air in rads

[aks_72]

CAN SOME ONE HELP.
I have a gravity fed hot water but pumped central heating system, all was working well. last week i drained system to fit new extra rads. Then my problem started.
All the ground floor rads are fine. One rad on the first floor and two rads fitted in the attic (second floor) are not wanting to bleed I can take the bleed valves out and I get no air or water, how ever i did notice that the expansion tank was over flowing so i adjusted the float. this was fine before. Every rad warmed up fine so what's up and what can I do to rectify problem.

 

[JohnD]

It is possible that you have a blockage in the pipe that brings water down from your feed and expansion tank into the system. This often manifests itself when the system fails to refill after draining, or removing radiators for decorating.

If so, it is usually in the Tee where this 15mm pipe joins the 22mm or 28mm circulating pipes, probably close to the pump beside the hot water cylinder or the boiler.

When you drained it, what colour was the water, and did you find any sediment?

If you can lay your hands on a magnet, hold this against the copper pipe, and you will find it is attracted to any blockage because it contains iron-based sediment from th radiators.

Try this and report back with your findings please.