Air in radiator system

[237SJ]

My upstairs radiators frequently need bleeding. When they`ve been bled and I put the heating on, there are some almighty noises that sound like air shifting in the system. A day or so later, they need bleeding again. Sometimes after bleeding them, I`ve let more water into the system as well. How does air get into a central heating system? Where does it come from and how do I stop it happening?

[NazNomad]

When new water is added to a central heating system, a certain amount of air also enters the system. The action of the impeller of the pump will also 'create' a certain amount of air.
Electrolysis in the pipework will also create gases in the system.

[buildersmate]

Unlikely to be air - almost certainly it is hydrogen gas - a by-product of the chemical action of the water on the steel innerds of the radiator. The other by-product is magnetite - black iron sludge which gradually collects in the system.
What? - you have inadequate or no corrosion inhibitor in the system? - best get onto it before the radiators get eaten alive and start leaking.

[LadyEyre]

never bleed a radiator with the heating pump on, turn the heating off, then bleed.

[buildersmate]

Errrrrrrrrrrrr... why not? What urban myth is that?