Air-to-air heat pumps

Fixed systems that heat and cool rooms by moving conditioned air.

An air-to-air heat pump transfers heat between outside air and the rooms it serves. In heating mode it extracts heat outdoors and releases it indoors. Reversible equipment can also provide cooling, which is why air-to-air systems overlap with fixed AC.

Space heating and domestic hot water

Conventional air-to-air equipment heats the air in a room rather than water for a wet heating circuit. It normally leaves domestic hot water to another system. Some compatible multi-split platforms can connect a hot-water tank, so the product combination defines whether space heating and hot water are both included. A conventional air-to-air installation still needs separate domestic hot-water provision.

Whole-home heating design

For a whole-home design under MCS standard MIS 3005-D, the heat load is calculated using BS EN 12831-1. The outdoor unit or units must cover the calculated building load at the external design condition. The total connected indoor-unit capacity must also cover that load, and each habitable room needs an indoor unit with enough output for its calculated room load.

The design should also maintain the required room temperatures through multiple defrost cycles. MIS 3005-D presents this as a design requirement rather than a separate certified defrost-performance value.

Multi-split capacity

For a multi-split system, the outdoor unit’s available capacity cannot be found by adding the headline capacities of the connected indoor units. The manufacturer specifies permitted combinations, connected-capacity limits and the output available to each combination.

The room and building loads remain the basis of the design. A permitted product combination does not establish that its indoor units meet those loads.

Refrigerant work

Split air-to-air equipment uses refrigerant connections between its indoor and outdoor units. In Great Britain, work on stationary equipment containing F gas requires the appropriate qualification for the activity.

Boiler Upgrade Scheme position

As at 28 July 2026, Boiler Upgrade Scheme guidance for England and Wales lists air-to-air heat pumps as a separate grant category. Eligibility still depends on the home, product, installer and current scheme conditions.

Whole-home design record

The MCS whole-home design record includes:

the building and room heat loads, including the design temperatures
the output of each indoor unit at the relevant design condition
the permitted indoor and outdoor unit combination
how the system deals with defrost at the external design condition
the domestic hot-water system

Cooling load, condensate, planning and outdoor-unit siting are separate design subjects covered in the linked entries.

Applies to

Heat

Last reviewed

28 Jul 2026