Hot water but cold rads

[PokerP]

I have a Worcester Greenstar Ri
Woke up this morning with plenty of hot water but a cold house.
All the rads were luke warm.
From searching online I tried switching the 3 port valve to manual.
This seemed to result in the boiler heating and the rads became warm, however this only lasted for about 15 minutes before the boiler turned off, by which time the rads were nice and hot.
I was unable to get the boiler to come on again until i ran the hot water for a while, after which the same thing occurred; the boiler came on for another 15 minutes and all the rads were hot.
I assume the boiler is only responding to a request for hot water?

 

[SJB060685]

Actuator motor or switch buffered, providing you're getting a feed from programmer and room stat. Needs testing correctly to pin point exact cause with a multimeter.

 

[garygvl]

youve either got a faulty programmer, room thermostat or 3 port valve. as its responding to a hot water demand where the 3 port valve doesnt have to do anything either turn your cylinder thermostat up to maximum or if its just clamped to the cylinder by a net curtain wire just remove it from cylinder and leave it on airing cupboard floor so it doesnt pick up cylinder temperature and turn boiler off. leave the 3 port locked in the manual position. forget about heating settings on programmer just turn heating to off. turn boiler thermostat settings to sort of mid position and set hot water on programmer to constantly on. you will now get heating and hot water until problem is sorted. if you get too hot just turn hot water off on programmer