Heat pumps are increasingly in the public eye as an energy-saving alternative to fossil fuel central heating systems. They fall into two basic categories, depending on the method of heat-collection:

  • Air-source
  • Ground source.

Air Source

Air-source heat pumps comprise a compressor unit, mounted externally which draws ambient air through a heat exchanger, taking heat from the air, warming a refrigerant, which is compressed, creating further heat. This refrigerant is then piped to an internal hot water immersion tank and to a heating cluster for conversion into hot (tap) water and hot water for heating.


Schematic of heat pump

Ground Source

Ground-source Heat pumps offer an alternative heat collection system to air-source heat pumps. Like air-source, ground source offer an energy-saving, carbon-free alternative to fossil fuel central heating systems.

Warmth from the ground is collected either via a horizontal collector (near surface) or via a borehole or “vertical loop” system which in either case pumps a compressible refrigerant through the pipework, which absorbs ambient heat from the ground and compresses it, creating further heat. This fluid is then piped to an internal heating cluster for conversion into hot (tap) water and hot water for heating.

Ground source heat pump schematic


Heat-pump Advantages:

  • Heat kilowatt output is 4 times energy input (in theory. In reality over a year it is only about x1.6);
  • Provides hot water heating and central heating;
  • De-carbonises your heating solution;
  • Ground-source “Borehole” types can be used in summer to assist with cooling;
  • Eliminates price uncertainty of using fossil fuels.


Heat Pump Disadvantages:

Heat pumps are very difficult to justify if retrofitted into an existing heating system (e.g. to replace a boiler). In such cases, installation costs are very high, due to the removal of the existing boiler; locating and installing the external compressor unit; locating and installing the central hot water immersion and heating water clusters and running the tubing and wiring between the compressor and the clusters.

In the case of ground-source heat pumps, further costs are implied by the requirement to perform appropriate surveys before drilling boreholes or digging large horizontal swathes through the garden to lay a horizontal system.

Purchase and Installation costs up to £25,000 are possible and consequent pay-back times from energy savings can be 10years and more.

Heat pump consumption is spiky (see image), typically requiring its own connection direct to the mains consumer unit and a separate electrical account is sometimes advised. Above 16kW you should be considering 3-phase.

Heat pump consumption profile


Image shows 16kW heat pump in use in a 4 bedroom house over a given 24 hour period in mid-winter.

Consumption is approximately 60 kW over 22 hours.











Heat distribution

Whether air-source or ground-source, heating distribution within the building is effected using either:

  • your existing wet central heating radiator panels or
  • underfloor hydronic heating.

If the existing radiator system is to be used:

Heating water produced by the heat pump will be 40-50C not 60-80C produced by the boiler.  This temperature drop also reduces the heat energy emitted from the radiator (i.e. it will feel colder than before). To produce the equivalent heat of the old boiler either implies:

  • Increasing the radiator size to maintain output wattage at lower temperature or
  • Running the heat pump longer than originally foreseen.

Householders are usually unwilling to fit larger radiators (an additional expense) so they usually end-up running the system longer and therefore paying higher running costs which narrow the gap between their old boiler costs and their new heat pump – increasing payback timescales.

If underfloor hydronic heating is to be used:

  • Ground floors should be under laid with insulation (to prevent heat radiating downwards into the ground) and ideally a storage-type layer (such as brick) placed over the top in order to create thermal mass (and therefore radiate over time).  This adds significantly to the installation costs and adds several weeks disruption to the house – often completely impractical, unless the householder is willing to move out during the period of the works.
  • As the system contains water, pressurised gas and moving parts, ongoing maintenance is required with the potential for leaks, burst pipes etc.

Other disadvantages:

  • The compressor is noisy during operation, unsightly, and takes up space on an outside wall;
  • Moving parts and the compressor require periodic servicing;
  • Compressor effectiveness reduces as ambient temperature drops;
  • Difficult to retrofit into multiple-occupancy buildings such as blocks of flats, where the ground-source option may be impossible anyway, unless planned from the start of construction.


Heat pump summary:

Heat pumps are worth considering in well-insulated new-builds where the cluster/compressor locations can be planned-in to the house design and installation costs are absorbed as part of the build and radiators are correctly sized from the outset. Ground-source heat pumps are worth considering for commercial new-builds where heating and cooling savings could be considerable and horizontal (or vertical) excavation is already implied in laying the foundations of the building.

Heat pumps are difficult to justify as retrofits. The ground source, external compressor and clusters imply high installation costs.  If underfloor heating is to be used, the cost, timescale and disruption become excessive. If existing wet radiator panels are to be used, you will have to increase their size or reduce energy-savings by having to run the system longer. Insulation of an old house should also be reviewed to avoid further energy-inefficiency. Payback times for retrofits are lengthy.


Herschel Infrared Heating as an alternative to heat pumps:

  • If a house has an existing immersion heater as well as the boiler, (many do), then leave this in place or consider the cost of upgrading to a modern more energy-efficient Immersion making use of overnight Economy 7 rates.
  • The purchase price of Herschel Infrared heating panels throughout the house is competitive relative to the cost of purchasing Heat pump components (compressor, immersion, cluster).
  • The installation costs of Herschel infrared will be significantly less than the cost of installing a heat pump and can be carried out by a general electrician over only a couple of days with minimal disruption to the house.  A heat pump installation requires specification by a qualified heat pump and central heating specialist; a builder will be required to handle aspects of the structural changes to the house and refrigerants must be handled by a qualified refrigerant engineer.
  • In a well insulated new-build house, the energy consumption of Herschel Infrared heating or a heat pump will be at their most efficient – but because of the lower capital costs, the payback on the Infrared system will be quicker.
  • In older properties, heat pump installation costs will be high and without enlarging the existing radiators and proper insulation, the energy-efficiency benefits far harder to achieve, with payback terms of 10 years+.  On the other hand, the installation costs of Herschel infrared will be minimal and the Herschel Infrared will heat walls, floors and ceilings evenly keeping operating costs optimal, making the overall payback term far more attractive.
  • Herschel Far Infrared provides excellent comfort levels – including warming the floor – without needing specialist underfloor heating.
  • No ongoing maintenance is required for Herschel Infrared heating panels. Maintenance requirements for immersion systems are rare and no worse than the existing system being replaced.


Publisher: Herschel Far Infrared