Reliance Valves Dry Trap Tundish, Backflow prevention device for Fluid Category 5 contamination risk with AA type air gap and spring loaded mechanism, Reliance Valves Dry Trap Tundish is a type AA backflow prevention device, meaning it is an unrestricted air gap. It is designed to take any discharge that may occur from either a Pressure Relief Valve or a Pressure & Temperature Relief Valve fitted to an unvented hot water cylinder. Water flowing through a tundish is an indication of a fault with the system and alerts the occupants of the issue. It is typical practice to fit both a trap, to prevent odours coming up through the drainage system, and a tundish to prevent backflow into the potable water system.