Blocked header tank pipe?

[harrydeee]

I have a leaking 3 way valve on my vented cylinder CH system, so I decided to drain the system, flush it out and replace the valve. However, the water level in the header tank in the loft didn't go down when I was draining the radiators.
I emptied the tank manually and disconnected the outlet pipe, pressurised the pipe with air to see if there was a blockage that could be moved, but the pipe pressurised. There is a U bend at the bottom of the pipe where it enters the CH system in the airing cupboard, I thought it may be blocked there, so I cut the pipe about 6 inches above the bend and put a wire down there to see if it was blocked. I also removed the pump to have a go from that direction. There was sludge in the pipe, but I think the wire is now hitting the T section where the feed pipe joins the main CH pipe.
I have tried pressurising the pipe with air to see if air comes out of the open pipe near the pump, but it pressurises. It doesn't look like there is a one way valve in the T section of the feed pipe, so I can't work out why it is still pressurising. Am I going to have to cut out that section of pipework and replace it? Please can anyone help. The system is 20 years old. Thanks

 

[hi-spec plumbing heating]

you've answered your own question it's blocked on the t which is the common place for it to block where the feed joins the system.

 

[JohnD]

you might be able to poke it out with your wire but cutting out the blocked part is better. Otherwise there will be lumps of dislodged sediment flowing round and getting lodged in your boiler and pump. When you refill it, include a bottle of Sentinel X400 which will break up the sediment and sludge, loosen it, and send it circulating round in the water which will be very black with the suspended solids. If you also fit a Magnaclean. it will trap and retain all this circulating material and you can take it out. The Magnaclean carries on working indefinitely. If you don't use one, you will have to allow the X400 to circulate for a couple of weeks, then drain it out and rinse it to remove the loose sediment, but there will always be some left. After rinsing, refill using X100 corrosion inhibitor. The Fernox equivalents are also good.