Central Heating Airlock???

[vmx1200]

Hi
I wonder if I have an airlock in my central heating. The system is based around a Potterton Puma combi boiler. I fitted a number of replacement TRVs a couple of month ago bled the radiators and all worked fine until about a week ago.
Now I can only get one or two radiators to heat up and then only slowly, when I replaced the TRVs I closed all the lock shields and opened up one rad at a time to bleed it and you could hear the water rushing through the rad … not any more.
I suspected the pump so I removed it and powered it up on the bench, runs fine and impeller is not loose. When I put it back and switched on boiler you could hear the surge of water so I guess it is OK.
I then thought airlock so pressurised system to 2.5 bar closed inlet to rad and opened lock shield fully and then drain valve( combined drainand lock shield), water came out under pressure with slugs of air, repeated procedure till no more air and then reversed procedure ie closed lock sheild and opened TRV ..no air. Did same on other drain .. no air.
Thought all was sorted but still no luck … any thoughts (and yes the lock shields to the rads are open) …thanks

 

[twgas]

Had one of these recently where the pipe between the diverter and pump was blocked giving your symptoms.

 

[vmx1200]

Thanks for replying. When I took the pump head off there was no crud and the inlet and out let ports were clear. How do you get to clean the pipe you refer too. The boiler is in the loft really low down so it is almost impossible to get to any of the rear connections!!