Top Takeaways
>> Air source heat pumps are the future of UK household heating. The plan is that they will take over from gas-powered boilers
>> Heat pump technology can reach energy efficient levels of 400%. That means creating 4 units of heat energy but 1 unit of energy input
>> Learn more about heat pump costs, available grants, the size you would need, as well as heat pump limitations
The demand for more energy efficient heating systems is growing. People are tired of burning fossil fuels to provide heating and paying extortionate energy bills. I know I am. Maybe you feel the same too?
Air source heat pumps are quickly emerging as a popular choice for both homeowners and businesses. These rather ingenious units are a reliable, energy efficient and eco friendly alternative to traditional gas boiler systems.
This guide will give you a comprehensive introduction to air source heat pumps. We’ll touch on a number of topics briefly, with links to where you can get more information if needed.
Air source heat pump definition
An air source heat pump is an electrically powered heat pump that takes outside air, extracts the heat energy, warms it up to a desired temperature and transfers it into a building to provide heating.
Sound too good to be true?
Well, it’s not. It’s an amazing bit of engineering that can provide energy efficient heating at a lower cost.
What are air source heat pumps?
Heat pump technology isn’t new. It’s been around for many decades now. But their adoption in homes hasn’t taken hold due to technology restraints. Gas boilers were the heating system of choice but this is quickly changing in light of a more sustainable, climate friendly approach.
The air source heat pump trick is to harness the heat in the air from outside, warm it up and let it heat your home. Even cold air contains heat that can be extracted.
A heat pump is what’s known as an active heat exchanger. It’s regarded as ‘active’ as it needs a bit of energy input to do the job. However, it’s not a trick, it’s just science and more people need to know about it.
The principles behind heat pump technology are the same found in fridges and air conditioning units. It’s known as the refrigeration cycle. This is a type of thermodynamic cycle.
The only thing with heat pumps is that this technology is reversed. Instead of providing cooling like a fridge, it provides warmth like a…well, like a heat pump!
Through a series of temperature changes, pressure changes and transitions from gas to liquid for the refrigerant running through the coils, heat can be extracted and distributed.
What do air source heat pumps do?
As the name suggests, air source heat pumps provide heat.
Modern day air source heat pumps have been designed to replace your gas boiler. They are helping us decarbonise our heating supply and move away from fossil fuels.
An air source heat pump can provide heat in a couple of ways:
- Heat rooms through radiators and underfloor heating
- Provide hot water for showers, baths and sinks
How do air source heat pumps work?
All is sounding good so far. Air source heat pumps are energy efficient, come with reduced carbon emissions and have long-term savings appeal. But how exactly do heat pumps work?
In short, it’s all about heat flow.
The premise is this: heat always moves from warmer to cooler. It’s the second law of thermodynamics. This is the law unless an external energy form is applied to reverse the process.
Let’s look at the key heat pump components and the role they play.
4 key components of air source heat pumps
There are 4 main components that work together in an air source heat pump.
1. Evaporator
This is where heat is absorbed from the outside air via a heat exchange surface. Cold coils with very cold refrigerant gas running through them do this job. Heat pumps take heat out of the air that’s way below freezing. How?
It’s all because the refrigerant is at a temperature even lower than this. The heat absorbed causes the refrigerant to change state from liquid to a gas.
What’s happening: Heat flows from ‘warm’ outside air to the colder heat pump coils and refrigerant.
2. Compressor
The clever compressor technology then works to increase the pressure and temperature of the refrigerant gas. This is the ‘pump’ part of the heat pump.
The compressor is in work mode when the setpoint temperature of the room hasn’t been reached yet. When the setpoint temperature is met, the compressor will tick over to maintain this.
What’s happening: The gas refrigerant is now pumped to increase its pressure and temperature
3. Condenser
Our refrigerant gas is now heated. The temperature of the gas leaving the compressor can range from 120-170oF (50-80oC). This flows into the condenser component of the heat pump system.
It’s in the condenser where the heat is taken out of the refrigerant gas to be transferred to your home’s heating system.
For example, with an air to water heat pump, the heat is transferred to the hot water system.
What’s happening: The heat in the refrigerant gas is transferred to the internal heating system.
4. Expansion valve
As the gas leaves the condenser heat exchange system its temperature has dropped and it starts to change state back into a liquid. The pressure is still high though.
In order to lower the pressure, the refrigerant is put through a thermal expansion value.
When the refrigerant is allowed to expand, its temperature and pressure reduce significantly. This is perfect as the refrigerant now returns back to the start of its journey at the evaporator. Remember, it needs to have a really low temperature here to be able to absorb the heat from the outside air.
What’s happening: The cooled off refrigerant is put through an expansion value to further reduce pressure and temperature. It turns into a liquid to be fed around to the evaporator.
Types of air source heat pumps
There are two main types of air source heat pumps.
1. Air To Air Heat Pumps
Air to air systems transfer heat between the outdoor air and the indoor air. They are typically used for indoor space heating and sometimes cooling too.
They can be installed in two ways – as a ducted or ductless systems. Ducted systems send around heated or cooled air through a network of ducts in the walls and ceilings. The air is then delivered into a room via a vent.
A ductless systems, sometimes known as mini-splits, sends conditioned air straight to individual rooms via pipes and a handling unit.
2. Air To Water Heat Pumps
Air to water systems transfer heat between the outdoor air and a water-based heating system, sometimes known as a wet system. These include radiators and underfloor heating.
Air to water is the most common type of air source heat pump for residential homes and business premises.
You can read more on the types of air source heat pumps here.
More about heat pump refrigerants
Key, and common, to all these components is the special refrigerant. This is the medium to which all the heat energy is transferred.
We’re getting into the weeds of detail here but there are numerous types of heat pump refrigerants.
Traditional refrigerants tend to be synthetic hydrofluorocarbons (HFCs). In the industry, they’re widely known as F-gases (fluorinated gases). In general, these are not good at all for the environment. Some HFCs have a global warming potential 3,000 times greater than even carbon dioxide.
In modern air source heat pumps, R-410A is the most common type of refrigerant. It’s a man-made HFC refrigerant designed to take-over from R-22 (freon, a CFC), which had a terrible environmental impact on the ozone layer.
Although F-gases don’t impact the ozone layer, they’re not good for global warming and climate change. R-32 is touted as the next generation refrigerant but it still has a massive global warming potential compared to natural refrigerants, which are now being looked at more seriously.
Generally speaking, natural refrigerants have a lower impact on the environment. Believe it or not, carbon dioxide (R-744) is one of the most promising natural refrigerants alongside R-290 (propane). Other natural refrigerants include ammonia.
Global warming potential of common refrigerants:
Global warming potential (GWP) is judged on the equivalent warming to carbon dioxide. This is the reference point.
For example, if a tonne of refrigerant with a GWP of 1,000 gets leaked into the atmosphere, this is the equivalent of 1,000 tonnes of carbon dioxide.
| Refrigerant | Type of refrigerant | Global Warming Potential Value |
| R22 | CFC | 1 |
| R-410A | HFC | 2,088 |
| R-32 | HFC | 675 |
| R-290 (propane) | Natural | 3 |
| R-744 (CO2) | Natural | 1 |
| R-717 (ammonia) | Natural | 0 |
How much do air source heat pumps cost?
The cost of an air source heat pump in the UK will vary depending on the size, brand and complexity of the system installation in a home.
The cost to buy an air to water heat pump can range anywhere between £2,500 and £9,000.
You then have the installation costs which start from £3,000 and can go up to £10,000.
On average the cost to buy and install an air source heat pump in the UK in 2024 is £14,000. Knock off the government grant, and this comes down to around £6,500.
At the lower end of the scale, Octopus Energy and British Gas are having a heat pump price war! Both are offering full installations starting from the £3,000 mark. For further reading, take a look at my Octopus Energy review.
A ‘full installation’ quote for an air to water heat pump should include:
- Home consultation and survey
- The heat pump (up to 7kW)
- A hot water cylinder
- All pipes and cabling
- Labour of the engineer
- Warranty on parts
Additional heat pump costs
There may be additional costs to consider if you have to upgrade your radiators or if you’ll be installing underfloor heating too.
As heat pumps operate best in a well insulated home, you might want to look at bolstering your wall and roof insulation too.
These are not cheap additional costs, but they are necessary if you want your heat pump to be operating at maximum efficiency. In the long run, these investments will pay off.
Air source heat pump grants
In England and Wales you can apply for a £7,500 grant towards your new air source heat pump via the Boiler Upgrade Scheme. This is the government scheme to reduce carbon emissions across the nation. It’s only available to homeowners living in an existing property (non-new build) who are replacing a traditional gas boiler.
You have until 30th March 2025 to apply for the Boiler Upgrade Scheme grant, which is usually done via your installer.
In Scotland, the Home Energy Scotland grant will give you £7,500 of the cost of your heat pump.
What size heat pump do I need?
Getting the right size heat pump for your home is really important.
Many heat pumps are over-sized, meaning they’re not running as efficiently which is costing you more money.
Generally speaking, most homes will need a air source heat pump sized somewhere between 5-15kW. This is a wide range and not too helpful. The size of heat pump you’ll need depends on house size, room size, insulation levels, heating demands on more.
You can get a much better understanding of what size heat pump you’ll need by using your current annual gas readings. You can use my free air source heat pump size calculator here.
To get into the specifics, check out these articles on the heat pump size for a 3 bed house and 4 bed house.
How big are air source heat pumps?
Most air source heat pumps are sized between 1m (height) x 1m (width) x 0.5m (depth).
Air source heat pump dimensions will differ depending on the specific range and model e.g. Vaillant, Samsung, Mitsubishi and on whether it is a monobloc (single unit) or split (two units) system.
There’s generally not much difference in unit size between the two different systems. Either way they have similar technology inside that needs to be housed accordingly.
Are heat pumps a sustainable solution for our heating?
Are heat pumps an environmentally friendly answer to climate change? Can they help provide a sustainable solution to traditional fossil-fuel led home heating?
The truth is, they can.
Heat pumps, alongside energy generating solar panels and excellent home insulation, are the future. Don’t forget you can get great eco insulation. In fact, we’re rather pinning our hopes on heat pumps to provide climate friendly heating.
Although they’ve been around for a number of decades, heat pumps are now gaining attention as a sustainable solution for our heating needs. The government is jumping on this, as are new house builders and environmentally aware homeowners.
Instead of digging up coal, drilling for oil and extracting gas, heat pumps are able to harness latent heat from the ground (ground source heat pumps) and from the air (air source heat pumps) to make them an eco friendly heating alternative.
If their use can be scaled across the nation, heat pumps have the ability to significantly reduce carbon dioxide emissions. Especially true if paired with a renewable energy electrical source to power them. Yes they do require electricity for power, but for every unit of electricity they consume, they produce 3 or 4 times in energy output.
My view is that yes, heat pumps offer a long term, sustainable solution for heating our homes and even businesses. You can read up on the pros and cons of air source heat pumps here.
Can heat pumps replace gas boilers?
I think the answer is unanimous. Yes, heat pumps can replace traditional gas boilers!
Air source heat pumps provide an energy efficient and eco friendly alternative for room heating and hot water production.
By transferring heat from natural sources, heat pumps can look after your heating needs but with significantly lower carbon emissions and lower running costs over the long term.
The downside to heat pumps at the minute is their higher up-front costs. As they become more popular though, the cost of a heat pump will reduce. This will make them an even more attractive sustainable heating solution for homeowners.
Saying all that, if heat pumps are to replace gas boilers, the national grid will have to adapt to greater electricity demand.
What are the limitations of air source heat pumps?
Air source heat pumps do have a few limitations that should be considered before installation.
The first limitation is that ASHPs do require electricity to operate. So, you will still get a monthly electricity bill unless you’re self-sufficient with solar panels!
More and more of the electrical energy from the national grid in the UK is being supplied by renewable energy sources, which is great from a sustainability point of view. The better insulated your property is, the less the heat pump will need to be on and less electricity you’ll be using.
You may read that air source heat pumps are generally less efficient in extremely cold temperatures. Whilst this is true, a good air source heat pump will still work effectively in -20oC conditions. Around half of all homes in Norway have heat pumps installed, and Norway is much colder on average than the UK!
Selecting the appropriate size heat pump for your home is a crucial factor that impacts the system’s overall performance and efficiency. It’s essential to work with a good professional installer who can conduct a thorough assessment of your property beforehand. A poorly installed or improperly sized heat pump will impact efficiency, as well as affect energy consumption.
The final limitation to consider is service and maintenance. In general, heat pumps require less maintenance than gas boilers and should last around 20 years. However, it’s still important to regularly service your heat pump, clean filters and check components.
