An introduction to how heat pumps work

Heat pumps are a way of heating your home and providing hot water. They use environmentally friendly, money-saving energy sources rather than finite fossil fuels like gas and coal.

Heat pumps are a vital part of Scotland's drive to a net-zero emission economy by 2045.

Read our guide to the different types of heat pumps that homeowners can install to help cut Scotland's carbon footprint.

air source heat pump

Why should you consider installing a heat pump?

Heat pumps can warm your home and provide hot water. Significantly, they run on electricity; an essential element to cutting carbon emissions.

In 2019, renewable energy sources supplied 90.1% of Scotland's electricity. Just over three-quarters of Scotland's home energy consumption is taken up by heating systems, with a further 11% on hot water. Heat pumps are a vital bridge to heating our homes and providing domestic hot water from environmentally-friendly, low-carbon energy sources.

Most of Scotland's homes are currently heated with gas through boilers and central heating systems. Off-grid methods include electric storage heating, oil, or LPG central heating. The Scottish Government wants the country to have one million low carbon homes by 2030.

Heat pumps could help lower your energy bills, depending on your current heating system. They can also reduce your home’s carbon emissions while providing central heating and hot water.

Recent research commissioned by Energy Saving Trust and Home Energy Scotland showed more than half of those asked (51%) hadn’t heard of heat pumps. Some 40% of people thought heat pumps were too expensive to install, although one in five (20%) say they are likely to install a heat pump within the next five years.

Let's look at which heat pump could work best for your home — and how you could earn money on your investment as well as receive financial support for installation.

What types of heat pump are there?

https://energysavingtrust.org.uk/report/renewable-heat-in-scotland-2019/There were 2,480 new heat pump installations in Scotland in 2019, almost all of them in the domestic sector. The country's total number of heat pump installations across the country now stands at 17,140. (Source: Energy Saving Trust's Renewable Heat in Scotland report, 2019).

The three main types of heat pumps are air source, ground source, and water source. Read on to find out more about each.

Any of these could potentially replace your current central heating system. Heat pumps are most cost effective when replacing electric or coal heating systems. They may not reduce your energy bill when replacing newer gas boilers but do emit fewer carbon emissions.

The most appropriate system for your home depends on the building type, its position, grounds, and other factors. Let's explore the workings and energy-saving capabilities of each heat pump type.

What is an air source heat pump (ASHP)?

The air source heat pump is the most commonly installed heat pump in the UK. Its popularity is thanks to its relatively easy installation, lower associated costs, and financial support and benefits.

Air source heat pumps need electricity to run. They extract renewable heat from the environment transforming the air’s heat energy into heating and hot water. The heat output is greater than the electricity input, which makes them an energy-efficient heating system.

Heat pumps have two main components and work in a way similar to fridges. In simple terms, they move heat from one area to another. Air source heat pumps use a refrigerant — a fluid that evaporates at a very low temperature — to draw heat energy from the outside air. The evaporated refrigerant is compressed, giving off a lot of heat energy.

There are two types of air source heat pumps; air-to-water and air-to-air heat pumps.

With an air-to-water heat pump system, heat energy warms water for your hot water cylinder and radiators. Traditional central heating systems use a boiler to warm water and radiators, often using high temperatures. Air-to-water heat pumps are more energy efficiency at lower temperatures. They therefore offer slower heat release over more extended periods; perfect for underfloor heating and larger radiators.

With an air-to-air heat pump system, heat energy warms air that is then circulated around your home. An air-to-air heat pump does not provide hot water.

Benefits of air source heat pumps

  • You could potentially lower your energy bills.
  • You can keep warm while lowering your home’s carbon emissions.
  • You can get financial help with installation available.
  • There's no fuel delivery, like oil, LPG, gas bottles, or wood for biomass.
  • You could get potential payments through the UK's Renewable Heat Incentive.
  • In general, they are the cheapest and most accessible of the three heat pumps to install.
  • They're generally low maintenance.

Savings and costs of air source heat pumps

Air source heat pumps use electricity, so you will still have energy bills. But their energy efficiency compared to the older, less efficient heating systems they replace should further reduce energy bills.

A typical air source heat pump costs between £9,000-£11,000 to install, depending on house size, insulation, and desired temperatures. Most systems come with a two to three-year warranty, and a well-maintained system can last up to 20 years. Someone replacing an A-rated LPG boiler could save £380-410 a year on energy bills.

In Scotland, the Home Energy Scotland Loan can help with installation costs. You could also receive payments from the heat you generate via the Renewable Heat Incentive.

These are the potential annual savings of installing a standard air source heat pump in an average-sized, four-bedroom detached home in Scotland, England, and Wales.

Things to think about before installing an air source heat pump

It's worth considering a few things to fully understand the implications of installing an air source heat pump:

Size: The higher your heat demand, the larger the heat pump.

Insulation:  Insulation and draught proofing can reduce your heat demand, as well as improving the comfort of your home. Financial help is available to insulate your home.

Placement: The heat pump needs plenty of space to allow for good airflow and is usually fitted on the ground or an outside wall. Check with your local authority if you require planning permission.

Inside the home: Inside, you’ll need room for a compressor and controls, plus a hot water cylinder that is usually smaller than a standard gas boiler. Underfloor heating and larger radiators work best. Installers can provide you with advice on this.

Noise: Typically quiet, a heat pump will emit some noise similar to an air conditioning unit.

Usability: Heat pumps work most efficiently delivering low-temperature water. Therefore, a heat pump system should be run for extended periods with larger radiators (or underfloor heating) to reach your desired thermostat temperature. (If you have a gas, oil, or LPG fuelled boiler, this isn’t true, as your system delivers higher temperature water. It’s more efficient for these systems to be on less.)

Planning permission: Many systems will be classed as a ‘permitted development.’ Always check with your local authority if you need planning permission, although it is not a likely requirement.

Heating water: Heating water can limit the system's overall efficiency. Solar water heating or an electric immersion heater can help with the hot water supply. It’s best to talk to your installer about your needs because every home will have different hot water usage requirements.

Maintenance: Air source heat pumps require very little maintenance. Check annually that the air inlet grill and evaporator are debris-free and you should remove any plants growing near the heat pump. Your installer may advise checking the central heating pressure gauge in your home from time to time. You can ask them to list all the maintenance requirements. We'd also recommend a professional services the heat pump every two to three years.

What is a ground source heat pump (GSHP)?

If you have ample garden space outside, you could consider a ground source heat pump.

Ground source heat pumps (also known as geothermal heat pumps) extract heat from the ground via a loop of pipes buried underground. Inside these pipes, called a ground loop, the ground source heat pump circulates an antifreeze and water solution. The fluid absorbs heat from the ground.

The fluid then passes through a compressor in the heat pump, increasing the fluid’s temperature. The fluid’s heat then transfers to radiators and underfloor heating systems. It can provide hot water in your home, too.

The cooled solution returns to the underground pipes to reheat in a continual cycle, providing heating and hot water for the lifetime of the heat pump.

As underground temperatures are pretty constant (between 10-13°C) ground source heat pumps are more consistently efficient than air source heat pumps.

The ground loop length required depends on how much heat you need. Longer loops draw more heat but need more space. People with smaller gardens may be able to drill a vertical borehole 90-160m deep for their ground loop.

Benefits of ground source heat pumps

  • They can potentially lower energy bills and carbon emissions for your home.
  • You can get financial help for installation.
  • They keep you and your home warm, whatever the weather.
  • There's no fuel delivery, like oil, LPG, gas bottles, or wood for biomass.
  • You could get potential payments through UK's Renewable Heat Incentive.
  • They heat your home as well as providing hot water.
  • They require minimal maintenance.

Savings and costs of ground source heat pumps

Ground source heat pumps use electricity, so you will still have energy bills.

GSHPs are energy-efficient and can save £1,000 to £1,090 per year when replacing the average electric heating system. Most GSHPs come with a two to three-year warranty, and a well-maintained system can last up to 20 years.

They do represent a significant investment. A typical system costs around £14,000 to £19,000 to install. Running costs will depend on several factors, including your heat demand, electricity tariff, and how long the system is left running.

In Scotland, the Home Energy Scotland Loan is available to help with installation costs. And there are potential payments from the heat you generate via the Renewable Heat Incentive too.

These are the potential annual savings of installing a standard ground source heat pump in an average-sized, four-bedroom detached home in Scotland, England, and Wales.

Things to think about before installing a ground source heat pump

There is plenty to consider before installing a ground source heat pump.

Your garden: It doesn't need to be big, but it must be accessible to digging machinery and be suitable for a trench or a borehole. The space required varies (horizontal pipes need more room) and the ground must be appropriate for digging. Vertical boreholes are typically more expensive than horizontal trenches, as you’ll need a geological survey and more specialised equipment.

Insulation:  Insulation and draught proofing can reduce your heat demand, as well as improving the comfort of your home. Financial help is available to insulate your home.

Heating system: ground source heat pumps can perform better with underfloor heating or systems that use larger radiators.

Installation: It may take several weeks to install an entire system. Once installed, the pipes are not visible, and you can use your garden as you usually would.

Noise: Typically quiet, a heat pump will emit some noise similar to an air conditioning unit.

Heating water: Heating water can limit the system's overall efficiency. Solar water heating or an electric immersion heater can help with the hot water supply. It’s best to talk to your installer about your needs because every home will have different hot water usage requirements.

Usability: ground source heat pumps work most efficiently delivering low-temperature water at around 35°C. Therefore, a heat pump system should be run for extended periods with larger radiators (or underfloor heating) to reach your desired thermostat temperature.

Planning permission: Always check with your local authority if you need planning permission, although it is not a likely requirement.

Maintenance: ground source heat pumps are low maintenance. Annual checks may include inspections of the heat pump’s external pipes, fittings, and electronics. Have a professional service the heat pump every four to five years.

What is a water source heat pump?

Water source heat pumps work very similar to ground source heat pumps, except the pipes are submerged in water — a river, loch, large pond, or borehole — rather than underground.

The water body’s heat transfers into a fluid in the pipes, which then passes through a compressor in the water heat pump. The compressor increases the fluid's temperature so that you can use it for hot water and central heating.

Water transfers heat better than air, and water temperatures are generally stable year-round (7-12°C).

There are two types of water source heat pumps. A closed loop system has sealed pipes filled with an antifreeze fluid submerged beneath the water. The heat pump circulates this fluid as it takes heat from the water body; the liquid never comes into contact with water.

Open loop systems extract the water's heat by pushing it through the heat exchanger then returning the water to its source. They can be more efficient than closed loops.

Things to think about before installing a water source heat pump

You need a large watercourse nearby, like a large lake or a river, and one that won't freeze. The higher the heat demand, the more extensive body of water you'll need for the pipes.

Underfloor heating and large radiators work best with water source heat pumps, and your home needs space for the heat pump. Insulation and draught proofing can reduce your heat demand, as well as improving the comfort of your home.

Open loop systems will require water extraction and discharge permission (called abstraction) from the Scottish Environment Protection Agency (SEPA) in Scotland. Abstraction is separate from planning permission. Neither open nor closed loop systems require planning permission but check with your planning authority.

A water source heat pump could deliver all your home’s heating and hot water needs. It's best to consult an expert; there are many things to consider in determining whether your water body is suitable for a water source heat pump or not.

Why install a heat pump instead of a traditional central heating system?

Besides reducing your carbon footprint, there are lots of financial incentives available to help with installation costs. The figures show that you can save a lot of money in the long term, especially if you’re replacing an electric or oil-based heating system.

Scotland's homes require a lot of heating due to the country's climate. Installing an electricity-powered heat pump heating system can reduce carbon emissions two-fold.

Firstly, a heat pump can produce two-and-a-half to four times more heat energy than the electricity needed to run it. This means you burn fewer fossil fuels to create electricity or heat.

Secondly, by warming our homes with heat pumps powered by renewable energy. If Scotland continues decarbonising its electric grid, heat pumps have the potential to lower their carbon emissions even further. Installing a heat pump is a carbon emission and climate crisis win-win situation.

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