Air Source Heat Pumps: A Beginner's Guide

Air source heat pumps have caught the attention of more homeowners in recent months, with media headlines suggesting the government may look to ban the installation of gas boilers in new builds from 2025. The renewable technology has been heralded as a viable alternative.

What’s more, while the Feed-in Tariff scheme is no longer offered to homeowners installing new electricity-generating renewable technology, the Renewable Heat Incentive is still alive and well. Under the scheme, those with renewable heating technologies are paid back for the heat they generate.

The Renewable Heat Incentive, which sees households with qualifying heat-generating renewables such as air source heat pumps receive payment for a period of seven years, is a definitive bonus for those considering investing in the technology.

Other benefits include the comparatively low running costs and the fact that air source heat pumps work well with underfloor heating.

However, air source heat pumps are not a ‘one-size-fits-all’ solution and arguably not suitable for every home. 

It is essential to design and specify the system correctly and ideally the insulation, airtightness and emitters (typically underfloor heating) of the property are optimised, to allow you to get the most out of your air source heat pump.

Here, we explain what they are and what you need to consider before investing…

What is an Air Source Heat Pump?

Although an air source heat pump does physically replace a boiler as the heat source, its operational characteristics are very different. Key characteristics include:

  • An air source heat pump does not create heat — it simply moves it from one place to another through the vapour compression cycle (or refrigeration process) to make it into a more useable form. Heat from the air gets absorbed into a fluid, which is then compressed, raising its temperature. The higher temperature is then transferred into the heating system.
  • They rely on electricity to power the pump. The efficiency, or the measure of the heat energy output per kW of electricity, is stated as the SCOP (Seasonal Coefficient of Performance). As an example, a SCOP of 3.2 means that for every 1kW of electricity, 3.2kW is generated.
  • The air source heat pump gets its energy from the surrounding air, so as the ambient temperature drops, so does the efficiency. The bigger the difference between the outside air and the target temperature (either the indoor room temperature or domestic hot water), the lower the efficiency. It is therefore key to have a good understanding of the heat load of the property and the performance characteristics of the heat pump.
  • When it comes to space heating, air source heat pumps work best with underfloor heating, but low-flow temperature radiators will work as well.

How Much Does an Air Source Heat Pump Cost?

Expect to pay around £11,000 for a good quality system (including installation). Air source heat pumps can cost upwards of £7,000, however.

How do the Costs Compare?

  • Combi-boiler: £1,500–£4,000
  • Ground source heat pump: £13,000–£20,000
  • Biomass boiler: £14,000–£19,000

The real cost comparison comes when you compare running costs, which are largely going to be based on the heat demand of your home:

  • For a four bedroom home built to Building Regulations’ standard, expect running costs of £703 per year for an air source heat pump, based on a SCOP of 3.2.
  • A newish gas boiler, with an efficiency of 90%, will cost around £900 a year to run.

The above does not take into account payments you will receive under the Renewable Heat Incentive when investing in an air source heat pump. The current payment (until 30 September 2019) is 10.71p/kWh.

How do Air Source Heat Pumps Compare with Ground Source Heat Pumps?

There are a number of key differences between the two:

  • The efficiency: SCOP (Seasonal Coefficient of Performance) is the measure of efficiency. A typical SCOP figure for an air source heat pump might be 3.2; the comparable ratio for ground source heat pumps is more like 4, so for every 1kW of electricity, 4kW is generated. Thus ground source heat pumps are slightly more efficient.
  • The installation: Ground source heat pumps require extensive ground in order to be installed, or are installed within deep boreholes. Both types of installation result in excavation costs. Air source heat pumps are not installed in this way — instead the external condenser unit sits in a box on the outside wall – and are cheaper to install as a result.
  • The incentives: The Renewable Heat Incentive offers a more generous return for owners of domestic ground source heat pumps — 20.89p as compared with 10.71p for domestic air source heat pumps.

Can an Air Source Heat Pump Provide Both Heating and Hot Water?

One of the first decisions to make when buying an air source heat pump is whether it will provide space heating or domestic hot water — or both.

The key here is that the flow temperature is different:

  • Space heating will usually require a flow temperature of around 35°C to 45°C (for underfloor heating or low-temperature radiators).
  • Domestic hot water will, however, require an absolute minimum flow temperature of 55°C.

On new homes that meet recent and current Building Regulations most air source heat pumps can do both, but this is not always the case.

Using Two Heat Pumps

Another solution is to use two heat pumps: one that is optimised for the space heating and another for domestic hot water.

The advantages of using the two heat pumps is that each unit is optimised for the required flow temperature and there is no priority system that causes the space heating circuit to ‘cool’ while the domestic hot water is being reheated.

The domestic hot water heat pump typically uses a different refrigerant that can produce higher flow temperatures but conversely also requires a higher source temperature (above 4°C) to be efficient.

It tends to be lot smaller than a space heating heat pump and usually built into the hot water cylinder. It draws its air either from the room it is in or from the exhaust waste heat of a ducted mechanical ventilation system — hence the collective term ‘exhaust heat pump’ or ‘micro heat pump’. Examples available include:

Thermodynamic Systems Explained

If you do not have a ducted ventilation system and don’t want to draw heat from inside the property, you could consider a different type of ‘micro’ heat pump — a thermodynamic system for instance.

It is in effect an R134a air source heat pump with an outdoor panel evaporator, and if it is designed and applied correctly there is no reason why it should not work efficiently.

The outdoor panel contains refrigerant and relies on ambient temperature and solar incidence as a heat source. The panel is often mounted on a roof but can be wall-mounted. Bear in mind that it needs good exposure to sunlight and moving air, so it should not be tucked away behind the garage or shed.

The micro heat pump only draws about 400W (watts) of electricity, and produces around 1,500W of heat, so if you have photovoltaic panels fitted to the property, the micro heat pump will also be optimised to use the on-house generation and possibly heat your water for free.

Higher Temperature Heat Pumps

In order to achieve higher temperatures, some manufacturers have built the two different refrigerant systems (R410a and R134a) into one heat pump in a ‘cascade’ system that can create flow temperatures of up to 80°C.

These systems (such as the Daikin Altherma) are designed for hot water and should not be used as a high temperature boiler replacement unless the lower efficiency has been carefully calculated to make certain that it is the best option for the property.

In the pursuit of higher temperatures and better efficiencies there are also a number of other new technological advancements that are worth noting. Compressors have been developed that effectively allow the compressed vapour to be re-injected into the compressor to enhance the temperature. These systems are capable of getting flow temperatures of around 65°C and have the ability to modulate through the inverter compressor technology.

The advantage of this system is that it reduces the complexity of the heat pump and therefore the cost. The operating pressures put a larger load on the compressor and push the tolerance of the refrigerant — examples include the Dimplex A-class ASHP and Mitsubishi Electric Ecodan.

Choosing Smart Controls for Your Air Source Heat Pump

Modern air source heat pump heating systems require specialist design and commissioning to achieve and maintain efficiency. In the age of the ‘app’ and smart heating controls, these systems can easily be tampered with, resulting in lower efficiency and high running costs.

Some manufacturers have therefore developed controls that can be monitored and maintained remotely. This is especially useful in second homes and rental properties, as well as for those occupants with a healthy fear of technology, as the systems can be reset and adjusted without someone coming out to the property.

In the event of a breakdown, the system can be checked, faults diagnosed and the correct spares sourced before incurring the expense of going to site. The engineering accessibility is often an after-sales add-on product, so check costs and requirements before ordering.

In the absence of full remote control and monitoring it is worth trying to find a controller that stores the operating data on a memory card so that it can be accessed for analysis and perhaps new settings emailed to you for upload.

Useful Air Source Heat Pump Contacts

About the Authors: David Hilton and Tim Pullen

David Hilton is an expert in sustainable building and energy efficiency and is a director of Heat and Energy Ltd.

With thanks to Tim Pullen. Tim Pullen is Homebuilding and Renovating’s expert in sustainable building and energy efficiency. He is the author of Simply Sustainable Homes.

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