The ground floor radiators in the living room furthest away from the boiler, are hardly aired whilst those nearest the boiler seem hot enough. There is also an occasional knocking noise from the system. I suspected low pressure or a pump fault, but it seems OK. The living room radiators aren't full of air either, just tried bleeding them. Gut instinct would be a pump fault not sending hot water to the extremities, but indications show the contrary. We had the system powerflushed, and a new heat exchanger fitted, all within the last 18 months. Can anybody suggest what the problem might be? Thanks.