Central Heating Question...What is it?

I`ve just changed my noisy Central Heating pump with no problems, but have noticed a brass fitting located at the end of a copper pipe coming from the pump .

It`s a 1" brass cylinder with a knurled screw end with a red plastic unscrewable cap on top.

I`m assuming that it is some kind of venting system to remove air from the CH circuit but how does it work.

Unscrewing the red cap and nothing appears to happen (no gush of air or water )

I`ve got a normal bleed valve located at the other end of the pump which can be used for manually bleeding the system using a normal radiator key. .

Any suggestions what this other fitting is and how it works?

TIA

Automatic air vent. There is as float inside it which seals the outlet when it is full of water. Air should be released on unscrewing the red cap, providing there's air in the system. The red cap should be screwed down a week or two after filling the system & no more air is in it.

Excellent answer!. Thanks.....Now I know

See the selaed CH FAQ its the pressure relief valve - leave it alone.

Sorry I've reread you post and I think it probably is the auto air vent - in good working order.

The above concise reply should be on uk.D-I-Y plumbing/heating/FAQ.

If it's like one of these

then yes.

On the Honeywell equivalent one, you can also rotate the upper part of the valve body to isolate the valve completely

If it's

(number 11)

then the red cap is purely a protection.

Unscrewing the cap merely reveals a button inside that can be pressed to manually operate the vent.