I'm trying to sort out a "no hot water" problem when boiler is set to
heat "hot water only, central heating off". Just get normal flows of
cold water from all the hot water taps.
The hot water flow pipe going into cylinder from boiler is very hot,
the return pipe coming out of the cylinder to the boiler is luke warm
where it exits cylinder.
The cylinder itself (including top) feels cold everywhere.
The boiler fires up but goes off after few minutes (say every 5
minutes).
I can hear water swishing through the pump which is running.

Its a conventional indirect system, Potterton Fuelsaver 65F boiler,
cylinder on first floor, expansion tank in loft (which is full).

I thought this was an airlock in the cylinder in the non-coil part of
cylinder, so attached hose & forced water out through cylinder out of
vent pipe, left it coming out for say 5 minutes & thought this would
be enough time to clear any air.
As this didnt fix problem,my question is how much water might need to
be forced through hose,cylinder & out vent pipe to get rid of air
lock. Might I need to replace the entire cylinder contents. I went
onto think the problem is now in the coil boiler fed part of the
cylinder but its hot going into the cylinder coil.
Help...
Steve