Heating & hot water
Cylinder recovery time
- where
- recovery time (s; the calculator shows min)
- fluid density (kg/m³)
- heated volume (m³; any unit you pick is converted)
- specific heat capacity (J/(kg·K); any unit you pick is converted)
- water temperature rise (K)
- average useful heat transferred to water (W; any unit you pick is converted)
Explain this equation
Estimates how long a cylinder takes to heat its water through a given rise with a known heat input. Use it to compare coil, boiler or immersion recovery.
ρ × V × c_p × ΔT is the energy needed to heat the volume through the rise. P is the average heat actually getting into the water from the coil or element, not the boiler's rated output.
Time is energy needed divided by the rate it is delivered. The calculator works in seconds and shows minutes.
Sense check: 150 L through 50 K at 15 kW takes about 35 minutes. A 3 kW immersion on the same cylinder takes around 2 hours 55 minutes. Halve the power and the time doubles.
Watch out: coil output drops as the cylinder warms up, so use an average figure. Losses are ignored, so real times will be longer.
Enter your values to see the result.
Good to know
Use average delivered coil or immersion output; actual recovery depends on coil conditions and losses.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.