Fixed AC and air-to-air heat pumps
A fixed split system transfers heat through a refrigerant circuit between equipment inside and outside the home. In cooling mode, the indoor unit removes heat from room air and the outdoor unit rejects it outside. A reversible system can change the direction of heat transfer and warm the room in heating mode.
This makes reversible fixed AC a form of air-to-air heat pump. It is different from an air-to-water heat pump, which supplies heated water to radiators, underfloor heating or a cylinder.
Conventional air-to-air systems do not supply a wet heating circuit. They also normally leave DHW to a separate system. Some compatible multi-split platforms can include a hot-water connection, but that is a feature of the exact product combination rather than a standard part of fixed AC.
Portable air conditioners use the same broad refrigeration principle but package the compressor and heat exchangers into a room appliance. They are covered separately in the portable versus fixed air-conditioning guide.
Single-split and multi-split systems
A single-split system connects one indoor unit to one outdoor unit. Cooling or heating another room requires another system or a designed air-distribution route.
A multi-split system connects several permitted indoor units to one compatible outdoor unit. This reduces the number of outdoor units, while making the indoor units dependent on a shared compressor and refrigerant network.
Manufacturer selection data defines:
- which indoor units can be connected
- the permitted total connected capacity
- the output available to a particular combination
- diversity and simultaneous-operation assumptions
- heating and cooling mode restrictions
The capacities printed in the indoor-unit model names cannot simply be added to establish the output available from the outdoor unit. A permitted combination also does not show that each room has been sized correctly.
Multi-split controls vary by platform. Indoor units may have separate set temperatures while still having to operate in the same heating or cooling mode. Separate single-split systems do not share an outdoor unit, although they require more outdoor positions and separate refrigerant routes.
Efficiency, sound and suitability depend on the exact models, system combination, operating conditions and layout rather than the number of outdoor units alone.
Indoor-unit formats
High-wall units discharge directly into a room from high level. They need clear air distribution, suitable wall space and access to filters and service points.
Floor-console units sit low on a wall. They can be used where a high-wall position is unavailable, but the furniture layout and proposed position must still leave the airflow and service access clear.
Compact cassettes sit within a ceiling void and discharge through a visible grille. The void needs enough depth for the unit, drainage and service access.
Concealed ducted units connect to supply and return grilles. Duct resistance, fan duty, return-air provision, acoustic treatment and access all form part of the design. The unit may be concealed, but its grilles and service access remain visible.
The indoor-unit type does not set the capacity. A high-wall and concealed ducted unit can have similar headline output while requiring very different building work and air-distribution design.
Room coverage and whole-home systems
An indoor unit serves the room or designed air path into which it delivers conditioned air. An adjacent closed room does not have a defined heating or cooling output merely because air may sometimes pass through a doorway.
A one-room system can be appropriate where the requirement is limited to a bedroom, living room, home office or another identified space. Several room units or a properly designed ducted system can extend coverage. Each room still needs enough delivered output and airflow for its calculated load.
For whole-home heating under MCS standard MIS 3005-D, the design uses a BS EN 12831-1 heat-load calculation. The outdoor unit or units and total connected indoor-unit capacity must cover the calculated building load at the external design condition. Each habitable room within the standard’s scope also needs an indoor unit with enough output for its room load.
That is a whole-home heating route, not a general description of every reversible mini-split installation. It also does not replace the cooling design.
Cooling load and heating load
Cooling capacity is the rate at which the system can remove heat. It is thermal output in kW, not electrical input and not the physical size of the unit.
A cooling-load calculation considers room volume, glazing, orientation, solar gain, insulation, occupancy, ventilation, infiltration and heat from lighting or equipment. Two rooms with the same floor area can have different loads or reach their peaks at different times.
Heating design uses different indoor and outdoor conditions and accounts for heat loss rather than summer heat gain. A cooling selection does not establish winter output. A whole-home heating selection also needs a separate cooling calculation where cooling forms part of the design.
Product literature may list nominal, minimum and maximum capacity. These figures describe different operating points. The exact indoor and outdoor combination needs enough capacity at the relevant design conditions and a suitable operating range when the load is lower.
The cooling load and sizing entry covers the inputs and measurement boundaries in more detail.
Capacity and efficiency data
EER (energy efficiency ratio) compares cooling output with electrical input at stated rating conditions. COP (coefficient of performance) is the corresponding point-condition ratio for heating.
SEER (seasonal energy efficiency ratio) is a seasonal cooling metric under the applicable product test method. SCOP (seasonal coefficient of performance) is the equivalent seasonal heating metric. Values are comparable only where the test basis, climate basis and complete product configuration match.
The product energy label provides standardised information for comparison. It cannot account for the home’s load, weather, set temperature, controls, installation, cycling, defrost or operating hours, so it is not a forecast of the household’s electricity use.
For a multi-split, the relevant data belongs to the complete indoor and outdoor combination. A value for one tested combination should not be assigned to another arrangement without manufacturer evidence.
Ventilation, humidity and condensate
Fixed split AC primarily recirculates and conditions indoor air. It should not be treated as a substitute for the home’s ventilation system unless the design includes a separate, documented outdoor-air function.
Cooling removes moisture as water condenses on the indoor heat exchanger. The unit therefore needs a condensate drainage route. A gravity drain needs a suitable continuous route and discharge point. Where that is impractical, a purpose-made condensate pump can be used within its stated limits.
Planning and fixed installation work
Fixed AC adds indoor equipment, at least one outdoor unit, refrigerant pipework, electrical work and condensate drainage. Outdoor positions also affect sound, airflow, appearance and access.
Planning rules differ across the UK. The property type, designation, number and position of outdoor units and whether the system heats as well as cools can change the applicable route. In England, the permitted-development route for qualifying domestic ASHP (air source heat pump) work excludes equipment used solely for cooling and has several other conditions.
Work on stationary equipment containing F gas requires the appropriate individual qualification in Great Britain. The electrical installation, Building Regulations position and any DNO (distribution network operator) process are separate from the refrigerant qualification.
The fixed air-conditioning installation guide covers these boundaries from room assessment to commissioning.