With the rise in heating fuel prices we have seen a steady increase in the adoption of air source heat pumps (ASHPs) as an economical alternative to conventional heat sources. Currently around 8,000 to 10,000 are installed in domestic homes in the UK each year.

The Renewable Heat Incentive – which sees households with qualifying renewables receive payment for a period of seven years – has also helped to bolster take up. As with all modern technologies, air source heat pumps are not a ‘one-size-fits-all’ solution and the advances in the technology need to be understood before rushing out and specifying one for your home.

ASHP technology has developed in recent years. Here, we explain the technology and explore some of the latest options currently available.

What is an Air Source Heat Pump?

Although the heat pump does physically replace a boiler as the heat source, its operational characteristics are very different and must be accommodated and understood. Keys points include:

  • An ASHP 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
  • Traditionally, a compressor will compress gas to achieve a higher temperature — the output temperature and the efficiency will depend on the amount of pressure and the type of gas
  • The most popular heat pumps currently use R410, a refrigerant gas that traditionally has an output flow temperature of around 55°C
  • Efficiency is stated as the SCOP (Seasonal Coefficient of Performance). A typical figure for an ASHP might be 3.2 (i.e. 1kW of electricity creates 3.2kW of heat); the comparable ratio for ground source heat pumps is more like 4, so for every 1kW of electricity, 4kW is generated)
  • The ASHP 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 (the source temperature) 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
  • For space heating, ASHPs work best with underfloor heating, but oversized or low-flow temperature radiators will work as well
earth save air-source heat pump on a self build

A 4kW Earth Save air-source heat pump was installed in this energy-efficient self build home on the Isle of Man

The Quick Read on Air Source Heat Pumps

  • Air source heat pumps are being developed to work with the most highly insulated new homes
  • Modern air source heat pumps need specialist design and commissioning in order to achieve maximum efficiency
  • Other innovations include thermodynamic systems, smart controls, and different pumps for hot water and space heating

Where to Start When Specifying an Air Source Heat Pump

In a boiler-fired central heating system we would expect to have a difference of around 50°C between the target room temperature and the water flow temperature. Therefore, if the room temperature is set at 20°C, we would expect the flow temperature to be at over 70°C on traditional radiators.

This is fine in a boiler-fired central heating system, but most heat pumps cannot get to those flow temperatures. It is essential to design and specify the system correctly and assuming that the insulation, airtightness and emitters (underfloor heating or radiators) of the property are optimised, we can then look at the air source heat pump options available.

  1. First, determine if the unit will be providing space heating and/or domestic hot water.
  2. Space heating will usually require a flow temperature of around 35°C to 45°C. Domestic hot water will, however, require an absolute minimum flow temperature of 55°C.
  3. On properties that meet recent and current Building Regulations most air source heat pumps can do both, but on higher performance properties the space-heating load will be very low and could be almost the same as, or less than, the domestic hot water demand.

How Much Does an ASHP Cost?

According to the Energy Saving Trust, an air source heat pump costs from £7,000–£11,000, including installation; expect to pay around £11,000 for a good quality system.

How do the Costs Compare?

  • Ground source heat pump: £13,000–£20,000
  • Combi-boiler: £1,500–£4,000
  • 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 ASHP, based on a SCOP of 3.2
  • A newish gas boiler, with an efficiency of 90%, will cost around £900 a year to run
Oxfordshire self build open plan living space

This award-winning self build home in Oxfordshire features an Ecocent hot water system and mechanical ventilation heat recovery system, as well as an air-source heat pump

Can an ASHP Deliver Both Heating and Hot Water?

As property efficiency improves, so does the requirement for better air source heat pump design and operation. The unit also needs to accommodate the changing seasons, so many modern air source heat pumps have the ability to modulate their output to match the heat load. They either use a variable speed compressor (known as an inverter compressor) or a series of two, or sometimes three, compressors that can ramp up or down as the demand varies.

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

The domestic hot water heat pump typically uses a different refrigerant (R134a) 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:

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. Thermodynamic systems have suffered from mixed reviews (perhaps because they were introduced as ‘solar panels that work at night’…) but the technology is sound.

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 or fixed to any structural element of the building.

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. In colder areas or where there is heavy snow, the design constraints will be significantly more. This technology can be used quite successfully for hot water preparation as a separate system from the space-heating unit.

The advantages of using the two heat pumps is that each unit is specifically 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.

It has the added advantage that the outdoor space-heating unit will not run during the warmer months when you are perhaps sitting outside close to the unit. 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.

off mains oak cottage self build in Worcestershire

£14,000 of the total £200,000 build budget for this oak frame home in an off-mains gas area was allocated to underfloor heating and an air-source heat pump

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.

Refrigerant Gases and the Environment

European directives on refrigerant gasses prohibit the use of products that have any ozone depletion potential (ODP) and there are ever-increasing restrictions on products that have any global warming potential (GWP). As a result, manufacturers of refrigerated heating and cooling appliances have done a lot of research into alternative refrigerant gases with low (or zero) GWP.

R290, or refrigerant quality propane, can achieve a flow temperature of around 65°C at good efficiency, but the units are more expensive as they require two compressors for any type of modulation. Propane is flammable and can combust under extreme pressure, so the air source heat pump unit should be a monoblock type that is located outside if you do not want to add ventilation airbricks to the property.

CO2 has been used successfully as a zero GWP refrigerant in commercial heat pumps and Sanyo had mixed success with a domestic-sized CO2 air source heat pump, but this was discontinued when Panasonic acquired Sanyo.

The CO2 heat pump operates at extreme pressures and produces high temperatures of around 80°C at high temperature difference between source temperature and target temperature. Due to the operating conditions, it needs to run consistently and not have frequent stop-start cycles.

ASHP Options for Highly Insulated Homes

Mitsubishi Electric’s 4kW CO2 air source heat pump tackles the needs of the highly insulated new build home; it matches the hot water requirements while still meeting the lower heating demand at an efficiency of 300 per cent.

The monoblock ASHP is plumbed to a bespoke thermal store that stores the water at around 70°C. Hot water is then delivered through an indirect heat exchanger at around 65°C, which eliminates the need for periodic legionella sanitation.

With a 4kW unit it is fairly easy to balance the hot water demand and the space-heating load. If the space-heating load increases but the hot water demand stays the same, it will require special design awareness to make sure that the efficiency is maintained. This technology is one to watch for the future.

modern white living room with wood burner and blue sofas

Heating is rarely used in this contemporary self build in Surrey — the house, which has an EPC A rating, is heated with a Terra Therma air-source heat pump

Choose Smart Controls

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 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. A smart controller for the whole house is also an option, but these controllers will usually only monitor and store the room setting and hot water data and so will not have any information on the commissioning settings or fault codes from the heat pump.

About the Authors: David Hilton and Tim Pullen

David is an expert in sustainable building and energy efficiency and is the training centre sales manager at HRP Ltd. He also delivers seminars at the National Self Build and Renovation Centre

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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