Air Lock In Gravity Domestic Hot Water Loop?

[TJBONE]

I am having a new kitchen fitted which required me to move the central heating circ pump which lives behind the sink unit adjacent to the boiler. The boiler is a Baxi Boston 2 60 RS. Before disconnecting the pump I closed all the radiators, isolated the expansion tank supply and drained the water from the radiator circuit from the low level drain cock outside the back door. I reconfigured the pipes and refitted the pump in the new position (12” to the left, not a big move) and re-commissioned the pump, refilled the system and opening up all the radiators it worked fine all the radiators got hot. The problem is that the gravity domestic hot water loop does not now work. Looking at the boiler the pipes for the gravity loop enter and leave on the left hand side. The pumped radiator loop is on the right side.

I can see from the hot water tank, on the second floor, that the expansion tank in the loft is tied into the domestic gravity hot water loop (i.e. the coil in the tank). This header tank must also supply the radiator circuit (through a connection in the boiler?) as the header tank emptied when I drained the radiator circuit.

I can only think that when refilling the system the radiator side filled OK (vent valve at pump used to remove air around the pump) but that the gravity hot water side has an air lock in a horizontal run of pipe as it runs from the boiler upstairs to the coil in the tank. When I ask for just hot water the boiler switches on and runs for a minute or two then switches off the outflow pipe at the boiler gets hot but that is not communicated to the pipe up at the hot water tank where the ciol enters the tank. I guess the boiler switches off on high temp as the hot water can't go anywhere?

How can I fix it ?

[mark6266]

answer a) back fill with out side tap and hose pipe and 2 jubily clips and ptfe tape.
drain down issolate F&E ball valves
connect hose to out side tap. connect other end of hose to boiler drain off make sure both ends are water tight. and turn on tap slowly have a frend in the loft to tell you when to turn off the tap ( when water enters the header tank up through the header tank.
answer b) check to see if there is a summer use rad on the system (heats up when hot water demand is on .normaly a rad in the bathroom ) bleed rad intill no air comes out.
answer c) use aqua vac on vent to try and pull the air out
method try b) 1st c)2nd a)3rd
regards mark(allinplumbing@fsmail.net)