What safety controls does an unvented cylinder need?

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.

How do I size the D2 discharge pipe?

The D2 pipe (from tundish to outside) must be at least one pipe size larger than D1 (from T&P valve to tundish). For cylinders up to 200L with 15mm D1, use 22mm D2. For larger cylinders with 22mm D1, use 28mm D2. BS 6700 Table 2 gives maximum equivalent pipe lengths.

What is a tundish and why is it needed?

A tundish is an open-ended funnel fitting in the discharge pipe that provides a visible air break. It must be installed in the same room as the cylinder and be visible for inspection. If water is dripping from the tundish, it indicates the T&P valve is operating and the system needs investigation.

Where should the discharge pipe terminate?

The D2 discharge pipe must terminate at a visible external location, minimum 100mm from the wall surface, at least 300mm above ground level. It must not discharge over a public pathway, and the discharge point must be visible for inspection. A trapped gulley below ground is not acceptable.

How is the expansion vessel pre-charge pressure set?

The expansion vessel pre-charge should be set to the incoming supply pressure minus 0.5 bar (after the PRV). For a 3 bar PRV, set the expansion vessel to 2.5 bar. Check with the cylinder manufacturer’s specific instructions, as settings may vary.

TradeCalculator

Pressurised Safety Calculator — G3 Discharge Sizing

Calculate PRV settings, expansion vessel acceptance, T&P valve discharge pipe sizing, and tundish requirements per Building Regulations Part G3.

Cylinder Size

Unvented cylinder capacity

Incoming Mains Pressure

Measured mains water pressure

Storage Temperature

Cylinder thermostat setting

How We Calculate This

This calculator sizes the safety components for pressurised (unvented) hot water systems in accordance with Building Regulations Approved Document G3 and BS 6700.

Component sizing

PRV: Set to reduce incoming pressure (typically to 3 bar)
Expansion vessel: Volume × 0.04 expansion × 1.1 safety
T&P valve: 90°C / 6 or 7 bar rated
D1 pipe: Same size as T&P outlet (15 or 22mm)
D2 pipe: One size larger than D1 (22 or 28mm)
Tundish: Visible air break in same room as cylinder

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.