How Quickly Does a Heat Pump Water Heater Recover?
How Quickly Does a Heat Pump Water Heater Recover? A heat pump water heater will typically take several hours to recover a substantially depleted cylinder, although the exact recovery time depends on the cylinder capacity, heat pump output, incoming cold water temperature, air temperature and required hot water temperature. A smaller cylinder with a relatively powerful heat pump may recover much faster than a larger unit with a lower heating output. In everyday use, the cylinder is also rarely completely cold, so actual recovery after showers or a bath can be considerably shorter than the manufacturer’s full heat-up time.
What does hot water recovery time mean?
Recovery time is simply how long the water heater takes to replace the heat that has been removed from the cylinder.
If you take a shower, hot water leaves the cylinder and is replaced by cold mains water. The heat pump then needs to heat that incoming water back to the required storage temperature.
A full heat-up test is different. This measures how long the appliance takes to heat an entire cylinder from a specified cold water temperature to a specified final temperature.
When comparing heat pump water heaters, it is important to check which figure the manufacturer is quoting.
At the compact end of our range is the 100 litre wall mounted air source hot water heater, intended for smaller domestic and selected light commercial hot water requirements.
For larger homes, compare the 200 litre home heat pump water heater with the 300 litre home heat pump water heater.
For applications requiring considerably greater hot water storage, consider our 400 litre high-capacity air source water heater or 500 litre high-capacity air source water heater.
What determines how quickly the cylinder recovers?
Several factors have a direct effect on recovery time.
The most important are:
- Cylinder capacity
- Heat pump heating capacity in kW
- Incoming mains water temperature
- Required stored water temperature
- Temperature of the air entering the heat pump
- Amount of hot water already remaining in the cylinder
- Whether an immersion heater or boost mode is used
A 200 litre and 300 litre heat pump water heater with the same heating capacity will not have the same full recovery time. The larger cylinder contains more water and therefore requires more energy to achieve the same temperature rise.
Why does heat pump output matter?
The heat pump’s heating capacity determines how quickly it can put energy back into the stored water.
For example, a unit producing 3 kW of useful heating output will recover a cylinder faster than a 1.5 kW unit under otherwise identical conditions.
However, the electrical input should not be confused with the heating output.
A heat pump might consume around 1 kW of electricity while providing several kilowatts of useful heat because it is also extracting energy from the surrounding air. This relationship is expressed by its Coefficient of Performance (COP).
Does recovery take longer in winter?
It can.
An air source heat pump water heater relies on heat from the air. As the incoming air temperature falls, the heat pump’s heating capacity and COP can also fall, depending on the design of the appliance.
A unit using relatively warm air from a utility or plant room may therefore have more consistent recovery performance throughout the year than one relying on cold outside air.
This is why manufacturer recovery figures should always be read alongside the stated test conditions.
Does the hot water temperature affect recovery?
Yes, considerably.
Heating water from 10°C to 50°C requires a 40°C temperature rise. Heating the same quantity from 10°C to 60°C requires a 50°C rise, so more energy is needed.
The final part of the heating cycle can also become harder for the heat pump as the water gets hotter.
This is one reason heat pumps generally achieve better efficiency when producing domestic hot water at sensible storage temperatures rather than continually operating at their maximum temperature.
Higher water temperatures can increase usable mixed hot water capacity, but they can also increase recovery time and reduce COP.
Will the immersion heater make recovery faster?
Usually, yes.
Many heat pump water heaters have an electric immersion heater that can operate alongside or instead of the heat pump during boost operation.
This increases the amount of heating power available and can reduce recovery time.
The disadvantage is increased electricity consumption. An immersion heater uses direct electric resistance heating, whereas the heat pump can provide several units of useful heat for each unit of electrical energy consumed.
Boost heating is therefore useful for occasional high demand but ideally should not be required every day simply because the cylinder is undersized.
Do I need the whole cylinder to recover before using hot water?
Not always.
Hot water cylinders naturally develop thermal stratification, with hotter water towards the top and cooler water towards the bottom.
Depending on the cylinder and heat pump design, this can mean useful hot water becomes available before the entire cylinder has reached its final target temperature.
This is why a quoted full recovery time of several hours does not necessarily mean you will have no usable hot water during that period.
Is a long recovery time a problem?
Not if the heat pump water heater is correctly sized.
A heat pump does not need to recover as quickly as a combi boiler if the cylinder already contains enough stored hot water to cover periods of peak demand.
For example, the cylinder might supply several morning showers and then gradually recover during the following few hours when very little hot water is required.
This is why cylinder capacity and recovery time should always be considered together.
How do I compare heat pump water heater recovery times?
Look at the manufacturer’s full heat-up or recovery time, but check the cylinder volume, starting water temperature, final water temperature and air temperature used for the test. Figures based on different test conditions cannot be compared fairly.
Also consider the heat pump output, COP, cylinder capacity, usable hot water volume and immersion heater boost capability.
The best heat pump water heater is not necessarily the one with the shortest recovery time. A correctly sized system should provide enough stored hot water for peak demand and then allow the heat pump to recover the cylinder efficiently before the next major period of use.