Branch Connections & AAVs

Why Group Venting Capacity Matters

The benefits of Air Admittance Valves (AAVs) for group / branch venting is worth explaining further.

Typically a drainage system in a building is made up of vertical pipes (stacks) and horizontal pipes (branches).

These pipes are called ‘wet pipes’ as they collect all the waste towards the external main sewer system. In order to maintain the integrity of the water trap seals, VENTILATION is required to balance dangerous pressure fluctuations, and the solutions adopted are either a network of ‘dry pipes’ or the use of AAVs (which are proven to be much more effective).

Drainage System Design
Building drainage systems, comprise of appliances such as WCs, sinks, baths, etc., which have water trap seals. The water trap seal provides the essential barrier between the sewer system and the inhabited area. Branch pipes are typically 32mm, 40mm, or 50mm for most appliances, rising to 75/100mm for WCs, run horizontally with a gradient. The branch pipes then discharge into vertical stack pipes, typically 100-150mm in diameter.

The buildings drainage system is designed to remove waste water, be self-cleaning, watertight and to prevent foul odours.

To ensure that the water trap seals are protected, the traps require ventilation. To prevent self-siphonage and induced siphonage, this is traditionally achieved by using a network of vent pipes for group venting of the branches, or individual vents for each appliance.

It is proven that using Studor AAVs offers a more effective method to provide ventilation, as the AAVs can be placed at the “point of need” (i.e. where air is required), and so react quickly and efficiently to maintain the ventilation requirements of the building’a drainage network.

How to Select the Most Appropriate AAV

Firstly, over ventilating a branch is NEVER an issue, but under-venting IS, which links to the second point: Which part of the branch needs protection / ventilation?

As the first point is very much self-explanatory and intuitive, the second point is probably the most important one to consider, as it underlines the fundamental aspect that all traps connected to the same branch interact with each other.

Basically a pressure fluctuation in a branch, if not properly balanced, will always be active and so will deplete a water trap seal; this illustrates clearly that the whole branch needs protection. The ventilation capacity is fundamental, as it should not just account for a single appliance’s trap, but for all traps connected to that same branch.

Unfortunately, too often the solution adopted is based on installing a simple “anti-vacuum trap” as a quick fix. However, for sure, this may not provide protection for the branch.

The ideal solution is the installation of an approved AAV that protects the whole branch – basically an AAV that has ‘group-venting’ capacity, i.e. the capacity to ventilate all traps connected.

To further elaborate on the ‘group venting’ capacity, the drainage design standard, EN12056-2, dictates that:

• a minimum of 1.5 l/s is required (based on the calculation for System 1 with k = 0.7) to ventilate a bathroom branch with 3-4 appliances connected (excluding the WC);
• only European standard EN12380 approved AAVs should be installed.