system?

E

 

My suggestion was based on aesthetics...nothing more.

 

Hi,
This is all the TS asked for, IE a simple extension to his 12V Wall wart output cable

Why are we suggesting rewiring the complete system?

E

Why am I suggesting replacing a cable that no longer is adequate?? Very simple: Lack Of Reliability!!
Surface wiring is subject to being disrupted! in addition, wall warts are subject to being bumped and accidentally unplugged. And while a cooling system that stops working in the hot season is uncomfortable, a heating system that stops working in the cold season can become unhealthy, at least.