What is an air admittance valve, and why does my house smell when the toilet flushes?
The one-way valve that lets air into a drainage stack without letting smells out, why it fails, and the smell-when-you-flush symptom that identifies it.
- Difficulty
- intermediate
- Time
- 30 min
- Read
- 5 min
- Safety
- caution
Short answer
An air admittance valve is a one-way valve on top of an internal soil stack. It opens to let air in when water rushes down, and closes to stop smells coming out. When its rubber diaphragm perishes or jams open, you get sewer smells whenever a toilet flushes or a bath empties. Replacing it is straightforward and inexpensive.
Water rushing down a soil stack drags air with it, and that air has to be replaced or the vacuum sucks the water out of every trap it can reach — which is what makes a house smell of drains. Traditionally the replacement air came down an open vent pipe extending above the roof. An air admittance valve does the same job at the top of an internal stack, in a loft or a boxed-in duct, without needing a pipe through the roof.
Safety
Step by step
- Recognise the symptom.A drain smell that appears when a toilet is flushed, a bath or a basin empties, or a washing machine discharges — and fades again afterwards. That timing is the signature. A constant smell with no relationship to water use is more likely a dry trap or a leaking joint.
- Find the valve.On top of the soil stack, wherever the stack terminates inside the building: in the loft, in a boxed duct in a bathroom or airing cupboard, in a cupboard behind a toilet, or occasionally under a sink on a smaller branch. It is a plastic dome or cylinder, roughly fist-sized, sitting on the top of a pipe.
- Confirm the diagnosis.Have someone flush the toilet while you are next to the valve. You should hear a soft click or suck as it opens and closes. A smell reaching you at that moment is the valve. Nothing audible and no smell means look elsewhere.
- Check it has not been buried.An air admittance valve needs free access to air. A valve that has been covered by loft insulation, boxed in tightly, or built over during an extension cannot draw air, and the traps siphon instead. Sometimes the fix is simply uncovering it.
- Check the orientation.They must be fitted vertically, above the highest connection they serve, and above the flood level of the highest fitting. A valve lying at an angle in a loft after somebody knocked it does not seal, and is a common cause.
- Replace a failed valve.They push or screw onto the top of the stack with a rubber seal. Replacing one is a genuinely simple job: buy a valve of the correct size and pattern, lift the old one off, clean the pipe end, and fit the new one. They cost very little and last years rather than decades.
- Check the other traps while you are at it.A valve that has been failing for a while has probably siphoned some traps dry — often a rarely used shower, a spare toilet or a washing machine standpipe. Running water into every waste in the house refills them and removes a second smell source.
- Know when a valve is not permitted.Every drainage system needs at least one open vent to atmosphere somewhere, usually the last one before the sewer, and building regulations restrict where air admittance valves may replace an open stack. A house with every stack valved and nothing open is a fault, not a design choice.
Tips
- Air admittance valves are consumables. If yours has been in place for fifteen years and something smells, replacing it is a reasonable first move even before you diagnose thoroughly.
- They are sometimes called Durgo valves after one of the original manufacturers, and sold as AAVs. Same thing.
- Note the stack diameter before buying — 110mm for a soil stack, smaller for a waste branch. They are not interchangeable.
- In a loft, tie a note to it or leave a gap in the insulation so a future insulation top-up does not bury it again.
If it doesn't work
Smell only when a toilet flushes
Cause: The air admittance valve is jamming open or its diaphragm has perished — Fix: Replace the valve. It is an inexpensive push-fit part and the commonest cause of this exact symptom.
Gurgling from basins and baths when the toilet flushes
Cause: The valve is not opening at all, so the stack pulls air through the traps instead — Fix: Also a valve replacement — a stuck-shut valve produces gurgling and eventually dry traps, where a stuck-open one produces smells.
Smell from a rarely used shower or spare bathroom
Cause: The trap has evaporated or been siphoned dry — Fix: Run water into it for a minute. A little cooking oil poured in afterwards slows evaporation in a room used only occasionally.
Valve replaced but the smell continues
Cause: Another source: a failed pan connector, a cracked stack, or a dry trap elsewhere — Fix: Check the pan connector behind the toilet and look for staining on the stack. A stack cracked in a loft smells continuously rather than only on flushing.
Smell appeared after loft insulation was topped up
Cause: The valve has been buried and cannot draw air — Fix: Clear a space around it. Valves need free air and enough room to operate.
Questions people ask
Can I just cap the pipe instead?
No. The valve is doing an essential job. Cap it and every trap in the house becomes vulnerable to siphoning, which produces exactly the smell you were trying to stop, plus gurgling and slow drainage.
Should the stack go through the roof instead?
An open vent above roof level is the traditional and most robust arrangement, and the building regulations in each UK nation require at least one somewhere on the system. Air admittance valves are permitted for internal stacks where an open vent is impractical, and they are perfectly acceptable when correctly sited.
Is sewer gas dangerous?
In domestic quantities it is mainly unpleasant, but it should not be tolerated — it indicates a fault, and in an enclosed unventilated space the concentrations can matter. Deal with it rather than living with it.