Gurgling Drains and Slow Plugholes

What the noise actually is

When water fills a pipe faster than it can drain, it forms a plug. Behind that plug a partial vacuum develops, and the only air available is whatever can be dragged through the nearest water seal, which is a trap under a sink, a bath or a toilet. The bubbles pulling through that trap are what you hear.

The important consequence is that the trap loses some of its water each time. A trap that has been repeatedly siphoned eventually stops sealing, and then sewer air comes back into the room. That is why a gurgling drain and a drain smell so often appear together.

Working out where the restriction is

Test each appliance on its own and note what makes a noise elsewhere. If flushing the toilet makes the shower gurgle, the restriction is downstream of the point where those two join. If emptying the kitchen sink makes the external gully bubble, it is in the kitchen run outside. If everything gurgles when anything drains, the fault is close to the main outgoing run and affects the whole property.

Do this before ringing anyone. It takes ten minutes and it tells an engineer more about the location of the fault than any description of the symptoms.

• One appliance gurgles only when it drains: its own trap or branch.
• One appliance gurgles when a different one drains: shared branch restricted.
• Toilet bubbles when the washing machine empties: main run restricted.
• Everything gurgles and the chamber is high: blockage in the outgoing drain.
• Gurgling only in strong wind: usually a vent problem, not a blockage.

Blocked vents rather than blocked drains

The stack on the outside of the building has an open vent at the top, and that vent is what lets air in so water can fall smoothly. Birds nest in them, leaves settle in them and old cast iron terminals corrode and collapse into them. A blocked vent gives you gurgling and smell without any slow drainage at all, because the pipe itself is clear.

The same effect happens with a failed air admittance valve inside a boxed in stack, common in extensions and loft conversions. These are inexpensive to replace but are frequently overlooked because they are hidden behind panelling.

Why a slow plughole matters more than it looks

A plughole that takes a minute to clear when it used to take ten seconds has lost most of its effective bore. The pipe is not half blocked, it is closer to nine tenths blocked, because flow through a pipe falls away very sharply as the diameter reduces. The gap between an irritating sink and a completely stopped one is smaller than it feels.

That is why we suggest dealing with it at the noise and slow flow stage. Clearing a partially restricted run in daylight during the week is a straightforward job, whereas the same run at full blockage on a Sunday evening involves standing water and a much less pleasant hour.

What we do on a visit

We test each appliance, lift the chambers to see where flow stops, and jet the affected section to full bore rather than punching a hole through the deposit. Where the symptoms suggest a defect rather than a build up, particularly where the same run has gurgled repeatedly, we put a camera down and record it so you can see what is causing it.

If the cause turns out to be a vent or an air admittance valve, we say so and fix that instead. There is no point jetting a pipe that was never blocked.

Keeping it quiet afterwards

Run water through any appliance you rarely use, such as a spare bathroom or a utility sink, at least monthly so the trap does not evaporate dry. Clear hair from bathroom wastes routinely, keep fat out of the kitchen run, and clear the external gully each season. Where a property has an older shared run, an annual jet is a sensible bit of housekeeping rather than an extravagance.

Questions we get asked