What is a heat pump?

You have heard all the buzz words, ‘eco friendly’, ‘green technology’, ‘renewables’, ‘low-carbon’ and now they are being associated with the push towards swapping your trusted gas boiler with a new ‘heat pump’ system. But what is a heat pump and what benefits do they provide? The answer is not straight forward, so in this article we will first define what a heat pump is, go through the various types and then explore what heat pump technologies maybe suitable for your home.

There are many definitions of what a heat pump is, but basically a Heat Pump transfers heat (can heat or cool) with the use of refrigerants.

Heat pumps utilise refrigeration cycles, meaning refrigerants are evaporated from liquids to gases to take in heat (produce cooling) and are condensed from gases to liquids to give out heat (produce heating).

Types of heat pumps- Heat pump are certainly not a new technology, standard household fridge freezers are a good example. However, generally when we use the term ‘heat pumps’ we are generally talking about devices that provide space heating and cooling, and domestic hot water, so in other words, they are considered ‘green’ alternative for gas boilers.

Heat Pump Types

Air to water Heat Pump-Mono-Block

Also known as ‘air source heat pump’

Monoblock heat pumps sit outside and are connected straight onto the properties heating and hot water generation systems.


-relatively low investment cost in comparison to other heat pump technologies

-Often the easiest heat pump technology to integrate into existing domestic properties.

Things to consider

-Outdoor space required for the unit

-Outdoor unit noise emissions

Air Source Heat Pump- Split System

Also known as simply ‘air source heat pump’

Air source heat pump slit system are different to the monoblock systems in that there have a separate indoor unit in addition to the out door unit. The indoor unit is connected to the outdoor unit with refrigeration pipework. The indoor unit is then connected onto the heating system.


-The external pipework contains refrigerant and not heating system water. Therefore the risk of heating pipework freezing is eliminated.

Things to consider

-Outdoor space required for the unit

-Outdoor unit noise emissions


Ground Source Heat Pump

Ground source heat pumps utilise the thermal mass of the ground with the use of bore holes or a ‘slink’ system. A heat sink slinky system is an underground, shallow pipe loop installed over a large area.

There are also water source heat pumps which work in a similar way but have a slinky style system installed submerged in a large open water source.


-No outdoor space of noise emitted

-High efficiencies compared with the other heat pump technologies.

-Operates at very low ambient temperatures

Things to consider

-Suitability of bore hole of slinky for specific property. Ground conditions and space

-High initial cost 

Air-to-Air Heat Pumps

Also known as AC split system or DX system (direct expansion).

These are considered standard air conditioning systems. They produce heating and cooling through indoor fan coil units.


-No indoor plant space requirements other than the actual fan coil units and controllers.

Things to consider

-Indoor refrigeration pipework required. Specialist required to work on the system (f-gas registered).

-Outdoor space required for the unit

-Outdoor unit noise emissions

Commercial Heat Pump Systems

There are several different types of commercial heat pump systems available. These include Chillers, VRF/VRV heat recovery systems. 

Selection of commercial heat pump system involves the analysis of many variables, advise should be sort from a Chartered CIBSE Engineer or competent engineering consultant.

It is worth noting that all the heat pump technologies mentioned above can also be run in reverse to produce cooling. Whether or not a heat pump system can also provide cooling will also depend on the main unit controls and the internal heat emitters installed.

What type of heat pump is suitable for my home?

The most common heat pumps being installed for domestic properties at the moment is air to water heat pumps (or air source heat pumps). These generally feed standard steel panel radiator systems but also work very well with underfloor heating systems. Most systems will also feed the hot water generation for the property which allows you to take full advantage of the technology. See Heat pumps vs gas boilers for comparisons on running costs, efficiencies and CO2 emissions.

Importantly, it should be noted that air to water heat pumps operate at lower temperatures than standard gas-boiler systems. This means that if you are considering swapping your gas boiler for an electric heat pump, you must carefully consider if it can be connected onto your existing heating system or whether new radiators and pipework is required. The lower temperatures will mean that your existing radiators will emit less heat. Typically air to water heat pumps operate at 55 deg C flow temperatures. To test if your existing radiators are suitable, you could turn your existing gas boiler flow temperature down to 55 deg C and observe whether your house is warm enough on a cold day. 

Due to the lower temperatures utilised by air to water heat pumps, it is advised that they are only used within houses that are well insulated. This means some older house maybe unsuitable. Possible fabric upgrades should be considered before a heat pump system is considered, such as loft insulation, cavity wall insulation and double/ triple glazing windows.

For further information on heat pump system selection for your specific house, we recommend you look at the Building Services Portal heat pump calculator. It is free to use and after answering a few simple questions about your specific property, it provides you with useful selection information, space requirements and estimated costs. It is a good place to start if you are considering changing you gas boiler for a heat pump.

Air to air heat pump systems can also be utilised for domestic properties. They tend not to be favoured where home owners wish to keep their radiator systems or when fan coil units are not desired. Some people may not like the idea of the warm air being blown into a room during the heating system and prefer the radiant emitted from standard radiators. Also the units are not generally considered great for kitchens and bathrooms where there is intermittent high-humidity. These rooms could be heated with other electrical means such as towel rails.

Typically, heat pump technologies require the use of hot water cylinders and therefore the internal space for this is required. If you currently have a combi boiler you will need to find the additional space for the hot water cylinder.

Boiler upgrade scheme

There is government financial support available for house holds planning on upgrading their existing gas boilers for heat pump technologies. The scheme supports home owners with a £5,000 contribution towards the installation of an air to water heat pump or biomass boiler system. Alternatively, they offer £6,000 towards the cost of installing a ground (or water) source heat pump system. There are limitations to the scheme such as an approved MCS (microgeneration certificate scheme) installer must be used. See link below for further details;

Boiler Upgrade Scheme (BUS) – Property owners | Ofgem

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