all of our radiators downstairs are stone cold. upstairs seem to heat up. the pump seems to be working ok as far as I can tell, its vibrating. is the problem pump related (impellers), sludge or header tank? The water in the header tank is now cold. and when you press the ballcock valve down no water comes into the header tank. the valve may be stuck. is this the source of our problem or just an additional one? help please