two questions I have are why put the "u" in the feed pipe? and why do they have to be so close together?i imagine its to aid filling of the loop with reduced potential for airlocks? or is it to keep the pressure similar between the two points?
Again many thanks for the help.
The u is to insure water is drawn in from that pipe rather than having air travel up it.
The distance is to insure constant positive pump pressure around the circuit. The only neutral or negative zone will be the max 150mm section. As the pump pushes water through the boiler and back to the vent any air there will rise up out of the vent. The pump wont pull air because it is easier for it to draw water from the feed which is between the vent and the pump. The pump then pushes water around the whole circuit (through the 3 port to hot water coil or to rads or both). The only point on the circuit that the pump isnt pushing is the section between vent and feed.
On older open vent boilers it was common to add the feed to the boiler direct as this insured that the boiler was least likely to air lock. I suspect as has been suggested that your boiler and system is likely to have been a gravity based system at some point and has been badly converted to add the pump and 3 port.
On a gravity system one way valves are fitted to insure hot water rises in the direction around the circuit that is intended. The brass cylinder / valve referred to as possibly a one way valve is below the floor in one of your pics. I will try to find the pic you posted of it.
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Sealing the system and getting rid of the feed and vent tank is another effective way to go, but will have greater initial parts and materials costs and may have implications for any parts of your system that cant take the increased operating pressure of a sealed system ( 1.2/1.5 bar cold - 2.5 bar hot)
Correct the pipe layout as I illustrated and try running the system again.
When filling the system with the pipework altered if you are competant enough, wire the central heating pump to a plug and just run the pump with the boiler off untill all the radiatirs are bled and the air pips I illustrated have stopped letting out air. Then wire the original pump flex back to the pump and fire the system up with thd boiler.