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Thanks Andy. I'll let you know about the inhibitor as the tank is always fine and desperate fiddling with pump speed in the past has little obvious effect to date. Steve

Standard old 1980's unpressurised system with open header/exp tank. If the system is left off to cool for even a weekend I get air in the system which seems to stick at the brand new pump, making it noisy and some rads don't heat up. Switching pump on and off a few times makes glug glug noises...