Air to air heat pump buying guide

Air to air heat pumps are installed as compact outdoor units containing a fan, evaporator, compressor and condenser. They are connected to indoor air handling units which distribute the heat inside the home.

How does an air to air heat pump work?

Air to air heat pumps work in five stages to extract heat from the air and distribute it around the home.

1. To start, the outdoor fan-fitted unit draws in air from outside.

2. This air is sent towards an evaporator which contains a liquid refrigerant. The heat from the air is gathered by the refrigerant. This process increases the temperature of the liquid and transforms it into gas.

3. The gas is then drawn into a compressor, which is powered by an electric motor, where it is compressed causing the temperature to increase again.

4. The gas then passes into a heat exchanger where it is condensed in order to transfer the heat indoors. The refrigerant gas then condenses back into liquid.

5. When this happens, the refrigerant passes through an expansion valve which lowers the pressure of the liquid back to its original state before the process starts again.

There are many reasons to invest in an air source heat pump but it is also important to be aware of the limitations of these systems.

Easy to install in old buildings as well as new builds, air to air heat pumps reduce the environmental impact of heating and can be adjusted to meet the heating requirements of each room. Filters can be installed to purify the warm air entering the home. These heat pumps can also be used as air conditioners when the weather heats up.

On the downside, beware of potential efficiency losses when temperatures drop below a certain level and the noise level of the heat pump, both of which can vary depending on the model. Finally, it's worth noting that installing the indoor system can be tricky and costly.

Air to air heat pumps can only distribute heat through a blower system either through ducts or conduits.

Ducted systems

In this case, each room of the house is fitted with a small indoor unit. Warm air is created by the heat pump and set out through a series of air ducts through to the individual air handling units.

The price of this type of installation depends on the layout of your home. You will therefore have to add this cost on to the price of the air to air heat pump itself, unless you already have a duct system in place. The indoor heads are powered by electricity and fitted with filters and thermostats which can be used to control the temperature and fan speed in each room.

The indoor heads can be fitted to the wall like traditional radiators or can be built into walls or ceilings. Like radiators, the heads need to be chosen to match the volume of the room and the type of insulation in place.

Ductless systems

One indoor unit is installed (in a boiler room or utility room, for example) to send out warm air through a series of conduits which run to every room that needs to be heated.

While only the wall ventilation grilles will be visible for this type of system, installation is tricker and this will need to be factored into your budget.

There are a few factors to take into account when working out your heating requirements.

Coefficient of performance (COP)

The COP of a heat pump refers to the difference between the energy input and heat output of the device. For an air to air heat pump, this usually sits around 3 to 4. A COP of 3 kW means that the heat pump releases 3 kW for every 1 kW of input power. The higher the COP, the more efficient the heat pump.

However, to put the COP into context, it needs to be considered with the air temperature where the heat is captured. For example, a COP of 3 at 7°C means that when it is 7°C outside, the air to air heat pump releases 3 kW for every 1 kW consumed. But when the temperature drops, the COP will drop with it.

Heat output of an air to air heat pump

The heat output of an air to air heat pump is measured in kW and describes the unit's capacity to produce heat. To work out what kind of heat output you need, you need to consider three pieces of information:

• the volume of your home in m3;

• the building coefficient (from 1.6 for a very poorly insulated home to 0.75 for a very well insulated home);

• the difference between the indoor and outdoor temperature where you live.

You can then multiply these three figures (volume x building coefficient x temperature difference) to find out what kind of heat output you need, The heat output of air to air heat pumps is usually 5 to 20 kW.

But do not hesitate to seek professional advice to help you make the right choice. A heat pump with an unnecessarily high output will be more expensive to buy and will provide the same performance as a heat pump with the right output. What's more, a heat pump that is too big will use up more energy stopping and starting more frequently causing its parts to wear more quickly. On the other hand, if you go for a heat output that is too low, the heat pump won't produce enough heat and you will have to rely on an additional heater.

Heat pump inverters

Inverter technology can be used to run the compressor of your air to air heat pump continuously without stopping and starting. This technology means that the motor is able to deliver a greater range of speeds to regulate heat production.

In turn, this helps to reduce power consumption and provide greater comfort indoors as the unit is able to maintain a constant temperature. The heat pump will also have a longer service life and, what's more, inverter technology won't affect the price of the unit.

Reversible heat pumps

Reversible air to air heat pumps can also work as air conditioners to cool down your home in summer.

A range of standards are in place to guarantee performance while grants are available to help you with the costs of installation.

Heat pump grants

A few different grants are available across the UK to help you install a domestic heat pump including Domestic RHI, Boiler Upgrade Scheme, Warmer Homes Scotland, Home Energy Scotland Loan and Nest Scheme, Wales. These grants are designed to help you cover the costs of setting up and running a heat pump.

CE certification

The CE marking shows compliance with key EU safety and quality standards.

Microgeneration Certification Scheme (MCS)

The Microgeneration Certification Scheme (MCS) sets out industry quality standards for heat pumps in the UK.

EN 14511-2

The European standard EN 14511-2 sets out the test conditions to determine performance qualities of air to air heat pumps.

Heat Pump Association

The UK Heat Pump Association is a trade association that represents manufacturers and distributors of heat pumps in the UK as well as heat pump component manufacturers. HPA can provide all further information on the uses and benefits of heat pumps.

1. Annual maintenance, performed by a certified professional, is required to maintain warranties and keep the heat pump compliant with requirements set out by the various grants.

2. The exterior unit must be cleared of any objects that could interfere with operation such as ivy, branches and so on.

3. The indoor air handling units must be maintained regularly and the filters must be cleaned or replaced as needed.

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