A worked estimate of how long a heat source takes to reheat a hot water cylinder, from energy and power.
A useful estimate is how long a heat source takes to reheat a cylinder of stored hot water. The method is two steps: find the energy, then divide by the power.
Step 1 - energy needed. Use energy (kJ) = mass (kg) x specific heat x temperature rise. For water, take the specific heat as 4.2 kJ per kg per °C and 1 litre as 1 kg. For this example, heat 120 litres through a 50 °C rise: 120 x 4.2 x 50 = 25200 kJ.
Step 2 - choose the useful heat input. Assume a constant 9 kW reaches the stored water throughout this example, equal to 9 kJ per second. An immersion heater might supply 3 kW. For an indirect cylinder, use heat transferred through the coil, not the boiler’s full rating; actual coil transfer depends on primary temperatures, flow and the temperature of the stored water.
Step 3 - reheat time. Time (seconds) = energy / power = 25200 / 9 = 2800 seconds. Divide by 60 to get minutes: 2800 / 60 = about 47 minutes.
Step 4 - check the assumptions. The 47-minute result assumes 9 kW of useful heat reaches the water continuously, with no draw-off or heat loss from the stored water. A real cylinder loses some heat, and coil transfer can change as the store warms. A coil may not keep up its quoted peak duty for the whole reheat, so the real time can be well over the estimate, not just a few minutes more.
Step 5 - compare like conditions. Use the actual volume, starting and target temperatures, primary flow and temperatures, available heat input and any draw-off. Check the manufacturer’s stated reheat conditions before treating a longer time as a fault. If performance remains poor under comparable conditions, investigate circulation, controls, heat transfer and losses using the appliance instructions.