Coal fired central heating system, hot water slows to a stop

[aardvark20]

Hi, My neighbour has a coal fired central heating system firing what looks like an indirect cylinder for her hot water.
At the moment the bathroom basin and bath hot taps don't let any water through and the kitchen sink hot starts ok but slows to a stop after a couple of minutes.
I first thought that maybe the expansion pipe had a blockage and has caused a vacuum in the pipework, but then thought that you might not get any water in the lower tap if that was the case.
The main tank has a lot of cr.. in it, maybe some of this has come into the system.
It looks like a seriously old system, although the cylinder has the blown lagging on it, suggesting it's not that old.
Any ideas would be greatly appreciated.
Thanks

 

[Servotech]

Could be an air lock (an old plumbers favourite in this is what the customer tells us) yet mostly it isn't.
Muck in the tank eh, so it could be a blocked feed pipe, possibly at the gate valve which is a favourite - then again it could be .......
You can't really speculate on a forum.
Good luck,
Servotech,