An anti-syphon trap is a specially designed waste trap that admits air at the trap weir to prevent the water seal being drawn out during fast discharge flows from connected appliances or other fittings nearby. They are used where conventional venting is impractical or where Building Regulations Part H1 limits on branch lengths and slopes would otherwise be exceeded. The trap usually contains a small mechanical valve or breather pipe that opens only when negative pressure develops. UK installations follow BS EN 12056 for gravity drainage inside buildings. Anti-syphon traps need to be installed the correct way up and within the temperature range stated by the manufacturer. They are sometimes used in retrofit work in older London flats to address recurring loss of water seal in distant basins or showers.
Anti-syphon trap
Trap with built-in air inlet preventing loss of water seal during heavy discharge flows.
