Wiring a room thermostat, could do with some wire help pleas

It is usually the design of the plumbing rather than the wiring that means the HW has to be on for the CH to work. If you have gravity hot water and pumped central heating it works like this;

Switch on the HW, the boiler fires, and the water circulates by gravity around the hot water cylinder.

Switch on the CH and that starts the pump up that circulates the hot water around the radiators. But you have to leave the HW on to keep the boiler running.

Programmers that operate this type of system are usually configured like yours, so that it's not possible to have the CH on without the HW.

Later systems have pumped hot water and heating and are controlled by motorised valves, so that they can operate independently.

So as long as you have your HW set to be on when you want CH, it will work.

Interesting info Stem, thanks for the lesson! I guess that this older design is less efficient than the newer ones with independent motorised valves, something to bear in mind at some point in the future I guess.

p.s. DP after having slept on it, I've ordered the Danfoss TP5000Si. - Not sure how I overlooked it, but the Siemens can't set different times/days ie. 5+2.

thanks everyone
valmiki