How Heat Pumps Work

Background

Heat pumps are based on conventional refrigeration technology, converting a high volume of low grade heat from the environment to a low volume of higher grade heat. The heating and hot water system operate exactly the same as they do with a boiler.

Heat Pumps can use various sources of free energy but the most common sources are ground (using solar heat from the earth) and air (ambient air). These energy sources are inexhaustible. However, the heat pump will also require electricity to run, which can also come from a renewable source.

They provide lower flow temperature water (up to 55oC generally) compared to a gas or oil boiler (generally 70-82oC). Nb Heat pumps are not air conditioning units, cooling optimised devices, or always able to provide domestic hot water as a sole source.

Different technologies

All heat pumps require the pump itself plus a distribution system, in the same way that a conventional heating system requires a boiler and a distribution system. Ground source heat pumps also require a ground loop.

How the heat pump works

In the same way that your fridge uses refrigerant to extract heat from the inside, keeping your food cool, a ground source heat pump extracts heat from the ground, and uses it to heat your home. A ground source heat pump has three main parts:

• The evaporator, (e.g. the squiggly thing in the cold part of your fridge) absorbs the heat using the liquid in the ground loop;
• The compressor, (this is what makes the noise in a fridge) moves the refrigerant round the heat pump and compresses the gaseous refrigerant to the temperature needed for the heat distribution circuit;
• The condenser, (the hot part at the back of your fridge) gives up heat to a hot water tank which feeds the distribution system. Coefficient of performance (CoP)
• Electricity is used to drive the compressor within the heat pump cycle. As long as the efficiency of the heat pump is greater than 100%, it is energy efficient.
• Efficiencies of 250-500% are generally available in the UK.
• Example: A typical 4kW GSHP is 320% efficient, as it gives 3.2kW of useful heat for every 1kW of electricity used. It has a CoP of 3.2.3
  

Different types of heat pump

Heat pumps are normally classified by their heat source and means of delivering the heat: ground source and air source are the most common types, although water source heat pumps are available.

Ground source

Ground source to water: provide useful heat throughunder floor heating systems and medium temperature radiators.

Land area required for ground loops – (guide only)

Semi Detached – 98 to 225 m2
Detached – 150 to 340 m2

Land area requirements vary depending on the insulation levels and heating requirements of the house.

The ground loop itself is comprised of lengths of pipe buried in the ground, either in a borehole or a horizontal trench. The pipe is usually a closed circuit and is filled with a mixture of water and antifreeze, which is pumped around the pipe absorbing heat from the ground. The ground loop can be:

• Vertical, for use in boreholes.
• Horizontal, for use in trenches.
• Spiral, coil or ‘slinky’, also for use in trenches.

Air source

Air source heat pumps can provide efficient benefits at temperatures as low as -20oC. Average temperatures in the UK range from 2 oC to 22 oC. The record low was -10 oC. They do not require a ground loop.

• Air to water: Commonly used to provide domestic hot water and space heating for dwellings.
• Air to air: Air is used as the low grade heat source and air is also how the heat is delivered to the space. These are commonly used as reverse cycle heat pumps (providing both heating and cooling).

Heating distribution systems

• Radiator systems designed for boilers tend not to be optimal for heat pumps, as they require higher water temperatures than heat pumps provide.
• Underfloor heating is the preferred distribution method.
• Large surface area radiators are an option. As a rule of thumb, a 20oC reduction in flow temperature (typical if compare a heat pump with a gas or oil boiler) requires a 100% oversize of radiator.

Benefits of heat pumps

• If installed with renewable electricity can be considered carbon neutral.
• Unlike gas boilers, there are no required annual maintenance and servicing costs associated with heat pumps.
• Although heat pumps are more expensive than gas and oil boiler equivalents, they only attract 5% VAT.
• Grants are available to reduce the cost for air and ground source heat pumps.

Heat Pumps faqs Savings potential of Heat Pumps