A reflux (non-return) valve contains a hinged flap that normal flow pushes open and reverse flow pushes closed. For properties whose fixtures sit low relative to the sewer, it is the main defence against surcharge-driven backflow.
The flap only seals if it can close cleanly — a valve fouled with debris or scale protects nothing, so periodic checking is part of owning one.
Blocks reverse flow from surcharging sewers into private drains.
On house drains of low-lying properties, in an accessible chamber.
A CCTV drain inspection is the standard way to confirm whether this is what's happening in your pipes: the camera shows the condition on screen, a locator pinpoints the exact position and depth, and you get a recorded report before any repair decisions are made.
A CCTV drain inspection shows exactly what's happening inside your pipes — no guesswork.