What is overpressure?
Overpressure is a pressure higher than the pressure in the surroundings. In building work the term turns up in three places: in the soil and waste drainage, in a sealed heating system, and in measuring and controlling air movement through a building.
1. Overpressure in drainage
Water falling down a soil stack does not travel as a solid plug but runs along the pipe wall with air in the middle. At the bottom, where the pipe turns, the water lands and briefly seals the full bore. The air below is pushed out and the air above is compressed: a pressure wave travels back up the stack.
You notice the result indoors: drain smells in the house. The pressure wave pushes the water out of a trap, after which the appliance is open to the drain.
What helps:
- A vent pipe carried up above the roof, so the air has somewhere to go instead of blowing out the traps.
- Large-radius bends at the foot of the stack rather than a sharp elbow.
- Adequate pipe diameter; an undersized stack runs full bore and produces stronger pressure waves.
- Traps with a sufficient water seal, and an air admittance valve where venting to open air is not possible.
2. Overpressure in a heating system
A sealed heating system runs under pressure, typically around 1 to 1.5 bar cold in a domestic installation. Water expands as it heats, and the expansion vessel absorbs that expansion. If the vessel is too small or its diaphragm has failed, pressure rises too far and the pressure relief valve discharges — usually set at 3 bar. That is a safety device, not normal operation: if the valve discharges regularly, the cause is almost always the expansion vessel.
3. Overpressure as a measuring and control tool
In a blower door test, the building is deliberately put under positive and negative pressure (normally 50 pascals) to establish its airtightness and locate leaks. Under positive pressure you feel the leakage streams from outside, under negative pressure from inside.
Overpressure is also used to control air movement deliberately: a stair core held at positive pressure keeps smoke out and stays usable as an escape route.
Related terms
- Vent pipe
- Expansion vessel
- Boiler
- Airtightness
- Blower door test
Looking for more DIY plans? Check out fredsdiyplans.com.