Amateur here. Not an electrician but can do basic first fixings on plumbing. Oil fired Worcester Bosch boiler Green star Utility 35/50 BFC. 5 zones plus DHW cylinder so six zone valves each two port. "Nest" stats (learning L2) on four zones - Front first floor Back first floor Front Ground floor back Ground floor and UFH in first floor bathroom. DHW directly off the boiler on a cylinder stat. All have been working fine for three or four years. Tend not to run the heating on the first floor full time front as it is spare bedrooms so only intermittent. On switching on this winter, found that First floor front zone would not come on the radiators despite the zone valve being open. Tested the light brown wire on the connection box and found no voltage so concluded that even though the valve was open and floppy the internal microswitch (not the obvious outer visible one) was not telling the boiler to fire up. Took off the motor head (Honeywell two port valve) and replaced it with one from Screwfix (!) Cowley and ,Lo! and Behold!, after replacement then got 240 volts on the brown wire. Thought my problems were over BUT the boiler would not fire up even though the motor opened up the valve. Checked the orange wire and that was live. The pump in the basement boiler room appears to be working and is lit up but no circulation evident. Took off the motor from the 2 port valve to the first floor front zone valve and made sure the valve was open (twisted anti clockwise 80 odd degrees?) No circulation. Tried to switch of all other zones (all of which work fine) so as to try to force the pump only on that zone system. Checked for an air lock and the AAV has water at the top (the top hand screw was not working so had to release the olive below just to check if air was present and none was present - water right away). It surely cannot be the radiator TRVs (four number) which are actually not on radiators but finned 12mm pipes controlled by a remote thermostat switch as there is a complete by pass pipes of flow and return off which they are directed from the flow and to the return pipes. Scratching my head now ....
