Estimates the reheat (recovery) time, energy and recovery rate to raise a stored hot-water cylinder from cold-feed to target temperature, sensible heat Q = m·cp·ΔT, cp = 4.186 kJ/kg·K.
Indicative sizing only: store DHW ≥ 60 °C for Legionella control (HSE L8/ACOP).
For design guidance only. Always verify with a qualified engineer.
About this cylinder recovery calculator
This free DHW cylinder recovery calculator estimates how long a stored hot-water cylinder takes to reheat (its recovery time), the energy needed, and the recovery rate in litres per hour, to raise the water from the incoming cold-feed temperature to the target stored temperature. It is aimed at UK building-services engineers, heating designers and plumbers sizing immersion heaters, boiler coils and indirect cylinders at tender or commissioning stage. Defaults follow a typical 200 litre unvented domestic cylinder heated from 10 °C mains to 60 °C by a 3 kW immersion.
How the recovery time is calculated
The method is the standard sensible-heat relationship Q = m · cp · ΔT. The temperature rise is ΔT = stored − cold feed; the stored mass is the cylinder volume in litres × a water density of 1 kg/L; and the heat required is that mass × the specific heat capacity of water, cp = 4.186 kJ/kg·K, × ΔT. Energy is converted to kWh (1 kWh = 3600 kJ), the effective heat input is the source power × the efficiency percentage, and the reheat time is energy ÷ effective power. The recovery rate is (Peff × 3600) ÷ (cp · ΔT), the litres raised from cold to target in one hour. The form used by UK trade cylinder-recovery calculators, and algebraically identical to volume ÷ reheat time. The 60 °C default reflects HSE L8/ACOP guidance on storing DHW hot enough to control Legionella.
Frequently asked questions
What efficiency should I enter?
For a direct electric immersion, use 100 %, effectively all the element’s output reaches the water. For an indirect coil fed by a boiler or heat pump, enter the heat-transfer efficiency of the coil into the store (commonly 80–90 %) so the effective input, and therefore the reheat time, reflect real performance.
Why is my recovery time longer than the manufacturer’s figure?
This calculator assumes a fully mixed store with no standing losses and a constant heat input. Real cylinders stratify, lose heat through the insulation, and a heat-pump or boiler output falls as the store warms, so allow a margin. Use it for indicative sizing, then confirm against manufacturer recovery data.
Should I store hot water at 60 °C?
HSE’s L8 Approved Code of Practice recommends storing DHW at 60 °C and distributing it at 50 °C or above to control Legionella, which is why 60 °C is the default target. Higher store temperatures increase the energy and recovery time for the same heat input.