Using the useful heat input to estimate recovery time, then checking actual cylinder performance.
How fast does the store recover
Recovery or reheat time tells you how quickly a cylinder is ready again after a big draw-off. The method is the same energy idea as before: find the energy needed, then divide by the heat input power. The numbers here are chosen to show the method.
The situation. A 120 litre cylinder (about 120 kg of water) needs a 45 °C temperature rise back to 60 °C. For this calculation, assume the coil supplies an average useful input of 10 kW to the water over that period and ignore heat losses.
Step 1 - energy required. Q = m x c x change in T = 120 x 4.186 x 45 = 22,604 kJ, about 22.6 megajoules.
Step 2 - reheat time. A 10 kW coil delivers 10 kJ each second. Time = 22,604 / 10 = 2260 seconds, which is about 38 minutes.
Compare with an immersion. The same job on a 3 kW immersion would take 22,604 / 3 = about 7535 seconds, or roughly 126 minutes. The shorter time follows from the assumed 10 kW useful input compared with 3 kW for the same heating task.
Real life. A coil’s transferred power changes with primary and stored-water temperatures, flow and available heat-source output. Use its performance at the actual conditions, with the relevant heat losses and control operation. Do not apply a second arbitrary coil-efficiency penalty after already using useful heat transferred to the water, or assume the real time is only slightly longer. Compare the manufacturer’s recovery data with the demand pattern.