Heat Pumps - Air vs Ground vs Water Source
Updated: Aug 1
Heat pumps have been used to heat homes for over 30 years. However, in recent years the technology behind them has advanced tremendously. Nearly 20 million households across the world purchased heat pumps in 2019- according to an IEA study. This figure has been growing year on year! If you're reading this you've probably already heard about heat pumps, and now you're thinking...which heat pump is best for my home?
How do heat pumps work? Before we can get into which heat pump is best for your home, you need to understand how they work. In simple terms, a heat pump uses the reverse process of an air conditioning unit. Whilst an air conditioning unit absorbs the thermal energy in the room and transfers the heat outside, a heat pump absorbs the thermal energy from an outside source and transfers it into the homes heating system. The thermal energy source can be from either the air, ground or water. Heat pumps absorb thermal energy using a liquid called a refrigerant, most commonly
R-410a. This liquid has a boiling point of -48.5c, meaning even on the coldest UK night this liquid will absorb thermal energy. Heat pumps work by pumping the refrigerant around a system. At the start of the system is an evaporator, this is where the refrigerant will come into contact with the thermal energy source. Due to the contrast in temperatures, the refrigerant will absorb the heat from the source and begin to boil – causing it to evaporate and turn into a gas. The refrigerant gas will then be pushed through a compressor, causing it to increase in pressure and temperature. The high temperate refrigerant then passes through a condenser. The condenser transfers the refrigerant’s heat to the home’s water based heating system, providing hot water for the home. The cooled refrigerant will finally pass through an expansion valve which will reduce the refrigerant’s pressure, turning it back into a liquid state. The process will then repeat again.
Which heat pump is best for my home?
To understand which heat pump is best for your home, you need to understand how each type extracts thermal energy and how much they cost to install. Air Source Heat Pump – Air source heat pumps are the most common type of heat pump. Positioned on the exterior of your home, air source heat pumps propel air through the evaporation section using an ultra-quiet fan. The refrigerant liquid inside the heat pump absorbs the thermal energy from the passing air, starting the process. Air source heat pumps are usually fully contained, meaning that all of the parts for the heat pump are contained within the unit. The only feeds coming in and out of the heat pump will be the flow and return pipes into the property. This makes air source heat pumps quick and easy to install – meaning installation costs are low. One disadvantage of air source heat pumps is that their efficiency is lower in colder conditions, due to the reduction of thermal energy in the passing air. However, reputable companies will factor this in when designing your heat pump and heating system, to ensure it can heat your home sufficiently all year round. Air source heat pumps cost around £7,500 - £13,000 including fitting costs, depending on the size of the property. We would recommend an air source heat pump for the following properties;
- Summer holiday let
- Low heat demand homes
- Properties with a small outside area
- Projects with a smaller budget
Ground Source Heat Pump – Ground source heat pumps are the most efficient type of heat pump. They work by installing pipes under the ground, filled with a refrigerant liquid. As the refrigerant liquid passes through the ground, it absorbs the grounds heat. Due to the ground maintaining a fairly constant 10C temperature, ground source heat pumps have a consistent efficiency all year round. There are two methods for installing the pipes in the ground, or 'ground array'. Borehole systems involve drilling 80-100 meter vertical holes in the ground and lowering the pipes down. Whereas, horizontal systems involve digging 1 meter deep by 1 meter wide trenches horizontally across the ground and laying the pipes within the trenches - don't worry the ground will be re-turfed after! Both types of ground array will have a small concrete unit built at surface level, which will house the manifold and other serviceable parts. Because of the cost of the ground array install, ground source heat pumps often cost around twice that of an air source heat pump, from £15,000 - £30,000. However the grants available are larger than for air source heat pumps.
We would recommend a ground source heat pump for the following properties;
- Large homes with a high year-round heat demand
- Properties with a large garden or available field
- Projects with a larger budget
- District Heating systems (supplying multiple properties from one ground array)
Water Source Heat Pump – Water source heat pumps extract thermal energy from water using a submerged heat exchanger (often plastic pipes), or by extracting the water for use in a heat exchanger on land. Similar to the ground, deep water sources tend to have a consistent temperature all year round, making them a great source of thermal energy. Depending on the size of the water source, a large amount of piping can be submerged in the water, making it the perfect method for large properties or district heating. One advantage of water source heat pumps are that they require minimal ground works compared to ground source systems, however it can be difficult to get permission to use water sources that aren't directly owned by the property. Due to the varying design of water source systems, the costs also vary dramatically, but when used in lakes/ponds situated close to the property they can be cheaper than ground source heat pumps.
We would recommend a water source heat pump for the following properties;
- Homes with an available water source near by
- District Heating systems
What to think about when choosing which heat pump to install on your home
When thinking about which heat pump you should have for your home, there’s a few things to consider. Costs – Heat pumps vary in price massively. Therefore, for lots of people the funds available will determine the heat pump you install - this is completely fine! All heat pumps work in the same way, so by having any type of heat pump you will save money on your bills. However, if you have the initial funds available, a ground source heat pump is the most efficient, so over the lifetime of the heat pump you will save more money than you would if you had an air source heat pump.
Homes Heat Demand – An air source heat pump’s efficiency is extremely high in summer but slightly reduced in winter. If you have a holiday let that only gets used in the summer, then an air source heat pump would be perfect – a small initial cost and high efficiency when in use. If you have a large family home with a high heat demand in the winter, a ground or water source heat pump might be worth considering, but an air source heat pump would still work just fine.
Available Space – If the first two points don’t determine which heat pump you require, then the space you have available may. If you are looking to have a ground source heat pump using trenches, you will need a large outside area that you can dig up to lay the pipes in – don’t worry this can be re-turfed after the installation. If you are looking for a ground source heat pump using bore holes, you will need to ensure that the ground is suitable for deep drilling and doesn’t contain obstacles such as very hard rock (granite) or mine shafts. If you are looking for a water source heat pump, you must have a deep-water source nearby and the owners permission. An air source heat pump is the easiest to install, and only requires a 1m x 2m space to place the heat pump – in some cases the heat pump can be attached to a wall off the ground.
How can we help? We understand that there are so many factors to consider when choosing your heat pump – that’s why we’re hear to help. Our team will assess your home, completing a detailed heat loss survey. This will provide us with your home’s energy demand, to help us understand what size heat pump will be required. We can then discuss your personal preference regarding the type of heat pump you would like, and provide you with our recommendations based on the location and setting. We provide all of this as a completely free, no obligations service. Fill out the short contact form on our website and a member of our team will get back to you.