Bathtub waste pipe only drains if I plunge it.

[Chaan]

Issue:
My bathtub waste pipe drains only when plunged, despite confirming there are no blockages downstream (snaked the line and visually inspected accessible sections). The problem persists even after clearing the P-trap and nearby pipe segments.

Proposed Solutions:

Air Admittance Valve (AAV): Suggested as a fix to address suspected venting issues causing negative pressure/airlock.

Anti-Siphon Waste Pipe on Connected Sink: Another plumber recommended installing an anti-siphon device on the sink (connected to the same drain line) to resolve the bathtub’s drainage problem.

Could I please request for Expert Insight?

Would the anti-siphon waste pipe on the sink effectively resolve the bathtub’s venting issue?

If not, why might this solution fail? I’m aware anti-siphon devices are typically for backflow prevention, but the plumber claims it could act as a vent.

Is an AAV still the more reliable solution?

 

Thank you for your guidance on prioritizing these solutions or suggesting alternatives.

[G and J]

So when you pull the plug out does some water sit in the bottom of the bath?

[Nickfromwales]

Anti-syphon devices are not for preventing back flow. A non return valve does that.

 

Can you post some pics of the waste pipe plumbing and trap under the bath, and where you have accessed it to rod it through please?

 

Baths also don't usually have P-traps.......odd that you say you have one? Again, a pic please