Prob 3way zone valve in airing cupboard that needs a new actuator motor (if it's a fully pumped system) or the valve might be jammed in one position,if so renew the complete valve- try removing the actuator and turn the 'D' shaped shaft by hand shouldn't be too stiff to turn ;or poss air lock in flow pipe vent at side of cylinder. Vent the end of the pipe with the bleed valve on. If it's not a pumped system then the circulating pipes are air locked. Check that the header tank in loft isn't dry.
Dave gas-man, August 2008
