Why does my shower drain slowly then suddenly clear up, despite using a spring unblocker?
We have a shower with a 6m long shallow drainage pipe run with a couple of right angle bends before it reaches the soil pipe. The layout and shallow fall are unavoidable. Inevitably the drainage pipe clogs sometimes and we use an old fashioned spring unblocker to clear it. But we occasionally also have a mystery blockage which causes water in the shower tray to build up considerably during a shower, before suddenly draining away and continuing to drain freely for the rest of the shower. What could be causing this? With a ‘normal’ blockage we would expect consistently slow drainage, not very slow drainage followed by rapidly draining water.
Advice from 2 local tradespeople
How to Fix or Minimize It
?Since changing the layout or fall of the pipe isn't an option, here are the best ways to solve an airlock issue:
?Install an Air Admittance Valve (Anti-Siphon Valve): If there is any accessible point along that 6m run (or just after the shower trap), a plumber can cut in an inline Air Admittance Valve (AAV). This allows air to enter or escape the pipe dynamically, preventing the pressure lock from forming in the first place.
?Support the Pipe Run: If you ever have access to the floorboards, ensure the 6-metre pipe is securely braced every 50cm so it cannot sag or create a secondary water trap.
?Flush with Hot Water & Bleach: Sometimes a thin layer of bio-film or slime on the top half of the pipe makes it easier for water to "seal" the pipe and trap air. Periodically flushing the line with boiling water can help keep the pipe walls clean, making it harder for a solid water seal to form instantly.
Build up of hair, skin cells.
Clogged vent pipe
Pipe corrosion
Blocked further down the drainage system.
