Types of Heat Pump

Heat Pumps are extremely versatile and as a result are suitable for a number of different applications. This means there are a number of ways to designate such heat pumps.

System Installed within.
Heat Pump Unit itself.
Motive/generating power input.
Temperature of operation.

The most common description is by way of the system into which it is installed:

By the system it is installed in e.g.

  1. Air Source Heat Pump System:
    Heat is extracted from ambient air drawn across its heat exchanger. Source temperature will be very dependent on prevailing ambient temperature and varies through the year depending on geographic area (typical average is 6-8°C).
  2. Ground Source Heat Pump System:
    A closed pipe-work loop of water/antifreeze solution is buried in the ground, either vertically via boreholes or horizontally in trenches (coiled or straight lengths) and the heat pump used to extract the heat. Supply temperature will be circa 0-5°C in order to extract heat from the source (c. 10°C) — this is most common form of “Ground source*?/em> where water is passed through the source inside a tube heat exchanger.
    *Closed Loop Ground Source Heat Pumps are themselves water to air or water heat pumps i.e. water source.
    Although this is the most common designation used to describe the heat pump unit it can lead to confusion as the heat pump only sees water as its source and not the ground directly, so it itself is a water source heat pump but can be interfaced with the ground via the water loop array.
  3. Water Source Heat Pump System
    This can be further subdivided:

    1. Closed Loop: a closed pipe-work loop of water/antifreeze solution is sunk into a river, lake or the sea. Supply temperature will be approximately 5°C less than the source at circa 0°C.
    2. Open Loop: where water is actually extracted from a ground Aquifer; and passed through the heat pump (sometimes using a protective passive water-to-water heat exchanger). Source temperatures will be close to ground temperatures unless a heat exchanger is used, hence circa 10°C.
  4. Water Loop Heat Pump System
    These systems use a closed water loop to supply heat and coolth to a building and the loop itself has heated or cooled by a variety of methods. This is not the same as water source.
  5. Exhaust Air System
    Air to Water: recovering heat from exhaust air to space heating and/or hot water before expulsion of air to atmosphere. Used in low energy Passivhaus type applications.

Heat Pump:

Alternatively the description can be of the heat pump unit itself – based simply on its source and delivery media (where the source is mentioned 1st and the delivery mechanism 2nd) and not the system in which it is installed e.g.

  1. Air-to-Water (A-W).
  2. Air-to-Air (A-A).
  3. Water-to-Water (W-W).
  4. Water-to-Air (W-A).
    Water Source (iii. & iv.) can be further subdivided:

    1. Closed Loop: this is the most common form of “Ground source?system when buried.
    2. Open Loop: much less common than closed loop ground source (e.g. ground Aquifer; open lake, river or sea).

This sort of unit designation would lead to better description and slightly less confusion. It also highlights that the key difference between water source heat pumps which work on closed and open loops in the ground is the temperature they work at rather than the medium itself (which is the same – water!).

Motive Power

Heat pumps are also sometimes designated by the fuel source used to drive the process which is most usually electricity (>95%), less commonly natural gas and much less commonly LPG.

Gas driven systems can cause more confusion because the gas can be used in an internal combustion engine to mechanically drive a separate compressor via a drive belt (very similar to automotive ‘air conditioning?unit) gas absorption heat pump.

Flow Temperature

For heat pumps generating hot water in their condensers (i.e. Air- or Water- to Water units) another sub description is the flow temperature capability.

BS EN 14511-2011
Low Temperature:
35°C
Medium Temperature:
45°C
High Temperature:
55°C
Very High Temperature:
>65°C

Just to confuse when most people refer to “high temperature” heat pumps they are often actually referring to Very High Temperature i.e. >65°C.

Note: For heat pumps producing warm air the temperature should not drop below 38°C to prevent risk of drafts.