HEAT LOSS HUB · TOOLBOX
Heat Pump Cylinder Reheat Calculator
How long does a heat pump take to heat a hot water cylinder?
The answer depends mainly on:
- how much water needs heating
- its starting temperature
- the required hot water temperature
- the heat output available from the heat pump
Use the free Heat Loss Hub Cylinder Reheat Calculator below to estimate the theoretical time required to heat a cylinder.
Calculate your heat pump cylinder reheat time
Enter:
Cylinder volume
The amount of water being heated.
Starting water temperature
For example, the incoming cold-water temperature or estimated temperature of a fully cooled cylinder.
Target hot water temperature
The temperature you want the stored water to reach.
Heat pump output
The approximate heat output available while heating the cylinder.
The calculator then estimates the energy required and converts this into a reheat time.
How is cylinder reheat time calculated?
Heating water requires energy.
The greater the cylinder volume and temperature rise, the more energy is required.
For example, heating 210 litres from 10°C to 50°C requires considerably more energy than reheating the same cylinder after only part of its stored hot water has been used.
The calculator uses the specific heat capacity of water to estimate the energy required and then compares this with the selected heat pump output.
Why might the actual reheat time be different?
The result should be treated as an estimate rather than a guaranteed reheat time.
Actual performance can be affected by:
- heat pump output at the current outdoor temperature
- hot-water flow temperature
- cylinder coil size
- heat exchanger performance
- heat losses
- controls
- water already remaining in the cylinder
- heat pump modulation
- cylinder temperature sensor position
A heat pump described as an 8 kW model also does not necessarily deliver exactly 8 kW under every operating condition.
That is why this calculator asks for the heat pump output actually available during cylinder heating, rather than simply asking for the model’s nominal badge size.
Why does a heat pump often heat hot water more slowly than a boiler?
Heat pumps normally operate with much lower heating outputs and water temperatures than traditional gas boilers.
That is not necessarily a problem.
A correctly sized hot-water cylinder provides stored hot water, so the system does not normally need to heat domestic hot water instantaneously.
Cylinder sizing, available heat-pump output and the required recovery time should therefore be considered together during system design.
Does a bigger heat pump reheat a cylinder faster?
If the cylinder and its heat exchanger can accept the additional output, increasing the available heat-pump output will reduce the theoretical reheat time.
However, the nominal size written on the heat pump is not necessarily the amount of heat it can deliver during domestic hot-water production.
Outdoor temperature and required water temperature influence heat-pump performance.
This is why proper heat pump selection involves considerably more than simply choosing a model based on its nominal kW rating — see what size heat pump do I need?
Does cylinder coil size affect reheat time?
Yes.
The cylinder coil has to transfer heat from the heat-pump circuit into the stored domestic water.
A cylinder intended for use with a heat pump normally has a larger heat exchanger than a conventional boiler cylinder because heat pumps operate at lower flow temperatures.
This calculator concentrates on the available heat energy and therefore does not attempt to model individual cylinder coil performance.
About the result
The calculator provides an educational estimate of cylinder reheat time.
It does not replace manufacturer performance data or a complete domestic hot-water design.
Actual heat pump and cylinder performance should be checked when specifying equipment for an installation.
Frequently asked questions
How long does a heat pump take to heat a 200 litre cylinder?
It depends on the starting temperature, required stored-water temperature and available heat-pump output. The calculator allows these values to be entered rather than relying on a single generic figure.
What temperature should a heat pump hot water cylinder be set to?
Operating temperatures depend on the system, controls and hygiene strategy. The calculator therefore allows the target temperature to be selected rather than prescribing one setting.
Does an 8 kW heat pump always deliver 8 kW to the cylinder?
No. Available output can change with outdoor and water temperatures and the characteristics of the heat pump.
Does cylinder size affect reheat time?
Yes. All else being equal, a larger volume of water requires more energy and therefore takes longer to heat.
Need help designing a heat pump system?
Heat Loss Hub provides independent heat pump design and technical advice covering heat loss, emitter sizing, domestic hot water and system design.