Main Types Of Air Source Heat Pumps Explained

Are you considering a switch to an air source heat pump? 

First of all, good call.

But I understand that discovering all the different heat pump types can be a little confusing. Especially if it’s a new type of heating system for you. 

This post will explain the two main types, as well as the pros and cons of air source heat pumps for each type to help you make an informed decision. 

Different types of air source heat pumps explained

When it comes to decarbonising the heating grid and providing the most energy efficient and cost efficient heat to your home, heat pumps are going to be integral. The holy grail of home heating will be heat pumps, solar panels and excellent insulation.

There are two main types of air source heat pump: 

• Air to air heat pump
• Air to water heat pump

On a wider view, you can also get hybrid heat pumps that slot in with a traditional boiler system and ground source heat pumps that are situated underground. But for this article, we’ll consider the main types of air source heat pumps.

Most popular heat pump types in the UK?

According to a 2020 public survey 60% of people were aware of ground source heat pumps and 57% were aware of air source heat pumps. 

Even though more people have heard of ground source heat pumps, air source pumps are by far the most popular type of heat pump in the UK. Roughly, for every ground source heat pump, there are six air source heat pumps installed in the UK

Although the two types of air source heat pump generate heat in the same way, the way this heat is released into your home differs. So, it’s important you get the right type installed and of course the right size air source heat pump for your home.

The most popular type for homeowners looking to replace their central heating system is air to water heat pumps. If you’re new to all of this, take a look at my guide to air source heat pumps. For now, let’s take a closer look at the two main types of air source heat pumps.

Air To Air Heat Pumps

What are air to air heat pumps?

An air to air heat pump is a type of air source heat pump that uses heat in the outside air as a source for an internal heating system. These systems are versatile and a single heat pump unit can provide both heating and background cooling.

What type of heat do they provide?

Air to air heat pumps only provide space heating. They can blow hot air into a room but they are not compatible with a traditional radiator system, which is water-based. Each room in an air to air system will have to have a unit to provide heating.

How do air to air systems work?

To provide heating, an air to air heat pump acts as an air conditioning unit in reverse. You can also liken them to a kitchen fridge in reverse. 

Air to air heat pumps have a unit outside that draws air in. This air passes over a heat exchanger, which extracts all the latent heat. Yes, even cold air contains some heat. The heat is absorbed by a refrigerant gas that circulates through the pipework. 

A second unit helps to warm the air further, which can then be transferred around your home via ducts (ducted system) or via pipework (ductless system) and released into your rooms. This can all be carefully controlled and monitored with a thermostat. 

Air To Water Heat Pumps

Air to water heat pumps transfer heat from the outdoor air to water, which can be used to provide hot water and heating to a building. They are often used in combination with underfloor heating or radiators. 

Air to water heat pumps are typically more efficient than air to air heat pumps, but they can be more expensive to install. They also operate at lower temperatures than traditional gas boiler systems which is why existing radiators often need to be upgraded (but not always). If you have large radiators, they might already be compatible. 

What are air to water heat pumps?

Like air to air systems, air to water heat pumps also extract heat from outside air to provide the basis for internal heating. Unlike air to air systems, air to water heat pumps can provide hot water to your taps, as well as mains heating via radiators. 

What type of heat do they provide?

Air to water systems can heat both space and water. This makes them much more like a traditional combi boiler system and are a great like-for-like swap when thinking about the future. 

How do the air to water heat pump systems work?

An air to water system is made up of an indoor and outdoor unit. The outdoor unit has a fan, heat exchanger and circulating refrigerant to absorb the heat. The refrigerant then gets compressed which increases its temperature.

The indoor unit is made up of a hot water cylinder, a circulation pump and the main controls. Once compressed, the refrigerant travels to the indoor unit, enters a second heat exchanger and heats up the water in the cylinder. The heated water can then be circulated through a ‘wet’ radiator system in your home. 

With underfloor heating systems, it’s possible to have a smart thermostat in each room to carefully control the temperature.

Air to water systems can provide cooling to some extent but is only really compatible in homes with underfloor heating. When in cooling mode, cooler water is run through the system, which helps extract thermal mass from the internal to provide a cooler environment. The warm energy is then released into the outside air. Cold water can’t be run through the radiator system as you’ll get condensation.

It’s also different from air conditioning, which is actively blowing cold air in. Heat pumps don’t do this. 

Pros and cons of the different types of air source heat pumps

Ok, so you know about the two main types of air source heat pumps. Now, let’s look at the main pros and cons for air to air and air to water heat pumps.

Pros and cons of air to air heat pumps

5 benefits to air to air heat pumps

1. Energy efficient – You can get 300-450% more energy out of a heat pump than the electrical energy that goes into running it.

For example, for every 1kW of electricity it consumes, the heat pump will produce 3-4kW of heat. Compare this to a maximum efficiency of 90% for a traditional combi boiler. Each heat pump will have published efficiency data known as Coefficient of Performance (CoP). Heat pumps can achieve this efficiency because they utilise the heat energy from the air outside.

2. Zero emissions – This renewable energy technology has zero emissions at the point of use.

3. Suitable for most properties – Can be easily installed in new homes or retrofitted to existing homes.

4. Space-saver – Can be wall-mounted to save space.

5. Long service – Heat pumps have double the life of traditional boilers.

5 disadvantages of air to air heat pumps

1. Upfront cost – Air to air heat pumps do require a high initial expenditure. They aren’t as expensive as air to water heat pumps though. Air to air heat pumps don’t qualify for the Boiler Upgrade Scheme.

2. Yearly maintenance – Generally heat pumps require little maintenance as there are few moving parts. However, it’s good practice to book in a yearly service.

3. Noise – Often touted as a disadvantage of heat pumps. Yes they do produce a low hum from the fans. This is generally between 40-60 decibels, which shouldn’t be a problem with the unit located outside. 

4. Insulated home – Your home has to be well insulated to install an air to air heat pump. This is because they operate at slightly lower temperatures. 

5. Electricity usage – If you swap a traditional boiler for an air to air heat pump your electricity bill will increase. The benefit is that your gas use will dramatically decrease. You will still have to run a separate system for your hot water.

Pros and cons of air to water heat pumps

5 benefits of air to water heat pumps

1. One system for heating and hot water – An air to water heat pump will support both your heating and hot water needs. If you’re upgrading your central heating system, your existing radiators and underfloor heating may already be compatible. 

2. Lower carbon footprint – Like air to air systems, air to water heat pumps are zero emissions at the point of use.

3. Energy efficient – Use outside air to heat water which is then pumped around your home. Air to water heat pumps can provide 4 times the amount of heat per unit of electricity it consumes.

4. Operate in cold weather – Can provide heating when the outside temperature drops to -15^oC, provided the house is well-insulated.

5. Long lifespan – Should last for 20 years or more with regular maintenance.

5 disadvantages to air to water heat pumps

1. Installation challenges – More difficult and complex to install than air to air heat pumps. 

2. May need to change radiators – This is not a given but you may have to install different sized radiators to meet the needs of the heat pump and your heating requirements. 

3. High upfront costs – There will be a higher initial expenditure. A bit like solar panels, you will make this back in energy savings over time. Heat pumps are coming down in price though as they become a more popular choice for homeowners. Government Boiler Upgrade grants are available for air to water systems only.

4. Regular maintenance – Just like a combi boiler, it’s best to service your air to water heat pump regularly. 

5. Require more space – For an air to water system, you’ll need a large outside unit (large washing machine to fridge size) and an internal hot water cylinder. They’re not as big as you think though. You can find more information on air source heat pump dimensions here.

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