Indirect gravity fed cylinder hot water problem

[Sevilo]

Hi - have a bit of a problem with one of the above. Customer says all was working ok until a few months ago with no new additions or changes to the system. It has been piped wrongly with the flow from the boiler going to the bottom of the cyliner - have had these before but usually work all the same. Feed and expansion were the same pipe which made things tricky so changed that and treated like a normal air lock in the coil. Getting water through a vent point ok and no air. Flow to the cylinder is roasting but return luke warm. Dont get how I am getting water out of the vent point on the return but no heat. If it was sludged up there would be no water, only thing I can think is to reverse the flow and return to the right way round and because it is piped in 15mm put gate valves on the feed and vent to reduce the flow and force heat back round again. Dont like the idea of having a gate valve on the vent though.
Any ideas - I hate airlocks:banghead:

 

[AWheating]

gate vavle on the the vent? you dont want to do that. sounds like you need to pipe it up correctly, problem sorted.

 

[Sevilo]

Thought so - there never are any easy answers are there !!!

 

[Sevilo]

quick update - injector t under floorboards - sludge build up and entire lenght of pipe sludged up cylinder practically replumbed. what a pain.