How to fit an air admittance valve
Updated: 23 October 2026
You have the valve in your hand and a plastic soil or waste pipe in front of you. How to fit an air admittance valve comes down to a seal that can move: the cap stays upright, the valve sits high enough for that diameter, and the rubber is greased rather than glued. A valve on its side, or level with the trap, lets the next flush empty the trap or lets sewer air into the room. Size and joint are on the [buying guide](/hub/diy-supplies/air-admittance-valve/sub/air-admittance-valve-buying-guide). Listings are in the [air admittance valve comparison](/hub/diy-supplies/air-admittance-valve). A qualifying purchase can earn us a commission at no extra cost to you.
You have the valve in your hand and a plastic soil or waste pipe in front of you.
You have the valve in your hand and a plastic soil or waste pipe in front of you. How to fit an air admittance valve comes down to a seal that can move: the cap stays upright, the valve sits high enough for that diameter, and the rubber is greased rather than glued. A valve on its side, or level with the trap, lets the next flush empty the trap or lets sewer air into the room. Size and joint are on the buying guide. Listings are in the air admittance valve comparison. A qualifying purchase can earn us a commission at no extra cost to you.
Our top picks
| Badge | Product | Pipe size | Joint | Standard stated | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  FloPlast FloPlast AF110G Push-Fit Air Admittance Valve - Grey | 110 mm; size line also Small | Push-fit | BS EN 12380 A1, KM512474 | 9.0 | about £23.85 | View offer |
| Best value |  FloPlast FloPlast AV40 40 mm Compression Air Admittance Valve - White | 40 mm; listing also says 32 mm | Compression | n/a | 7.9 | about £11.38 | View offer |
| Alternative |  FloPlast FloPlast AF32 32/40/50 mm Solvent Weld Air Admittance Valve - White | 32 mm; adaptor 40/50 mm | Solvent weld | BS EN 12380 A1, KM512474 | 8.4 | about £14.99 | View offer |
Alternative
FloPlast FloPlast AF32 32/40/50 mm Solvent Weld Air Admittance Valve - White8.4/10about £14.99
On this page
Set the height and keep it upright
The valve opens when a flush pulls a vacuum and closes once the pressure settles, so keep the cap pointing up. A valve leaned into a joist can sit half open, and you then smell the loft even though the joint is tight.
The letter A on a BS EN 12380 valve does not cancel the height rule. Designation A means the valve may sit below the flood level of the connected appliances. FloPlast’s FAQ still asks for at least 200 mm between a 110 mm valve and the highest wet entry, and at least 100 mm between a 32, 40, or 50 mm valve and the waste it vents. Measure from that connection up to the underside of the valve. A 40 mm valve level with a bath trap can still gurgle.
Push-fit seal, then solvent weld
Read the socket before you open a tin. A push-fit air admittance valve seals on a rubber fin. Solvent cement on that rubber can stick it shut. Silicone grease on the ring or the spigot is what Polypipe specifies for its soil push-fit rings, and what reviewers of the FloPlast AF110G describe using. Keep cement away from the seal.
Push the spigot home, then allow for movement. Polypipe’s soil instructions say to pull a push-fit joint back by about 10 mm so the pipe can expand. On the AV40, reviewers say the compression rubber is awkward to keep seated while you tighten.
On a solvent socket, dry both faces, apply the cement as the tin describes, and keep it off the moving parts under the cap. Let the weld cure before waste runs. Use the time on your tin.
Leave it reachable
Approved Document H, clause 1.33, for England and Wales, expects access and a valve you can remove so the pipe can be rodded. If you cannot reach the cap without cutting plasterboard, the rodding point has gone.
The British Plastics Federation’s January 2024 note says a roof-space valve should sit above the insulation, because dust sticks to a lubricated seal. Fibres around the cap can hold the seal open, which smells, or hold it shut, which gurgles. These sources do not give a service interval. A screwed plywood box with no gap is the pocket to avoid. If you want a cover, use a panel you can lift. The drain still needs positive-pressure relief, which is the comparison with an open vent.
When you should not finish the job
Stop if the drain is blocked. A slow WC, a full inspection chamber, or sewage in a low gully is a blockage. Sefton Council’s note says to check the chamber before you rebuild the indoor pipe.
Stop if this valve would be the only vent to a septic tank or a cesspool. It lets air in under suction and stays shut the other way, so it does not relieve a surcharge. Stop when several houses share the drain and you cannot see an open vent on the run. A cast-iron stack, with no plastic spigot to grease, belongs with a plumber.
A fit is working when a flush leaves the basin and bath traps full and the cap does not smell. If the smell is still there the next day, the seal may be stuck open, a trap may be dry, or the chamber may still be the fault.
Common questions
How high above the waste should the valve sit?
FloPlast asks for at least 200 mm above the highest wet entry for a 110 mm valve, and at least 100 mm above the waste for a 32, 40, or 50 mm valve. Measure from that connection up to the valve. A pedestal cupboard that leaves the valve level with the bath trap is too low for the 100 mm figure, even when the compression nut is tight.
Should I glue a push-fit air admittance valve?
Leave the cement tin closed for a push-fit socket. Silicone grease goes on the rubber ring or the spigot, then you push the joint home. Polypipe’s soil instructions add a pull-back of about 10 mm on their push-fit joints so the pipe can expand. Use solvent cement only on a socket made for it, and keep that cement off the moving seal under the cap.
How long should a solvent weld cure before I run water?
Follow the cure time on the cement tin. Let the weld cure before waste runs through, and there is not one waiting time for every brand. If the tin asks for longer than the rest of the afternoon, leave the WC until that time has passed. A greased push-fit joint has no cement cure. You still flush only once the valve is upright and the cap is clear of insulation.
Can I box an air admittance valve in?
You can cover it if air still reaches the cap and you can take the valve off to rod the pipe. Approved Document H expects that access. A screwed plywood box with no gap, or a valve buried under the loft quilt, packs dust onto the greased seal. The British Plastics Federation note places roof-space valves above the insulation. A lift-off panel with an air path meets the same point.
When should I stop and call a plumber?
Stop when the inspection chamber shows a blockage, when the valve would be the only vent on a septic tank or cesspool, or when the stack is shared and you cannot see another open vent on the run. A cast-iron joint that is not a plastic spigot is the same call. A new cap leaves a slow WC slow.
Related pages
Sources
Public UK sources and the amazon.co.uk listings compared on this page. Listings captured 19 October 2026. Updated 22.10.2026.
- Approved Document H (2015), clauses 1.31–1.33 (retrieved 22.10.2026)
- BPF Pipes Group, Air admittance valves for domestic properties, January 2024 (retrieved 22.10.2026)
- BSI, BS EN 12380:2002 (retrieved 22.10.2026)
- FloPlast soil and waste FAQ (retrieved 22.10.2026)
- FloPlast Kitemark schedule KM 512474 (retrieved 22.10.2026)
- Sefton Council, smelly drains note (retrieved 22.10.2026)
- P&H Engineering, Air admittance valves: getting it right (retrieved 22.10.2026)