Safe termination options for unvented cylinder discharge pipework

An unvented cylinder’s relief discharge can contain very hot water and steam. Select the whole route and termination to protect people, preserve drainage and make discharge apparent. The arrangements below follow Approved Document G for England; the cylinder and discharge-system instructions also apply.

External hardstanding or a trapped gully

These are separate arrangements. For a downward outlet above external hardstanding, finish no more than 100 mm above the surface and use a wire cage or similar guard to prevent contact while leaving discharge visible. Choose a position where the hot water cannot injure someone or create an access hazard.

For a trapped gully, place the outlet below a fixed grating and above the water seal. Do not apply the hardstanding height rule as though every gully outlet must sit above the grating. Keep the outlet clear so discharge can escape.

A suitable internal soil stack

An internal connection is possible only as a specified arrangement. Establish that the stack can withstand the discharge temperature. Use a separate branch with no sanitary appliances connected, the required warning marking and a suitable mechanical odour seal without a water trap. Any plastic components must meet the stated performance requirements; an ordinary waste connection is not enough.

Keep discharge visible at the tundish. A correctly designed stack connection is different from terminating an open pipe inside a wall void or cupboard. Never add an isolating valve that could shut the safety route.

Assess the complete installation

Approved high-level arrangements also exist, subject to their material and location conditions. Do not substitute a convenient roof, drain or hopper without checking those conditions. Confirm pipe sizing, fall, temperature suitability and protection together; a safe-looking outlet cannot compensate for an undersized or obstructed route.