What is an air source heat pump?
An air source heat pump is of the type that extracts heat from the air. See article explaining heat pumps. Within this type, there are several sub-types; Air to Water (A2W), Air to Air (A2A) ie modern Air Conditioning units. Within these sub types, there are usually quite a few different arrangements and components depending on requirements.
A2A Heat pumps
Usually there is 1 outdoor unit and 1 indoor unit, known as a 1 to 1 setup. It is possible however to have 1 outdoor unit to several indoor units. These indoor units can be wall hanging, floor mounted etc. Refrigerant is piped to the indoor units and requires FGAS approval. This is usually the most commonly installed system found in homes and offices.
A2W Heat Pumps
In this setup there usually is 1 outdoor unit, however if the property is large enough, several outdoor units may be required. There is sometimes also an indoor unit that manages the immersion backup, diverting heat to the hot water cylinder or to space heating (radiators / underfloor heating). These type of units are also capable of cooling, however you need all the correct indoor equipment for this to work. These units can be monobloc, ie self contained refrigerant, or split systems ie refrigerant is sent to an indoor unit(s), from there you can have further eqiupment doing various things.
Performance
Air Source Heat Pumps performance varies according to outside temperature. Simplified, the higher the temperature outside, the easier and more efficient it is to pump the heat into the home directly (A2A), indirectly (A2W) or hot water cylinder. Heat pumps efficiency is measure in terms of COP( Coefficient of Performance) or spread over the year SCOP (Coefficient of Performance) .