Air admittance valve help

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Hi guys,

Our ensuite has the usual shower, toilet and sink.

On occasions the toilet or sink would empty the trap in the shower.

There isn't an air admittance value near these 3.

Could one be installed under the sink. It is a very small ensuite. What is that thing on the right? Doesn't look like a valve but looks odd to me.

Edit: is it an anti siphon valve?

To confirm rightly or wrongly the system doesn't vent outside. The main bathroom has an aav and so does the downstairs loo. No problems with these.

Thanks

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That is an 'Anti Vac' trap, basically a trap with an AAV. If the WC is pulling the shower trap then you will need to look at venting the system elsewhere, ideally on the stack.

Should be an open vent somewhere on the system, usually at the highest point of the drainage system.
 
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Yes, it’s an anti siphon trap. If water is being pulled from the shower trap from another appliance sounds like induced siphonage. How long is the shower run, and how close is it to the basin and toilet wastes?
 
Thanks for replying. All within 1m of each other. Very tight.

Quick question the shower trap has a cup in it. After reading am I right in thinking it doesn't matter if water is sucked from underneath the cup as it's the water in the cup and the lid seal that stops the smells?

Screenshot_20220405-104555.png
 
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Not sure quite how that trap works, but if it's gurgling it means one of 2 things. Either a vacuum is being created behind outgoing flow and it's pulling water out the trap, or there is positive pressure being created as the outgoing discharge is displacing the air in the pipework, and it is looking for a route to vent out. Given you've said there are AAV's on the other bathrooms and no mention of an open vent, it may well be the latter.
 

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