many thanks to brian for his earlier help
further to my earlier request i have checked my grundfos selectric pump & it is working as it should so i now suspect either the manual roomstat or the diverter valve
any suggestions would be greatly appreciated

john p, April 2011

Assume Combi. with diverter valve not working. May be pressure diaphragm has given way (cheap & simple to fix once diagnosed) or the microswitches operated by diaphragm have gone open/closed circuit. Typical symptoms are Set Heating plus Hot water (gives hot water for a while) or Set Hot water very slow heat & nearest radiators warm up when boiler set to hot water or Hot water much better if all rad valves closed.

Can replace entire assembly or once diverter valve removed, mark alignment, dismantle & check diaphragm (may be split), check that microswitches operate. Be sure to label/photograph wiring, completely isolate boiler electrically and shut off & drain boiler. You may find it difficult to access diverter valve pipe connections:- best to measure nuts & buy secondhand hefty spanners to fit the usually quite large nuts.
If unsure, the work should only be undertaken by a qualified professional.

Geoff Ipswich, April 2011

hi john if you have got gravity fed hot water you wont have a 3 way diverter valve Im confused.

but you have, so you need to turn heating on go to the valve get someone to turn the thermostat on and see if you can hear the motor slide across there is also a slide on the side if you hold your finger against it you will feel it move. if it does then it should slide across and hit a microswitch that turns on your boiler if it dont then either wall stat or valve check power to wall stat and continuity across terminals or loop a wire across terminals to create a circuit 240 volt !!!!!! be warned.
if it aint the stat its the valve. best I can do for you im afraid. good luck

Brian Emian Plumbing & Heating Norfolk, April 2011