Air Source Heat Pump Water Heater Sizing Guide

Correctly sizing an air source heat pump water heater is important because the unit must provide enough domestic hot water (DHW) for the property while still allowing the heat pump to operate efficiently. Unlike a conventional electric immersion heater or gas-fired water heater, a heat pump water heater generally has a lower instantaneous heating output and relies heavily on stored hot water capacity, recovery time and efficient heat transfer.

The correct size, therefore, depends on more than simply the number of occupants. You should consider hot water demand, cylinder volume, peak usage, recovery time, heat pump output, incoming mains water temperature, storage temperature and the number of baths or showers used during busy periods.

What Size Air Source Heat Pump Water Heater Do I Need?

As an initial residential sizing guide:

 

These figures should only be treated as a starting point.

For those that want to jump straight to our products, here are the webpages in question; otherwise, please read on

Our smallest unit, suitable for flats and smaller properties, is a 100-litre model detailed here – 100 litre wall mounted heat pump water heater

Then our 200L floor-mounted ASHPWH is detailed here – 200 Litre Air source heat pump water heater 

Our 300L floor-mounted ASHPWH is detailed here – 300 Litre Air source heat pump water heater 

Our 400L floor-mounted ASHPWH is detailed here – 400 Litre Air source heat pump water heater 

And lastly, our 500L floor-mounted ASHPWH is detailed here – 500 Litre Air source heat pump water heater 

A household with several showers spread throughout the day may require considerably less stored water than a property where several people shower consecutively each morning. Properties with large baths, rainfall or drench showers or unusually high domestic hot water consumption may need greater storage capacity.

Size the Water Heater for Peak Hot Water Demand

One of the most important considerations is peak DHW demand rather than simply total daily water consumption.

For example, a household may use 180 litres of hot water during an entire day, but if 120 litres are required within one hour in the morning, the water heater needs sufficient stored hot water to satisfy that demand.

This is particularly important with heat pump water heaters because the compressor cannot normally replace hot water as quickly as a high-output gas boiler or water heater

Typical peak-demand appliances include:

* Showers
* Rainfall showers
* Baths
* Kitchen sinks
* Utility sinks
* Multiple bathrooms operating simultaneously

The larger the short-term demand, the greater the importance of hot water storage capacity.

Allow for Shower and Bath Usage

A normal shower might consume approximately 7 to 10 litres of mixed water per minute.

A higher-flow shower can use more than 12 litres of water per minute.

For example, an 8-minute shower operating at 9 litres per minute uses:

8 × 9 = 72 litres of mixed water

However, not all 72 litres comes from the hot water cylinder. Cold mains water is blended with stored hot water through the shower mixer.

A bath can require approximately 80 to 150 litres of mixed water, depending on the bath size and filling level.

Homes that take multiple baths consecutively therefore usually require significantly more stored water than properties that mainly use efficient showers.

Stored Hot Water Temperature Affects Usable Capacity

The amount of usable hot water available from a cylinder depends upon its storage temperature.

For example, water stored at 55°C can be blended with cold water to produce considerably more water at a comfortable shower temperature of around 38°C to 42°C.

This is known as the mixed water volume.

A simplified calculation is:

Usable mixed water = cylinder volume × (storage temperature − cold water temperature) ÷ (outlet temperature − cold water temperature)

For example, consider the following:

* 200 litre cylinder
* 55°C stored water temperature
* 10°C incoming cold water temperature
* 40°C shower temperature

The theoretical mixed water volume is:

200 × (55 − 10) ÷ (40 − 10)

= 300 litres of mixed water

In practice, usable volume will be affected by stratification, heat loss, control settings, cylinder design and the minimum useful outlet temperature.

Heat Pump Output Is Just as Important as Cylinder Size

An air source heat pump water heater uses a refrigeration circuit containing:

Evaporator → compressor → condenser → expansion device → evaporator

The heat pump extracts thermal energy from ambient or ducted air and transfers that energy through the condenser into the stored domestic water.

The thermal output of the heat pump determines how quickly the cylinder can recover after hot water has been used.

A larger cylinder provides more stored energy, but if the heat pump compressor is too small for the demand, the system may take too long to reheat.

How to Calculate the Energy Required to Heat the Water

A useful calculation is:

Energy required (kWh) = litres × temperature rise × 0.001163

For example, heating 200 litres of water from 10°C to 55°C requires:

Temperature rise:

55 − 10 = 45°C

Energy:

200 × 45 × 0.001163 = 10.47 kWh

Approximately 10.5 kWh of thermal energy is therefore required.

If the heat pump produces 2.5 kW of thermal output:

10.47 ÷ 2.5 = 4.19 hours

The theoretical recovery time would therefore be approximately 4.2 hours.

Actual recovery times can be longer because heat pump capacity changes with ambient air temperature, water temperature, refrigerant operating conditions and system losses.

Understanding COP When Sizing a Heat Pump Water Heater

The Coefficient of Performance (COP) describes how much useful heat the heat pump produces compared with the electrical energy consumed.

For example, a COP of 3 means that approximately:

1 kWh electricity → 3 kWh thermal energy

If the cylinder requires 10.5 kWh of heat and the heat pump operates at a COP of 3:

10.5 ÷ 3 = 3.5 kWh of electricity would theoretically be required.

However, COP should not be confused with heating capacity.

A water heater may have a high COP but still have a relatively modest compressor output. Both energy efficiency and recovery performance therefore need to be considered.

Ambient Air Temperature Affects Performance

The air temperature entering the evaporator affects both heating output and COP.

Warmer air generally allows the refrigeration circuit to extract heat more easily.

As the incoming air temperature decreases:

* COP can fall
* Heating capacity may decrease.
* Recovery time can increase.
* Compressor operating time may increase.

This is why manufacturers normally provide performance data at specified air inlet and water temperatures.

When comparing air source heat pump water heaters, always compare performance under similar test conditions.

Consider Where the Heat Pump Water Heater Will Be Installed

Installation location can influence sizing and performance.

Units may use:

* Internal ambient air
* External air
* Ducted outdoor air
* Extract air
* A combination of intake and exhaust ducting

Potential installation locations include:

* Utility rooms
* Garages
* Plant rooms
* Service cupboards
* Commercial plant areas

The installer must consider the manufacturer’s required air volume, airflow rate, duct dimensions and minimum room volume.

Heat pump water heaters also produce condensate, so a suitable condensate drain is normally required.

Allow for Recovery Between Hot Water Demand Periods

A properly sized system should consider when hot water is actually used.

For example:

Morning: four showers
Daytime: very little demand
Evening: showers, bath and kitchen use

The heat pump may have several hours during the daytime to recover the cylinder.

In this situation, a slightly smaller cylinder could potentially satisfy demand because the compressor has time to reheat the stored water.

Where demand occurs continuously, a larger cylinder or greater heat pump output may be required.

Do Not Size the System Only by Number of Bedrooms

Bedroom count is sometimes used as a quick guide, but occupancy provides a better indication of domestic hot water demand.

A four-bedroom house occupied by two people could use considerably less hot water than a three-bedroom property occupied by five people.

Sizing should therefore consider:

occupants + bathrooms + showers + baths + flow rates + usage patterns

rather than bedroom count alone.

High Hot Water Demand Properties

Additional storage capacity should be considered where a property has:

* Large baths
* Multiple bathrooms
* Rainfall showers
* High-flow shower heads
* Consecutive shower use
* Large families
* Frequent visitors
* High kitchen hot water demand

In these circumstances, moving from a 200 litre cylinder to a 300 litre heat pump water heater may provide significantly better comfort.

Immersion Heater Backup

Many air source heat pump water heaters incorporate an electric immersion heater.

The immersion heater can serve several purposes, including:

* Backup heating
* Faster recovery
* Raising water to higher temperatures
* Anti-Legionella cycles
* Emergency operation if the compressor is unavailable

However, immersion heating normally has a COP of approximately 1, whereas the heat pump may achieve substantially higher efficiency.

A correctly sized system should therefore minimise unnecessary reliance on the immersion heater during normal operation.

Legionella and Hot Water Temperature

Stored domestic hot water temperature also needs to be considered from a water hygiene perspective.

Many heat pump water heaters include an automated anti-Legionella or thermal disinfection cycle, during which the cylinder temperature is periodically increased.

Depending on the appliance, the compressor may achieve part or all of this temperature increase, while an immersion heater provides additional heat where necessary.

Always follow the manufacturer’s instructions and applicable UK water hygiene requirements.

Residential Air Source Heat Pump Water Heater Sizing Example

Consider a household with:

* Four occupants
* Two bathrooms
* One bath
* Two showers
* Normal shower flow rates
* Most demand occurs in the morning and evening.

A 200 to 250 litre air source heat pump water heater would typically be considered an initial sizing range.

If the property regularly has four consecutive showers followed by a bath, a 300 litre model may be more appropriate.

The final selection should then be checked against:

* Heat pump thermal output
* Recovery time
* COP
* Maximum water temperature
* Ambient operating temperature
* Immersion heater capacity
* Available air volume
* Ducting requirements

Commercial Air Source Heat Pump Water Heater Sizing

Commercial systems should not normally be sized using a simple litres-per-person rule.

Applications such as:

* Hotels
* Care homes
* Gyms
* Leisure centres
* Changing rooms
* Restaurants
* Offices
* Student accommodation
* HMOs

will have very different hot water demand profiles.

Commercial sizing should establish:

1. Total daily DHW demand
2. Maximum hourly demand
3. Peak simultaneous demand
4. Required storage capacity
5. Hot water storage temperature
6. Cold water inlet temperature
7. Heat pump recovery output
8. Available recovery period
9. Required redundancy
10. Number of heat pump water heaters

Larger applications can use several commercial air source heat pump water heaters operating in parallel to provide greater storage capacity and increased recovery output.

Our commercial air source heat pump domestic hot water heater range is detailed here: Commercial air source heat pump water heaters

Oversizing an Air Source Heat Pump Water Heater

Bigger is not always better.

An excessively large cylinder volume can result in:

* Greater standing heat losses
* More stored water than necessary
* Increased capital cost
* Greater installation space requirements
* Longer full-cylinder heating times

The objective should be to provide sufficient storage for peak demand while allowing the heat pump compressor to recover the cylinder efficiently between demand periods.

Undersizing a Heat Pump Water Heater

An undersized system can cause:

* Running out of hot water
* Excessive immersion heater operation
* Long recovery periods
* Poor user comfort
* Continuous compressor operation
* Higher electricity consumption

This factor is particularly important because heat pump water heaters normally recover more slowly than conventional high-output fossil-fuel water heaters.

Key Information Required Before Selecting a Heat Pump Water Heater

For accurate sizing, establish:

* Number of occupants
* Number of showers
* Shower flow rates
* Number and size of baths
* Peak simultaneous demand
* Daily hot water usage
* Required storage temperature
* Incoming cold water temperature
* Heat pump thermal output
* Cylinder storage volume
* Recovery time
* COP
* Ambient operating temperature
* Available installation space
* Air intake and exhaust requirements
* Electrical supply
* Immersion heater capacity

The Most Important Sizing Principle

The correct air source heat pump water heater size is the combination of sufficient stored hot water capacity and sufficient heat pump recovery output to meet the property’s peak domestic hot water demand.

For residential properties, occupant numbers provide a useful starting point, but shower flow rates, bath usage, storage temperature and recovery time should determine the final size of the air source heat pump volume required.

For commercial applications, sizing should be based on a calculated DHW demand profile, peak hourly consumption, storage capacity and heat pump recovery rate rather than a simple litres-per-person allowance.