I've just moved into a new house. All radiators work except 1 in the hall, and in the kitchen where the radiator had been removed. I've put a new radiator into the kitchen and it heated up nicely when it initally filled up but this has also now gone cold. I've tried balancing by switching off all other radiators but no joy, and they don't need bleeding. The outlet pipe of the rad in the hall is hot, but the inlet is cold. Any ideas?