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Unvented Cylinder Calculator — Part G3 Sizing
Size unvented hot water cylinders to Building Regulations Part G3. Calculate recovery time, expansion vessel size and safety controls.
Boiler or heat source output to coil
Mains cold water temperature (UK avg 10°C)
Target storage temp (60°C recommended)
Water expansion fraction. 0.017 = physical expansion 10→60°C (density 999.7→983.2 kg/m³). Some designers allow up to ~0.04 toward 100°C as a conservative margin — that is a rule of thumb, not a Part G3 figure.
Incoming mains cold water pressure (used to select the relief valve; vessel pre-charge is set to match the cold-fill pressure, min 1.5 bar — Heatrae Sadia / Megaflo install-manual practice)
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How We Calculate This
This calculator sizes unvented hot water cylinders in accordance with Building Regulations Approved Document G3 and BS EN 12897.
Formulas
- Energy stored: (Volume × 4.186 × ΔT) ÷ 3600 = kWh
- Recovery time: (kWh ÷ (coil kW × 0.85 efficiency)) × 60 = minutes
- Expansion volume (Ve): Cylinder litres × 0.017 (10°C→60°C)
- Expansion vessel: Ve × Pmax(abs) ÷ (Pmax(abs) − Pcharge(abs)) — acceptance-volume method
- ΔT: Storage temperature (60°C) minus cold inlet (10°C) = 50°C
The 0.85 coil efficiency factor accounts for heat transfer losses through the coil. Standing-loss figures are indicative typical values for UK unvented cylinders tested to BS EN 12897; modern ErP-rated cylinders may be lower. Always refer to the specific manufacturer's data sheet for precise heat-loss and recovery values.
Frequently Asked Questions
As a guide: 1 bed/1 bath = 120 L, 2 bed/1 bath = 150 L, 3 bed/2 bath = 210 L, 4 bed/2 bath = 250 L, 5 bed/3 bath = 300 L. The actual size depends on occupancy, simultaneous demand, and whether you have a bath (which uses around 80–100 L per fill). For a more detailed comparison, see our Hot Water Cylinder Calculator.
Under Building Regulations Part G3, unvented hot water systems over 15 litres must be installed by a competent person. This means holding a recognised G3 qualification (such as the City & Guilds unvented course). Building Control notification is required.
First find the expansion volume (Ve): cylinder litres × the expansion fraction. Water expands by about 1.7% when heated from cold (typically 10 °C) to storage temperature (60 °C), as its density drops from 999.7 to 983.2 kg/m³. The vessel is then sized by the acceptance-volume method: V = Ve × Pmax(abs) ÷ (Pmax(abs) − Pcharge(abs)), where Pmax is the relief-valve setting and Pcharge is the vessel pre-charge (set to mains pressure), both in bar absolute. This is larger than Ve alone because the bladder can only accept part of its volume between pre-charge and relief pressure. For heating system expansion vessels, see our Expansion Vessel Sizing Calculator.
Part G3 requires a temperature and pressure relief valve discharging via a tundish, visible in the same room, to a D2 discharge pipe. Domestic cylinders use a G½ (½-inch) valve with a 15/22mm tundish, so D2 is 22mm copper minimum. D2 must be at least one size larger than the D1 pipe leaving the tundish, fall continuously, and discharge to a safe, visible low-level point. AD G3 sizes D2 from the valve outlet and the equivalent pipe-run length: a G½ valve allows up to 9m in 22mm, 18m in 28mm or 27m in 35mm, with each bend counting as 0.8m. Larger commercial G¾/G1 valves need 28mm or 35mm D2.
Unvented cylinders should store water at 60°C to prevent Legionella growth (which thrives between 20-45°C). The thermostat is usually set to 60-65°C, with a high-limit thermostat cutting the heat source at around 80-85°C.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.