Air in hot water system

[swindontony]

When the hot water part of my heating system fires up, there is always a lot of gurgling as if there is loads of air in the system, when the timer stops I undo the air vent in the airing cupboard and release the air, this happens every day. What I want to know is where all this air is coming from, hope this makes sense, thanks in advance for any answers.

[buildersmate]

If it is a sealed , pressurised system (and no small feed-and-expansion tank in the loft to replenish the water), it is unlikely to be air but hydrogen. This is the byproduct of corrosion between the water in the system and the inside of the steel radiators. You need to put some corrosion inhibitor in the system.
If it is an open vented type (with the little tank in the loft), it is probably air and it is getting in through this loft tank. Is the level of the tank correctly controlled by the ball valve? Does the water level in this tank seem to rise and fall quite a bit whilst the system starts up?

[swindontony]

Thanks for your answer buildersmate, it has a header tank in the loft, i have checked this before and it seems ok, but will check again just before it fires up and see what happens to the water level.

[buildersmate]

So you have a safety valve in the airing cupboard on an open-vent system. Less common, but possible. Are you sure that isn't leaking in air, even if it isn't leaking out water?
Have you got an air separator fitted - usually copper body with four copper pipes leading into it?