How do I fit an aerator/ Automatic float air vent

[Stevel]

I have air in my CH system that I can't seem to get rid of.

My existing system has an old (upside down bottle) aerator which has rather rusted over, so have purchased a new one to replace it with.

The question is: how? I am presuming that if I unscrew the present one, I am going to have a lot of water spouting out! Is there a quick way I can replace this without draining the whole system?

Many thanks in advance for any advice!

 

[ChrisR]

Unless you freeze it, or bung up both the feed and vent pipes at the f&e tank, you'll need to drain to the level of the aav.