Baxi Bermuda 555lfe

Baxi Bermuda BBU. 3 way valve. Feeding 9 rads in Bungalow. HWT in loft space, expansion tank in loft space. Replaced 5 year old Pump due to \\\"carbon growth/fur\\\" on blades (making clicking noise). Flushed system out. Removed each rad and pressure flushed out each one. Replaced rads and powered up system. Now system runs for 30 minutes then the pump is full of steam/hot air and circulation stops. During 30 minutes no rads heat up. When opening distant drain off tap, hot water rushes to drain off point. Close the drain off, then the pump runs for 30 mins after that it then needs to be bled. No apparent leaks visible all joints dry, no hissing or steam from BBU. Thoughts are a blockage but when you open bleed points the hot water runs well. When selecting hot water only....it does not pump sufficiently to lift boiler hot water HWT in loft, pump is rotating in right direction and installed correct way up. Any ideas? What is the best way to test if air is getting into system? Where is most likely point of problem?

where is pump in relation to cold feed?

Picture the back boiler, 15 inches to the right hand side on the boiler output is a T joint. The vertical of the T joint feeds the lower input to the pump. The other side of the T joint is the expansion pipe from ground to roof level and into the expansion tank. On the lower pipe input to the boiler, again 15 inches from the right hand side of the boiler (this time on the cold input side or return to boiler) rests the cold water feed. It is a 15mm T into a 22mm pipe. The other side of the T is the consolidated return pipe carrying both hot water return from HWT indirect coil and radiator "cooler" piped return.

have u checked for air release points in loft?

Have you water in the f&e tank?

Have you checked that the cold feed is not blocked where it tees into the 22mm?