Where D2 can end: a guarded outlet over hardstanding, a trapped gully, a suitable soil stack or at high level, and the terminations that fail.
An unvented cylinder's relief discharge can be very hot water and steam, reaching the outlet close to the stored water temperature. Terminate it where it cannot scald anyone or cause damage, and keep discharge visible at the tundish so an unexpected release gets noticed. A deliberate service test is different from an unexplained discharge. Approved Document G describes several acceptable arrangements for England: a low-level termination is common, but it is not the only one. The cylinder and discharge-system instructions also apply.
External hardstanding
For a downward outlet above external hardstanding, finish no more than 100 mm above the surface. Fit a wire cage or similar guard so nobody can touch the discharge while it stays visible. Choose a position where hot water cannot injure someone or create an access hazard.
Trapped gully
Place the outlet below a fixed grating and above the water seal. The 100 mm hardstanding rule does not apply here: this is a different arrangement. Keep the outlet clear so the discharge can escape.
High level
Two high-level options are described:
- Into a metal hopper and metal downpipe, with the end of the discharge pipe clearly visible.
- Onto a roof that can withstand near-boiling water, at least 3 m from any plastic guttering that would collect the discharge. Hot water and steam damage plastic rainwater goods, asphalt and roofing felt.
Internal soil stack
An internal connection is allowed, but only as the specified arrangement. The stack must withstand the discharge temperature, with a separate branch serving no sanitary appliances, the required warning marking and a mechanical odour seal without a water trap. Any plastic pipework must meet the stated performance requirements; an ordinary waste connection is not enough. Check the whole arrangement before using it, and keep the discharge visible at the tundish.
Terminations that fail
- Just above a doorway step where people stand: someone could be scalded.
- An open end in a concealed wall void or cupboard: with no protected collection or drain connection, it can scald or flood unseen and it hides the warning. On a service, record it as non-compliant and require a safe termination with discharge visible at the tundish.
- A route that rises before it falls: after the tundish's air break, D2 relies on gravity and needs a continuous fall. A larger diameter does not fix it.
- Any isolating or shut-off valve on the discharge: even one meant only for servicing would, once closed, stop the relief valve discharging.
A hidden section of D2 is not a defect in itself, as long as the tundish stays visible and the pipe is still sized, falls and terminates correctly. If you cannot provide a safe discharge route with a visible tundish, do not go ahead with the installation until that is solved.