What is a positive pressure?
Positive pressure is generated in two forms within the drainage system: a) constant pressure – which is the slow build-up of pressure due to a long-term blockage within the system; and b) positive transients – which are produced at every discharge of the system, mainly at the base of the stack. The positive transient is the most harmful to the trap seal, as it travels through the system at the speed of sound, 320m/s, with an average volume of 2 litres of air.
