central heating or hot water not both unless...?

[dcollier]

:?: hi folks - been trying to fathom out this for 9 months - have a fully serviced and ok 18 months old gloworm 12hxi condensing feeding a tank - if i switch on hot water either by main hw control panel or by 1xhr. boost on my controller horstmann c27 i get lovely hot water but the central heating will never come on despite the boiler's numerous attempts. it takes up to 12 hours with hot water off before central heating will work.
if i leave hot water off central heating works perfectly. the only way i can get both hw and ch is by ensuring that the central heating is actually burning,up and running first and then using boost x1hour on the c27. i can only be sure of warm radiators and a shower on a cold and frosty morning by putting a 1hour boost on at night when i go to bed - otherwise having hot water switched off permanently.
this cant be right, can it?

[htg engineer]

Check the motorised valve it may be sticking, also check the three port valve for any signs of a leak and check the spindle turns freely.


htg