What Size Air Source Heat Pump Water Heater Do I Need?
What Size Air Source Heat Pump Water Heater Do I Need? The size of air source heat pump water heater you need depends mainly on your daily hot water demand, the number of people using it, shower and bath usage, cylinder recovery time and the temperature at which the hot water is stored. As a general guide, smaller households may use a 100 to 150 litre unit, while 200 to 300 litre heat pump water heaters are commonly considered where several people need hot water each day. Larger domestic or commercial applications may require 400 litres, 500 litres or more. The correct size should always be based on actual hot water demand rather than cylinder capacity alone.
How is a heat pump water heater sized?
An air source heat pump water heater is usually sized differently from a combi boiler.
With a combi boiler, much of the emphasis is on how quickly hot water can be produced as it is required. A heat pump water heater normally heats water more gradually and stores it in a cylinder ready for use.
You therefore need enough stored hot water to cope with periods of peak demand while giving the heat pump sufficient time to recover the cylinder afterwards.
Important factors include:
- Number of occupants
- Number and duration of showers
- Shower flow rates
- Bath size and frequency of use
- Number of bathrooms
- Stored water temperature
- Incoming cold water temperature
- Heat pump heating capacity
- Cylinder recovery time
Two four-person households can have completely different hot water requirements, so occupancy alone should not be used to select the cylinder.
What cylinder size do I need per person?
A litres-per-person figure can provide a starting point, but it should not be treated as a fixed sizing rule.
As a broad guide, you might consider:
| Household | Indicative cylinder capacity |
| 1 to 2 people | 100 to 150 litres |
| 2 to 3 people | 150 to 200 litres |
| 3 to 4 people | 200 to 250 litres |
| 4 to 5 people | 250 to 300 litres |
| Higher demand | 300 litres or more |
These figures are only indicative. A household taking short showers at modest flow rates could require considerably less hot water than another household of the same size using large baths and high-flow showers.
Shower flow rate can make a big difference
One of the most useful things to establish is how much water the showers actually use.
For example, a shower delivering 8 litres per minute for 10 minutes uses 80 litres of mixed water.
A 12 litre per minute shower running for the same 10 minutes uses 120 litres.
If several people shower one after another, the difference becomes significant.
Remember that the water delivered at the shower is a mixture of hot water from the cylinder and cold mains water. A cylinder containing 200 litres of hot water can therefore provide more than 200 litres of usable mixed shower water, depending on the stored water temperature, incoming cold water temperature and required shower temperature.
Smaller flats, apartments and houses can benefit from the compact dimensions and storage capacity of our 100 litre wall mounted heat pump water heater.
For properties with a higher daily hot water requirement, consider our 200 litre residential heat pump hot water heater or 300 litre residential heat pump hot water heater.
For high-demand domestic or commercial applications, our larger options are the400 litre commercial heat pump hot water heaterand 500 litre commercial heat pump hot water heater.
Does the storage temperature affect cylinder size?
Yes.
Hotter stored water can be mixed with a greater proportion of cold water, increasing the amount of usable hot water available from the cylinder.
For example, water stored at 60°C can provide more mixed water at a 40°C shower temperature than the same cylinder stored at 50°C.
However, increasing the storage temperature is not necessarily the best way to compensate for an undersized cylinder.
A heat pump generally achieves a better Coefficient of Performance (COP) at lower water temperatures. Higher storage temperatures can also increase standing heat losses.
The cylinder size and operating temperature should therefore be considered together.
Why is recovery time important?
Cylinder capacity only tells you how much water is stored. It does not tell you how quickly that water can be reheated.
This is particularly important with a heat pump water heater because its heating output may be lower than the output available from a conventional boiler or large immersion heater.
A 200 litre unit with fast recovery could suit an application better than a larger cylinder with a low heating capacity.
When comparing models, look at both the cylinder volume and the manufacturer’s hot water recovery performance at realistic air and water temperatures.
Is it better to oversize a heat pump water heater?
A small amount of additional capacity can provide a useful margin for unusually high demand, but simply buying the largest cylinder available is not necessarily the best approach.
An oversized cylinder costs more, occupies more space and heats a larger volume of water. It may also have greater total standing heat loss.
An undersized cylinder creates the opposite problem. It may regularly run short of hot water, requiring additional recovery cycles or frequent use of the electric immersion heater.
Correct sizing sits between these two extremes.
What about larger 400 and 500 litre heat pump water heaters?
Larger 400 and 500 litre air source heat pump water heaters can be suitable where domestic hot water demand is considerably higher.
Applications could include large houses, multiple bathrooms, guest accommodation and certain commercial premises.
At these capacities, sizing should be based on a proper assessment of peak hot water demand, recovery time and usage patterns rather than simply the number of occupants.
How do I choose the correct size?
Start with the amount of hot water that will actually be used during the busiest period of the day, then check whether the heat pump can recover the cylinder before the next major period of demand.
Consider cylinder capacity, shower flow rates, bath volumes, stored water temperature, incoming mains temperature, heat pump output and recovery time together.
For an air source heat pump water heater, choosing the correct size is not simply about having enough litres in the cylinder. It is about having enough usable hot water when you need it, while allowing the heat pump to provide that hot water efficiently without regularly relying on the immersion heater.