Pressurised Safety Calculator — G3 Discharge Sizing

Part G3 requires three levels of safety: (1) Thermostat controlling normal temperature, (2) High-limit thermostat cutting heat source at ~80–85 °C, (3) Temperature and pressure relief (T&P) valve opening at 90 °C or rated pressure. Plus a pressure reducing valve (PRV) and expansion vessel. Use our Unvented Cylinder Calculator for full sizing and safety requirements.

The D2 pipe (from tundish to the safe discharge point) must be at least one pipe size larger than D1 (from T&P valve to tundish). For a G½ T&P valve, D1 is 15mm and the tundish is 22mm. Approved Document G3 sizes D2 by hydraulic resistance: the maximum permitted resistance expressed as a straight-pipe length is 9m for 22mm copper, 18m for 28mm and 27m for 35mm — and you must subtract 0.8m per 22mm elbow, 1.0m per 28mm elbow and 1.4m per 35mm elbow. If your run is longer, step up to the next pipe size. (The discharge-pipe sizing table is in Approved Document G3; the underlying water-supply standards are BS 8558 / BS EN 806, which superseded the withdrawn BS 6700.)

A tundish is an open-ended funnel fitting in the discharge pipe that provides a visible air break. Approved Document G3 requires it to be fitted vertically, in the same space as the cylinder, and within 500mm of the T&P (safety) valve, and it must stay visible for inspection. If water is dripping from the tundish it indicates the T&P or expansion-relief valve is operating and the system needs investigating.

Approved Document G3 requires a low-level downward discharge to terminate no higher than about 100mm above the external surface, or above the floor of a trapped gully (below the grating but above the water seal). Fit a wire guard if a person could come into contact with it, keep the outlet visible for inspection, and do not site it over a public access route or anywhere a person could be scalded. Terminating it high up (for example 300mm or more above ground) is not compliant — the requirement is the opposite, a low-level outlet that cannot reach a person.

The expansion-vessel pre-charge must match the static cold-fill (cold working) pressure of the system — i.e. the PRV setting, typically 3 bar — and is usually set about 0.2 bar below it so the diaphragm is just clear of the water at rest. So for a 3 bar cold-fill pressure the pre-charge is about 2.8 bar, not 2.5 bar. Setting it well below the cold-fill pressure reduces the usable acceptance volume and causes the expansion-relief valve to weep on every heat cycle. Always confirm against the cylinder manufacturer’s instructions.