Moving heat instead of creating it
Most heating systems create heat by burning fuel or by passing electricity through a resistive element. A heat pump does something different. It collects heat that already exists in the outside air, the ground or a body of water and moves it indoors. Because moving heat takes less energy than generating it from scratch, a heat pump can deliver more heat to a building than the electrical energy it consumes.
That sounds like a trick, but it follows from ordinary physics. A refrigerator already does the same thing in reverse. It moves heat from the cold inside to the warmer kitchen, which is why the back of a fridge feels warm. A heat pump is, in essence, a refrigerator whose cold side is the outdoors and whose warm side is your home.
The cycle in four steps
A sealed circuit contains a refrigerant, a fluid that boils at a very low temperature. It travels around a loop with four main components.
- Evaporator. The refrigerant passes through a heat exchanger outside the building. Even cold outdoor air contains heat, and the refrigerant is colder still, so it absorbs that heat and turns from liquid to gas.
- Compressor. An electric compressor squeezes the gas. Compressing a gas raises its temperature and pressure, so it now holds heat at a temperature useful for the house.
- Condenser. The hot gas passes through a second heat exchanger indoors and gives up its heat to the water in the heating circuit or to indoor air. As it loses heat, it turns back into liquid.
- Expansion valve. The liquid pressure drops sharply, cooling it further, and it returns to the evaporator to begin again.
The electricity mostly powers the compressor and fans or pumps. The bulk of the heat delivered comes from the environment.
What “efficiency” means here
Heat pump performance is usually described by a ratio. The coefficient of performance (COP) compares the heat delivered with the electricity used at a given moment. A COP of 3 means three units of heat for one unit of electricity. A seasonal figure, often called SCOP or SPF depending on the standard, averages performance across a year. Figures on product data sheets are measured under standard test conditions, so real homes may do better or worse.
The key point is that efficiency is not fixed. It drops when the gap between the outdoor temperature and the temperature you need grows. Heating water to a high temperature on a very cold day is harder than warming it gently on a mild day.
Types of heat pump
- Air source. These take heat from outdoor air. They are generally the simplest and cheapest to install and are the most common type. Some deliver warm air (air-to-air) and others heat water (air-to-water), which then feeds radiators or underfloor heating. Many can also run in reverse to cool a building.
- Ground source. These use buried pipes or boreholes. Ground temperature is steadier than air temperature through the year, so efficiency can be more stable, but installation usually costs more and needs space or drilling.
- Water source. These draw on a nearby lake, river or other water body, and are less common for homes.
When they work well
Heat pumps tend to do best when the building needs less heat and the heat can be delivered at a lower temperature.
- A well-insulated, reasonably airtight home. A smaller heat demand means a smaller, cheaper system and lower running costs. Home insulation and basic draught proofing often go hand in hand with a heat pump installation.
- Low-temperature emitters. Underfloor heating and larger radiators can warm a room at lower water temperatures than the small radiators designed for boilers, so the heat pump runs more efficiently.
- Steady, longer running. Heat pumps often work best running for long periods at modest output, rather than short, intense bursts. This is where good heating zones and timer settings need care. Aggressive set-back and boost cycles may reduce efficiency, and manufacturer or installer advice is worth following.
- Correct sizing and commissioning. A poorly designed or set-up system can disappoint even in a suitable house.
When they work less well
- Poorly insulated homes. If the house loses heat quickly, the system has to deliver a lot of heat at a high temperature, which lowers efficiency and raises bills.
- High-temperature requirements. Replacing a boiler without changing small radiators can force the system to run hot. Some radiators may need upsizing.
- Very cold weather. Modern units can operate in low temperatures, but efficiency falls as it gets colder, and some use backup electric heating in extreme conditions.
- Space and noise. Outdoor units need room and clearance, and the noise they make can matter in tightly packed streets or near bedroom windows.
- Upfront cost. Purchase and installation can cost more than a gas boiler, although grants or support schemes exist in some places. Whether it pays back depends on energy prices, the building and the system chosen, so be wary of simple promises.
The environmental picture
A heat pump’s emissions depend largely on how its electricity is produced. As the electricity grid draws more on renewable energy sources, the carbon cost of each unit of heat falls. Some households pair a heat pump with rooftop solar, or choose a green tariff, though it pays to read what such tariffs actually mean. Refrigerants are also a consideration because some are strong greenhouse gases if they leak, so installation by a qualified professional and proper servicing matter. The overall climate benefit is generally considered positive compared with burning fossil fuels for heat, but it varies with local circumstances.
Questions to ask before choosing one
- How much heat does my home lose, and could it be reduced first?
- Will my radiators and pipework suit lower-temperature heating?
- Is there somewhere suitable for the outdoor unit?
- What will the system cost to run compared with what I have now?
- Is the installer qualified and able to provide a design calculation?
Individual situations differ, and a proper survey from a qualified installer is the most reliable way to judge suitability. This article is general information only.