Ventilating Building Drainage Systems: Are Air Admittance Valves Only Half the Answer?

This #TechnicalTuesday Nick Orman discusses air admittance valves and the challenge of effectively ventilating building drainage systems.

Traditionally, buildings’ drainage systems in the UK have had soil stacks, which vented sewer gases to the atmosphere outside the building, usually above roof level. These were to prevent negative pressures caused by flow into already full pipes which can empty the water seal in appliances within the building, and might cause air from the sewers to later enter the property. In addition, at a time when it was thought that diseases might be spread from sewer gases seeping through the ground, they provided ventilation of the underground building drainage system.

When soil stacks were mounted on the outside of the building, it was easy to provide an open stack; however, when soil stacks were moved inside the building, this necessitated passing the soil stack through the roof without resulting in future leaks. In a few cases provision of an open stack was not possible.

What is an Air Admittance Valve?

An Air Admittance Valve (AAV) is a spring-loaded diaphragm that acts as a non-return valve, which is fitted to the top of a soil stack. If a negative pressure occurs inside the building’s drainage system the valve opens, letting air in, keeping the pressure at or near atmospheric pressure and therefore preserving the water seal in the traps on each of the individual appliances. It can therefore be fitted inside the building in a roof space, or elsewhere, avoiding the need for the soil stack to be vented outside.

Do AAVs solve the whole sewer gas problem?

The AAV only protects the property from the effects of negative pressures in the drainage system. So, in that sense they are perhaps only half a solution; if ventilation is not adequate in the drainage system a positive pressure can develop. For example:

• Where the system is pumped: in this case, as the water level in the wet well rises (between pump cycles) the air will be displaced into the drainage system increasing the air pressure.
• If the drainage system is ultimately connected to a combined sewer system: in this scenario, which is very common, during extreme rainfall the water level in the downstream can back up the branch sewers, compressing the air that is trapped due to inadequate ventilation creating positive pressure.

In extreme cases this positive pressure can blow air from the sewer through the water traps and into the property. This not only can lead to an unpleasant smell, but it can also spread diseases.

For this reason, the Building Regulations Approved Document H Drainage and Waste Disposal (para H1, 1.29)* recommends stacks to be ventilated, and (in H1 para 1.33) it states that where AAVs are used they should not affect the amount of ventilation necessary. This regulation requirement is normally achieved by ensuring 1 in 10 stacks have open vents. At least one of these should be on the property nearest to the point of connection from the development to the sewer system.

I get a sewage smell in my property. Is it due to lack of ventilation in the sewer system?

Not necessarily. Occasionally the valves do get stuck in the open position. In the majority of cases, where there have been persistent sewage smells in a property, this has been the cause. If this occurs it is usually a simple matter to replace the valve. For this reason, the Building Regulations Approved Document H (H1 para 1.33)* recommends that AVVs are fitted in accessible areas such as lofts. In one case I am aware of, where the smell was particularly severe, the homeowner had to break out the tiles in his bathroom to get access to the valve!

Do I need an AAV if my soil stack is outside?

I often see AAVs fitted on outside stacks, but this is neither necessary nor advisable. Provided the stack is terminated at least 900mm vertically above any opening, and is within 3m of the stack, an open stack is permitted. Indeed, the Building Regulations Approved Document H (H1 para 1.33)* recommends that they are not fitted outside or in dust laden atmospheres.

* These references and the text are identical in the England and the Wales editions. There are similar provisions to para 1.29 in Scotland and Northern Ireland.