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Heat pumps: the principles

A heat pump extracts heat from a source - the ground, air or water - and effectively concentrates it so it can be used at a higher temperature for space heating or for heating water. It is a form of renewable energy because the heat extracted is continually replaced by heat from the environment, ultimately derived from the sun.

Any heat pump works like a refrigerator in reverse, using exactly the same principles; instead of removing heat from a box (your fridge cabinet) and transferring it to the environment (your kitchen), it takes heat from the environment and transfers it to a heat store, usually a water tank, from where it can be distributed around your house.

The most common systems in the UK are ground source heat pumps and air source heat pumps. The vast majority of systems are installed in offices and community buildings, such as leisure centres. However, they are increasingly seen as suitable for houses in rural locations that are not on the gas grid, as a cheaper and more eco-friendly alternative to heating with electricity, oil or LPG. For dwellings sited near rivers or lakes, water source heat pumps are worth considering.

How does a heat pump work?heat pump principles

Any heat pump system has three basic features:

  • A heat source and a means of extracting the heat (i.e. a heat exchanger, or collector)
  • A heat pump to drive the working fluid/refrigerant (i.e equivalent to the coolant in a refrigerator) around the refrigeration circuit
  • A heat distribution system, such as an underfloor heating system
The basic circuit of a heat pump system. As very cold refrigerant fluid (dark blue) passes through the evaporator it absorbs heat from the source and is warmed, causing it to vaporize. The pressure of the cool gas is raised by the compressor, which warms the gas and forces it through the condenser, where it gives up its heat. Consequently, the gas condenses back to a liquid and the cycle is repeated. The process is driven by energy (i.e. electricity) that powers the compressor.

The heat pump requires an input of energy, usually in the form of electricity. Typically, for every kilowatt of electricity used to extract heat from the source, between 2.5 and 5 kW of energy, in the form of heat, is delivered to the heating system.

The efficiency of a heat pump is described as its coefficient of performance (COP), which is the ratio of the heat output to the energy input. So, for example, a COP of 4 means that the system delivers 4 kW heat for every 1 kW energy input. Note that values of COP will vary with the temperature of the source heat and the temperature required in the heating system.

These systems are most efficient when delivering heat at relatively low temperatures, typically 35-50 degrees celsius and are best suited for use with underfloor heating, low surface temperature (LST) radiators, or for preheating hot water to feed a high-temperature water heating circuit (e.g. one using solar thermal collectors). They are not suitable for delivering heat to conventional radiators, nor can they, alone, provide hot water at the necessary high temperature for washing, etc.

Because the heat pump typically runs overnight, to store heat for distribution during the daytime, it can operate using electricity metered on an economy 7 tariff. Further, if the electricity is from a renewable source, the system has zero carbon output.

Is a heat pump the answer?

From the outset, it is vital to understand how heat pump systems perform, and whether they will suit the pattern of energy consumption and occupancy of your household. Assess the likely heat loss and take steps to minimize this before going ahead with installation. Then the system can be designed to suit the needs of the building.

  • They are best suited to regular occupancy with a fairly constant demand for relatively low-level heat.
  • The lower temperatures mean that effective insulation of the building is essential to realise the full potential of the system
  • Underfloor heating is expensive to retro-fit; costs are reduced with new-build or during extensive renovation of an existing property
  • Beware of manufacturer's claims and obtain a written statement of the system's COP at your designed operating temperature
  • You will need sufficient land for a ground heat source, or a suitable external wall to fit the collector for an air source pump
  • Some systems can work in reverse, to extract heat from the building and help cooling in hot weather

A UK field trial of heat pumps

Heat pumps are a well-established form of heating in continental Europe, but experience with them in the UK is more limited. In 2010 the Energy Saving Trust (EST) published the initial findings of a field trial of 83 installations across the UK. This study raised several important issues both for customers and installers. For example, system efficiency varied by almost three-fold from the least to the most efficient systems, for both air source and ground source systems. Also, performance of the system can be greatly affected by customers' behaviour and understanding of how to control the system effectively. However, it was concluded that "well-designed and well-installed heat pump systems can perform well in the UK."

Other key findings included:

  • Simplest system designs performed most efficiently
  • Customers must understand the heating controls
  • Responsibility for after-sales service should lie with a single company to ensure consistency of advice.

Advice to customers

On the strength of these preliminary findings, the EST drew up the following tips for householders considering heat pumps intallations:

  • Ensure first that your home is optimally insulated
  • Ideally use heat pumps with underfloor heating or properly sized radiators
  • Insist on user-friendly controls
  • Ask the installer to explain how they have determined the type and size of system for your property
  • Get the installer to explain fully how the system will work with the existing heating system
  • Ask to be shown how to operate the controls
  • Always use an MCS accredited installer.