Heatrae Sadia C1200 central heating not working unless the hot water is on

[nonnynonus]

In the flat we have moved into, we have a Heatrae Sadia C1200 linked to a vented cylinder for the hot water which works to heat the hot water perfectly fine. Since its started to get cold we've needed the heating but it only works when the hot water is also turned on (which wastes a lot of energy). I don't know if I'm missing something obvious that can be fixed quickly, but i see no reason the heating shouldnt be able to work fine without the hot water?

 

[ProDave]

The first thing I would check / replace is the 3 port motorised valve.  They are notorious for the microswitches failing so they either don't hold position as they should, or don't provide the "call for heat" to the boiler.

 

[Sharpend]

As Dave says the three port valve will be first place of interrogation. You can check this by putting on hot water only and feeling the pipe leading to water tank heating up, then putting heating only on and a)listen for valve motor operation and b) checking which pipe leaving the valve is heating. Are they heating equally? Or does one take longer than the other?