Understanding air source heat pumps

An air source heat pump works by transferring heat from the outside air to an indoor space. A fan draws in air from outside and transfers it to an evaporator, which is situated in the heat pump. A refrigerant circulates inside the pump, which changes its physical state and evaporates even at low temperatures. The refrigerant vapour needs to be raised to a level where it can be used for your heating and hot water, so it’s compressed using a compressor. The heat produced in this process is transferred to the heating and hot water systems in your home.

By adding free heat (energy) from the air outside to the electrical energy needed to power the heat pump, the heat pump can typically produce up to 4 times as much heat energy as it needs to run. If you want to make your home even more eco-friendly, you could install solar panels to provide electrical power to run the air source heat pump. This will help you to reduce your energy bills and your carbon footprint.  

Air source heat pumps can be effective if your home suits this kind of technology.

Home insulation can make a huge difference to the effectiveness of your heat pump. A well-insulated house requires one kW of heat for every 25 m2, but a badly insulated house will only heat 10 m2 per kW of heat. This means that a house with poor insulation could require two and a half times more energy to heat it.

To get the most from your air source heat pump, your home should have good insulation and large radiators and/or underfloor heating. Windows should be double glazed and you should invest in draught excluders to make sure you’re keeping as much heat in your home as possible.

Heat pumps are more versatile than boilers as they can be powered by wind energy or solar power to save you more money. They’re an excellent choice for people who want to reduce their home’s ecological footprint or are looking to modernise their home.

Heat pump sizes range from around 4 kW to 16 kW. The size you need depends on a range of factors, including:

• Outdoor design temperature
• Desired room temperature
• Flow temperature
  
  

The outdoor design temperature is the average temperature outside so this is dependent on where you live. It’s usually based on the coldest temperature of the year in your area. For example, a home in Scotland will have a lower average outside temperature than a home in Cornwall.

The desired room temperature is the temperature you, the homeowner, would prefer. 21 °C is a popular home temperature, but if you want your home cooler than this, you could have a smaller pump.

The flow temperature comes down to the size of your radiators and the standard of your home insulation. If your radiators are too small, the flow temperature will need to be higher.

Your installer should be able to tell you what size air source heat pump you need. They can do this using specialist design calculation software that determines the pump size needed to match the heat demand. If a pump is too big, it’ll short cycle all the time. If it’s too small, it’ll use the backup heater too often to keep the heat at a constant level. Both of these issues could cost you more in the long term.

If you’re considering an air source heat pump for your home, there are plenty of benefits that you can find below.

• Efficiency
  
  

Air source heat pumps generate less CO2 than gas heating systems, making them better for the environment. This is especially the case when they’re powered by renewable energy created from solar panels. This could significantly reduce the carbon footprint of your home.

• Running costs
  
  

Franck Energy has calculated that the average yearly running cost of an air source heat pump is £520 based on average home usage in the UK. This could bring you an annual saving of up to £216 if you currently use a gas boiler.

If your pump installer is MCS-certified, you could apply for the Renewable Heat Incentive*, which is run by the UK government. The scheme means that you could receive quarterly cash payments spread over seven years for installing a renewable heating technology such as an air source heat pump. You need to apply for the scheme within one year of the commissioning date of your system. It’s another cost benefit to having an air source heat pump system.

*Valid until end of March 2021

• Easy to install
  
  

Air source heat pumps can be retrofitted into your home and will work alongside the radiators or underfloor heating that you already have. This means there’ll be no messy renovations. Instead, your old boiler can be removed and the heat pump installed ready for use immediately.

• Heats and cools
  
  

While heat pumps can draw heat from the air to heat your home, they can also do the opposite and draw the heat out of your home to cool it in summer. This is an especially useful advantage that heat pumps have over boilers.

While there are clearly many benefits of having a heat pump, there are also some disadvantages that you may also want to consider.

• Installation cost
  
  

Air source heat pumps are generally more expensive to buy and install than a gas boiler.

The cost of an air source heat pump could depend on a number of factors, including the size of your house. When you’re getting quotes for the job, you should make sure the installer is MCS-certified and a member of one of the two consumer codes: Renewable Energy Consumer Code or Home Insulation & Energy Systems Contractors Scheme.

A heat pump will generally cost around £7,000 to £9,000 but, once installed, you should see a drop in your bills and the pump won’t need to be replaced as soon as a boiler would.

A further cost that you may have to factor in is additional home insulation. Air source heat pumps will work much more efficiently if your home is able to maintain the heat that the pump produces. This might mean upgrading your current home insulation to ensure your pump is heating your property as efficiently as possible.

• Noise
  
  

A heat pump may be slightly noisier than a boiler. This is because of the fan that is used to draw the air into the unit. 

Depending on the system and the quality of the installation, your air source heat pump should produce around 40 to 60 dB of noise. A computer is around 40 dB and a refrigerator is around 50 dB, so pumps are not very loud. The noise you’ll be able to hear coming from your pump could be either the fan or the compressor. Some heat pumps may have advanced acoustic design which will help to minimise noise.

The UK government has stated that the noise shouldn’t be higher than 45 dBs when it’s one metre away from the window of a neighbouring property. If you find that your pump is considerably louder than this, you could contact your installer to see if there’s anything they can do.

You should weigh up all the options involved when choosing an air source heat pump. There are lots of things to think about, including who will install it, whether it will be more cost effective than your current system, how much noise it will make and whether you have to make any other home improvements to get the most out of your new pump.