Soil pipes running through a home take the waste from appliances and direct it towards the sewer system. When waste water flows inside a soil pipe, as when a WC is flushed, negative pressure builds up inside the pipe causing a siphoning effect. Water always flows form an area of high pressure to an area of low pressure. The force of the water being exerted from a pipe is proportionate to the height of water pushing down. When a siphon is introduced, there is nowhere for air to get into the siphon as it’s full of water and a vacuum is created inside the pipe due to the difference in pressure between the internal and external environment. If this pressure is left unchecked, water would be siphoned out of the traps in the bath or sink which would allow drain odours to enter the bathroom or kitchen. What we see in the picture is the traditional way of preventing this from happening, which is to have a vent pipe connected to the drain that is open to the atmosphere. As the siphoning pressure builds up inside the soil pipe, air is drawn in through the open vent pipe to balance the negative pressure.
Approved document H – Drainage and waste disposal requires that the foot of the discharge sack should have as large a radius as possible. Ventilating pipes should be finished at least 900mm above any opening within 3m and should be finished with a wire cage or perforated cover.
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