What Size Hot Water Cylinder Do I Need?
Quick Answer
120L for 1–2 people, 180L for 3–4, 250L+ for 5+
Based on: average UK hot water usage of 40–50 litres per person per day. Indirect (coil) cylinder with boiler or heat pump.
Sizing Guide
- 1–2 people, 1 bathroom: 120–150 litres
- 3–4 people, 1–2 bathrooms: 180–210 litres
- 5+ people, 2–3 bathrooms: 250–300 litres
- With heat pump: Increase by 20–30% (lower flow temperatures need more storage)
Direct vs Indirect
- Indirect: Heated by a coil from the boiler or heat pump. Most common in the UK.
- Direct: Heated by immersion heater(s) only. Used where no boiler is present.
- Unvented (mains pressure): No header tank needed. Good shower pressure. Requires annual service and G3 qualified installer.
- Vented (gravity fed): Fed from a cold water tank in the loft. Simpler but lower pressure.
Physical Dimensions
- 120L: ~500mm diameter × 900mm tall
- 180L: ~550mm diameter × 1,100mm tall
- 250L: ~580mm diameter × 1,400mm tall
- 300L: ~580mm diameter × 1,600mm tall
Check the airing cupboard dimensions before ordering.
Heat Pump Considerations
Heat pumps operate at lower flow temperatures (40–50°C vs 60–70°C for a boiler), so you need a larger cylinder to store the same usable energy. A 210–300L cylinder is typical for a heat pump installation.
Last updated: April 2026. Unvented cylinders must be installed by a G3-qualified engineer and inspected by Building Control.