Answer by Dale Blundell (Atkins)
Air to water heat pumps transfer heat from the external air to wet heating systems. They can either be single packaged units with ducting, or split units with separate evaporator and condenser.
Air to air heat pumps provide warm air that is blown into rooms through ducting. It is unusual for these to also provide a source of hot water. Sometimes these are reversible, thus providing air conditioning in warmer periods.
GSHPs are sometimes also referred to as Geothermal heat pumps, but this is a bit misleading as they do not use the much higher temperatures found in true geothermal heating. Ground source heat pumps work on the same principle as air source heat pumps, except the evaporator is a pipe system in the ground, not an air-based heat exchanger. Closed loop systems are most common, and come in horizontal and vertical options:
Closed Loop Horizontal system: Pipes laid in loops within horizontal trenches typically 1-2m deep, ideal where there is adequate space.
Closed Loop vertical system: Less space required than horizontal systems, but specialist drilling rig required to create boreholes typically 120-150mm diameter, and up to around 100m deep.
Open Loop vertical systems: In these systems, water is taken and returned to an underground aquifer, and is not constrained to sealed pipework. These are both more efficient, and cheaper to install, compared to closed loop systems, but the initial site investigation is more costly and performance is based on the properties of the aquifer. A license to abstract water may be required.
If used only for heating OR cooling, the local ground temperature will be changed, leading to a decrease in efficiency. It is best practice to run these to provide both heat and chilling over the year, periodically reversing the impact on the local soil conditions.
A water source heat pumps (WSHP) is designed to extract heat from either a moving or stationary body of water such as a river or lake. Again, as for GSHP these can be open, or closed loop systems, where, in the latter, a heat exchanger (the evaporator) is placed in the water body and the working fluid is pumped through that. In an open loop system the river water is pumped to the heat pump then returned to the river.