Choosing the right size unvented hot water cylinder is essential for reliable hot water supply and energy efficiency. Unvented cylinders operate at mains pressure and store a set volume of hot water that must meet peak demand between recovery periods. Here’s our guide to selecting the right domestic hot water cylinder.
Start with the property, not the current occupancy:
OSO Hotwater recommends sizing the cylinder to match the property’s capacity rather than basing the decision solely on the current number of occupants. This protects the homeowner’s investment and ensures the system remains adequate if household size changes.
Recommended sizes for unvented cylinders by property type:
- Studio or 1-bedroom flat (1–2 people, 1 bathroom): 120 litres.
- 2-bedroom house or flat (2–3 people, 1 bathroom): 150 litres.
- 3-bedroom house (3–4 people, 1–2 bathrooms): 180–210 litres.
- 4-bedroom house (4–5 people, 2–3 bathrooms): 250 litres.
- 5+ bedroom house (5+ people, 3+ bathrooms or en-suites): 300 litres.
Adjusting for heat source recovery speed:
If the cylinder is connected to a heat pump, particularly an air source heat pump, recovery times are longer than with a gas boiler. In this case, OSO recommends increasing the nominal cylinder size by one step to provide additional stored capacity between recovery cycles.
OSO’s DELTA GEOCOIL (DGC) is specifically recommended for heat pump installations, offering the lowest heat loss of any comparable cylinder and optimised for low-temperature primary circuit operation.