What size unvented cylinder do I need?

As a guide: 1 bed/1 bath = 120L, 2 bed/1 bath = 150L, 3 bed/2 bath = 210L, 4 bed/2 bath = 250L, 5 bed/3 bath = 300L. The actual size depends on occupancy, simultaneous demand, and whether you have a bath (which uses around 80-100L per fill).

What qualifications do I need to install an unvented cylinder?

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.

How is the expansion vessel sized?

The expansion vessel must accommodate the expansion of water when heated from cold (typically 10°C) to storage temperature (60°C). Water expands by approximately 4% over this range. The vessel is sized at 4% of cylinder volume plus a 10% safety margin.

What are the discharge pipe requirements for unvented cylinders?

Part G3 requires a temperature and pressure relief valve discharging via a tundish to a D2 discharge pipe. The tundish must be visible in the same room. The D2 pipe must be at least one size larger than the T&P valve outlet and discharge to a safe, visible location.

What temperature should an unvented cylinder be set to?

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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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.

Property Type

Select property size or choose custom

Heating Coil Output

Boiler or heat source output to coil

Cold Inlet Temperature

Mains cold water temperature (UK avg 10°C)

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: Cylinder litres × 0.04 × 1.1 safety margin
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 losses are based on typical UK cylinder performance to BS 1566 / BS EN 12897 standards. Always refer to the specific manufacturer's data for precise values.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.