What Size Air Source Heat Pump You Need (Real Example)

An air source heat pump can be an energy efficient, renewable way to heat your home, and generating your own sustainable heat is a great way to ensure you’re comfortable in your home all year round and help save money on energy bills.

Air source heat pumps must be sized appropriately for the space that they need to heat, as well as have the output capacity for anticipated demand for heating, cooling, and hot water if also required.

The general rule of thumb for sizing an air source heat pump is 1kW for every 10 square meters, or 30BTUs for every 1sqft. When determining the size of an air source heat pump, consider the number of rooms and windows, the layout and size of house, the climate and level of insulation.

Our 11.2kW air source heat pump was sized sufficiently to provide heating and hot water to our home.

Air Source Heat Pump
The output capacity of our air source heat, which determined its size, was worked out by an installer using the correct software with a range of variables such as internal floor space and local climate

Ensuring that the right size air source heat pump is installed essential, as using the wrong size heat pump may lead to extra energy bills or repair costs.

In this article we discuss and explain more about our ASHP, how an air source heat pump works, what sizes are typically available, what can occur if you install the wrong size, and how to estimate the size and capacity of your own air source heat pump.

How Big Should My Air Source Heat Pump Be?

Air source heat pumps (ASHP) generate heat at lower, more consistent temperatures than the average boiler.

They can help you save some extra money on your energy bills, while giving you an economically friendly, sustainable way to generate heat and hot water for your home.

An air source heat pump works by extracting heat energy stored naturally within the outside air, raising the pressure and temperature using a compressor, and circulating the heat around your home, whether that’s through hot air using an air to air heat pump or hot water using an air to water heat pump.

Air source heat pumps use electricity as the sole source of power for components such as the fans and compressor.

Air Source Heat Pump
Greater capacity heat pumps can have bigger, or a greater number of, fans to maximize airflow through the unit to capture heat energy from the air

A hot water cylinder can be used to reserve any hot water, ready to meet demand.

Air Source Heat Pump Tank
A hot water tank for use with an ASHP will also be sized appropriately

Similar in appearance to your typical air conditioning unit, air source heat pumps can also generate cooler air in the warmer months.

Typically, the larger the area required for heating, such as floor space inside a home, the greater the output capacity is required from an air source heat pump.

Higher output capacity ASHPs, which can extract more heat from the air to meet a higher demand for heating and/or hot water, are generally larger in size compared to lower capacity heat pumps.

Our air source heat pump is sized appropriately for our 5-bedroom house, where the heat pump must provide both heating and hot water for our home.

We don’t use it for cooling as we have separate air conditioning units for that.

Air Source Heat Pump
Our air source heat pump (right) is larger than one of our air con units (left), albeit the heat pump is providing both heating and hot water for our whole house while the air con unit is providing cooling for one room only

This is an 11.2kW maximum output capacity appliance for heating, as shown on the information plate on the side of the heat pump.

Air Source Heat Pump Plate
For our 5-bedroom house, the installer deemed that this 11.2kW max heating capacity heat pump would be sufficient

An installer chose this heat pump based on the level of demand required in our home, and the floor space, along with other factors.

The size of the air source heat pump is a result of requiring the right output capacity.

The dimensions of our ASHP are shown below for reference.

As an example, the dimensions of our 11.2kW air source heat pump (112 model) shown for reference

An air source heat pump should therefore be sized as big or small as it needs to be.

The demand for heating and/or hot water, among other factors such as local climate, will dictate the output capacity required.

This in turn can affect the size of heat pump required, and sizes and shapes can differ between manufacturers, models and output capacities.

Sizing Air Source Heat Pumps

While an air source heat pump size usually depends on several variable factors, including property age and size, types of rooms, and the number of windows, there’s a universally accepted scale that you can use to estimate what size air source heat pump you might need.

House typeASHP recommended output (kW)
2 bed house / flat5
Poorly insulated 3 bed house9
Well insulated 4 bed house9

In the above table, you can see that the size varies based on the house or apartment size.

According to Unique Heating and Cooling, the rule of thumb in most states is one ton of air-conditioning capacity for every 500-600 square feet (46.45-55.74 square meter) of floor surface area.

The size of air source heat pump is usually broken down and estimated using the above-mentioned variables, and so every house will be slightly different.

Sizing the right air source heat pump for a home can be a lengthy and complex process, and so it’s always recommended to have a qualified professional design and install any air source heat pump system.

What Happens If I Get the Wrong Size Air Source Heat Pump?

If an air source heat pump unit is too big or too small for your home’s needs, then you might come across a few problems.

If an air source heat pump is too big, it can waste resources and leave you with a higher energy bill. An undersized air source heat pump can be overworked, also leading to higher electricity usage, and potentially increased deterioration of parts.

Ensuring you have the correct size air source heat pump can help to prevent problems occurring and help to maintain the long lifespan of your ASHP.

An installer will be able to determine the right size of air source heat pump for your home and particular requirements.

How To Calculate the Size Of An Air Source Heat Pump

When it comes to size, getting it right for an air source heat pump is important.

Unfortunately, you probably won’t get an exact number without proper software and the necessary knowledge to figure it out. HVAC installer can often use Manual J to determine what size is appropriate.

Getting in touch with a certified engineer is always helpful since they can give you honest advice based on the particulars of your home. Make sure you confirm their credentials before allowing them to install the unit and check that the heat pump is from a registered and reputable manufacturing company.

However, before contacting an expert, there are ways you can estimate the size of your heat pump, to help you prepare the location you wish to install it or bear it in mind as a future expense.

First, you will need to work out the size of your home in meters squared. You can use the blueprints of your house, or you can measure it manually. Ensure you also note the number of windows and their locations for future reference.

If you already have a heating appliance installed in your home, you can determine how many kilowatts are necessary to heat your entire house to your desired temperature.

Depending on the device you have, you can often read the meter directly on the tank, or you can contact the company who supplied it, who can tell you your monthly usage.

Most experts agree you need 1kW for every 10 meters (32.80 feet) squared, but the calculation is not limited to this, and you will need to take in other factors. For example, even the number of people who live in the house and their preferred temperature levels can affect the size of heat pump.

What Size Air Source Heat Pump Do I Need?

The required size of air source heat pump can be estimated to a degree, but there are a number of factors such as floor space, layout, demand and local climate that can all influence the heating or cooling output capacity of heat pump needed, and therefore its size.

If in doubt, get in touch with an accredited professional who is a member of a local recognized organization. They can help provide the right output capacity and size of heat pump for your home.

Further Reading

Can An Air Source Heat Pump Heat A Whole House?

Where To Install An Air Source Heat Pump

Air Source Heat Pump Installation Requirements

Air Source Heat Pump Enclosures

Can You Cover An Air Source Heat Pump?

Can An Air Source Heat Pump Be Wall Mounted?

How An Air Source Heat Pump Works

Parts Of An Air Source Heat Pump Explained