Every time water flows through a soil pipe – for example, when a toilet is flushed – it carries air along with it. This creates negative air pressure or vacuum in the pipe, which in turn causes air to be drawn into the pipe through plugholes in baths, basins, showers and sinks to balance the air pressure. This can result in water draining slowly or noisily into the pipes, unpleasant odours escaping from the drain into the house and even wastewater coming back through the trap. Let’s look at how this can be avoided.
Vent pipe vs. air admittance valve
The problem of negative air pressure in soil pipes is most commonly prevented by installing a vent pipe on the property. With its open end above roof level, the vent pipe allows air from outside to balance the air pressure in the drains without any foul odours from the pipe becoming detectable.
Where a vent pipe isn’t a viable option, an air admittance valve can be used. Also known as a durgo valve, air inlet valve or by its abbreviation AAV, an air admittance valve is designed to let air into the system to balance the air pressure but not escape from it, preventing problems with odours.
How does an air admittance valve work?
When the pressure in the drain system is equal to or greater than atmospheric pressure, the valve remains closed. This prevents the escape of foul odours from the soil pipe. As soon as the air pressure inside the soil pipe falls below atmospheric pressure the valve opens, allowing air to enter the system. When the air pressure has equalised, the valve automatically shuts again.
When would I use an air admittance valve rather than a vent pipe?
If your property is of a similar height to the surrounding buildings, the chances are that a vent pipe will be acceptable. However, it may not be the best solution if neighbouring properties have windows – either on an additional floor or as roof windows – that are higher than the top of your vent pipe, as foul odours escaping the vent pipe may reach their windows.
Likewise, if you or a neighbour are converting a loft and adding roof windows, an air admittance valve may be required.
Installing an air admittance valve
Can I install an air admittance valve outside?
Air admittance valves are most commonly installed inside a property and are designed for this. Having said that, some air admittance valves are designed for outside use. If that’s what you want, check that the valve is kitemarked and rated for outside use in the UK, as a standard valve may not be suitable for frost, freezing temperatures or extreme weather conditions.
You may want to use an air admittance valve outside if converting your roof space places a window 3 metres or less from an existing vent pipe, as adding an air admittance valve to the vent pipe will prevent odours from being detected at a closer distance.
Where can I install an air admittance valve inside my home?
It goes without saying that we will want to keep an air admittance valve discreetly out of sight, and the good news is that this is possible. Providing there is good air circulation, an air admittance valve can be installed in a roof space or an airing cupboard, but make sure you can easily access it for maintenance.
The valve should be placed on a soil pipe that is at least 200mm above any water entry pipe or the highest point that wastewater will normally reach in the soil pipe. This is generally at the end of a soil pipe, but can also be added inline, using a soil pipe junction.
How is an air admittance valve fitted to the pipework?
Air admittance valves feature either a push-fit or solvent-weld connection. Push-fit valves are the most popular as they require no tools or chemicals to assemble them and an internal rubber seal creates an instant, watertight fit that can be pulled apart if required in the future.
However, solvent-weld joints are more permanent and offer greater peace of mind that they won’t deteriorate and start to leak with age.
What problems might I get with an air admittance valve?
Providing it is fitted correctly, there is little that can go wrong with an air admittance valve and this will be largely due to the valve malfunctioning or failing over time.
The first indication you may get that there is a problem is the symptoms we mentioned at the outset – bad odours in the house, gurgling and slow-draining pipes.
If this happens immediately after the valve has been installed, check that it has been fitted correctly. Have you overlooked a point of water entry such as a basin in a loft conversion, for example, that may be too high for the valve?
If the problem has only developed over time, remove and inspect the valve (this is why push-fit valves are the most popular). Check that the rubber seal isn’t clogged or stuck. If it is, release the valve and apply some pipe lubricant to the seal before reassembling and testing it.
A faulty or damaged seal can be replaced but if a replacement cannot be found, a completely new valve is inexpensive and easy to install in place of the old one.
If there is no fault with the valve, the symptoms might be caused by a blocked waste pipe preventing the waste and gasses from entering the sewer. This will require rodding from an access point; if you’re not confident about doing this yourself, we advise calling in a professional plumber or drain cleaning service, as rodding drains incorrectly can create further problems.
We hope you have found this information helpful and interesting. If you have any further questions you are always welcome to call the friendly team of drainage experts at Drainfast on 01420 555600 or email [emailprotected].
Also, look out for more articles in our ongoing series of blog posts, bringing you useful information, insights, guides and tips on all things drainage!
