Shower waste fitting vent?

[Serval]

The old shower waste that was removed from the bathroom had a section where the waste went from 40 up to 50mm with an adapter. The pipes were run above floor and boxed in. Along a 2.5mish run

There was this part just before the transition from 40 to 50 mm

Going to be running my new shower waste under the floor boards and want to use 50mm as the new shower is more powerful than the old, plus the tray is shallow.
Will need to use a short section of 40 from the trap before going up to 50.

The question is what is that part in the photo and will i need to fit one on the new pipe work?
It has something rattling about inside
so it seems like its some sort of vent to help with the drainage flow.
Any one able to confirm?
Also is there a similar part that can be fitted inline? Ie one that doesn't stick up.
I'll be putting the new waste under floor and think something like this might foul the floorboards depending on how if fit the shower tray

 

[Ben-gee]

Yes, it is an Automatic Air Vent (aav).

May not be necessary, but presumably it was put there for a reason initially, try it without and see how you get on.
(I personally wouldn’t fit one in an inaccessible place as they can fail quite easily)

 

[Serval]

Thanks for that. It was fitted during or before the previous owners were here.
So not sure why it was fitted.
Perhaps because the old waste run was sat directly on the floor so there was no fall along the majority of the pipe run apart from the short section that dropped below the floor before connecting to the stack.

anyway ill see how it works without one and go from there

 

[Serval]

One other question while im on the subject,

The waste is a straight run from the trap along the length of the room before hitting a 90 degree bend then another short straight into the stack.

Am i better off using two 45 degree bends rather than one 90 if feasible?

 

[Ben-gee]

Yes that would be better but a swept 90 is fine, don’t use a knuckle 90.