Cylinder Heat-Up Calculator — Recovery Time

The formula is t = (V × 4.186 × ΔT) / (kW × 3600), where V is volume in litres, 4.186 is the specific heat capacity of water in kJ/kg·K, ΔT is the temperature rise, and kW is the heat input. The result gives time in hours, which we convert to minutes.

An indirect coil in a cylinder doesn’t transfer 100% of the boiler’s output. Typical coil efficiency is 80-90% depending on coil surface area, flow temperature, and flow rate. A 28 kW boiler with 85% coil efficiency delivers about 23.8 kW to the stored water.

While the cylinder is heating, it also loses heat through its walls. A well-insulated modern cylinder loses about 1-2 kWh per day. During the heat-up period, standing losses add approximately 3-5% to the energy requirement, slightly extending recovery time.

Heat pumps typically deliver water at 45–55 °C compared to a gas boiler's 70–80 °C. The lower flow temperature means a standard indirect coil transfers far less energy per unit time, which is why a heat pump paired with an ordinary boiler-rated coil gives slow 2–4 hour recovery. Heat pump cylinders fit larger high-gain coils (often 3 m² surface, 20 kW or more) to restore fast recovery despite the low flow temperature — so set the Coil Rating to your cylinder's heat-pump coil figure, not a standard 15 kW. Use our Cylinder Coil Calculator to size the coil for a heat pump installation.

The recommended storage temperature is 60°C to prevent Legionella bacteria growth. Water should be stored at 60°C and distributed at 50°C minimum (or via TMV at point of use). Weekly pasteurisation to 70°C may be recommended for larger systems.