Hot water
Hot water cylinder reheat time
- where
- heat needed (kWh)
- volume to reheat (litres)
- water density (kg/litre)
- specific heat capacity of water (kJ/kg·K)
- target stored temperature (°C)
- starting water temperature (°C)
- reheat time (min)
- heat delivered to the cylinder (kW)
Explain this equation
Works out how long a hot water cylinder takes to reheat from cold. Use it to judge whether the heat pump and coil can keep up with the household.
Volume times density is the mass of water. Times c_p and the temperature rise gives the heat needed in kJ; dividing by 3600 turns kJ into kWh. Dividing by the heat delivered in kW gives hours, and 60 converts that to minutes.
Heat needed is mass times specific heat times temperature rise; time is energy divided by power.
Sense check: 200 litres from 15 to 50 °C needs about 8.1 kWh, about an hour at 8 kW. Halve the power and it takes twice as long.
Watch out: the coil often limits the heat input more than the heat pump does, and output drops as the cylinder warms. The real time will be longer than this ideal figure.
Estimates how long a heat pump takes to heat a cylinder from cold, or after a big draw-off, to the stored temperature.
Enter your values to see the result.
Good to know
Use the heat the coil can actually transfer at your flow temperature, which is often less than the heat pump’s rated output: check the cylinder’s heat pump coil rating. A 200 litre cylinder from 15 to 50 °C needs about 8 kWh. MCS 031 allows for a weekly or daily immersion pasteurisation cycle (about 10 K more) on top.
Common mistake
Assuming the full heat pump output reaches the water. As the cylinder warms, the coil temperature difference falls and the heat transfer drops off, so the last few degrees are slow.
Limits
Average-power estimate with no standing losses and full-volume mixing. Stratified cylinders deliver usable hot water sooner than this suggests.
Sources
- MCS 031 Issue 4.0 Heat pump pre-sale information and calculations (Pspecific = 1000 × Q / (24 × D); Pt = Pspecific × (21 − do); W/m²; Appendix A Table 1 degree days and outdoor low temperatures by postcode; Appendix B worked example)
- MCS MIS 3005-D Heat pump design standard (heat pump to meet the design heat loss at the design outdoor temperature; room temperatures and outdoor design temperatures; design to BS EN 12831-1)
- CIBSE Domestic Heating Design Guide (2026 edition; design to BS EN 12831-1, room temperatures, air change rates, pipe and emitter sizing)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.