Draining & filling the system problem

System Drain / Fill problems

I tried to drain down my central heating system as I had done in the past by opening the drain adjacent to the boiler. It normally takes about an hour to empty but this time the water in the loft feed & expansion tank failed to drain away, and after two hours there was still a trickle from the boiler drain.

I emptied the system by loosening a radiator valve.
I then did the pipe work alteration as I wanted to do.

The water in the header tank never dropped to the outlet.

A couple of hours later when I tried to fill up again the water would not drain down into the system.

I eventually filled the system up by connecting a hose from the mains to the boiler drain until water went over into the tank via the expansion pipe. The heating system is now working fine, but I am concerned that it will not feed from the tank if it needs to.
I am suspecting that the feed from the tank to the Mistral Oil boiler is blocked by sediment somewhere, if this is so how is the best way to clear it
.
Any other suggestions would be really helpful.

Thanks Dave.

blockage is usually at the point the feed pipe from the tank joins the system pipework

cold feed and vent are usually just before the pump on most wet systems. look at your pump and work back from it... ther'll be a teed of bit of 15mm and then a teed 22mm. the 15 is where the blockage ll be.... stick a magnet on it and thatll give a rough location