I have 3 heatiing circuits for the house and each has a pump to drive the hot water around the system. Oner circuit is not heating. The thermostat is working and I hear the the pump whirling but the furnace does not fire up (most of the time). Prior to this happening and when turning on the system after the summer I could hear the water swishing through the system. Could there be air or sludge in the pipes?