Air admittance valve required?

[Thomas C]

I have an AAV next to a ground floor WC, the first floor WC is connected to an internal soil stack that eventually vents externally out above roof after turning a few corners via the loft space. If the regs allow it then I would like to do away with the AAV serving the ground floor WC and maybe I can as it is part of the same vented 110mm waste pipe system - although not directly as shown in the very basic diagram I paste below. The reason I want to get rid of the AAV is to install a wash basin in its place. Grateful for any opinions on whether I can do this.

 

[blup]

Not a pro but the aav has presumably been installed to prevent siphonage from the gf wc. You could maybe disable the aav temporarily and see it makes a difference in practice.

Blup

 

[dilalio]

Put a trap with integrated AAV on the new basin.

 

[Thomas C]

I have found a section in document H which would allow for the removal of the AAV -
"1.30 Stub stacks – A stub stack may be used if it connects into a ventilated
discharge stack or into a ventilated drain not subject to surcharging and no
connected water closet has a floor level more than 1.3m and no other branch
into the stub stack has a centreline more than 2m to the centre line above the
invert of the connection or drain (see Diagram 5)."

At present the ground floor WC is connected to a small stub stack albeit that stub stack continues vertically upwards and terminates in an AAV 5ft off the floor level but it does not need to do this as the WC runs into a ventilated drain. I will remove the AAV leaving the stub stack tall enough to accept the WC and a new wash basin and blank the end.

 

[Madrab]

The AAV will probably have been added to minimise the risk of self syphoning as well as stopping the upper WC pulling on the lower WC when it's flushed.

Given the upper WC is open externally vented then as suggested using an antivac trap on the basin should be enough to break any pull from the bottom toilet.

 

[Thomas C]

Thanks for your replies.

I will look at getting an integrated AAV/anti VAC with new basin as advised.