Rooms, operating modes and design conditions
The design begins with the rooms to be served and whether the system will cool, heat or do both. A single bedroom used for summer cooling has a different scope from a reversible system intended to replace the main space-heating system.
For each room, record the intended temperature, expected occupancy and heat-producing equipment. Proposed changes to glazing, shading, insulation or room layout belong in the same design case if the system depends on them.
Cooling and heating use different calculations. Cooling design deals with solar and internal heat gains as well as heat entering through the fabric and ventilation. Heating design deals with heat loss at the relevant winter conditions.
A fixed system can meet one duty without being suitable for the other. The selected equipment therefore needs separate cooling and heating evidence where both modes form part of the intended use.
Cooling-load calculation
A room’s cooling load cannot be established from floor area alone. The calculation or assessment needs to account for:
- room volume
- glazing area, orientation and shading
- solar gain through glazing and the building fabric
- insulation and construction
- occupancy
- ventilation and infiltration
- lighting, computers and other internal gains
Rooms in the same home can reach their maximum loads at different times. A west-facing bedroom may peak later than an east-facing room, while a kitchen or office can have a substantial internal load.
The output is a room-by-room cooling requirement with its assumptions visible. The selected unit is then checked against that requirement using capacity data for the exact indoor and outdoor combination.
Where the system will provide whole-home heating under MCS standard MIS 3005-D, a BS EN 12831-1 heat-load calculation is also required. The MCS heating calculation is not a substitute for cooling-load work, and MCS 031 b) does not estimate cooling-mode performance.
Selecting the system combination
A single split has one indoor unit and one outdoor unit. A multi-split shares one compatible outdoor unit across several indoor units. Separate single splits and a multi-split can therefore serve the same number of rooms through different equipment and refrigerant layouts.
For a multi-split, manufacturer selection data defines the permitted indoor units, connected-capacity range, diversity, simultaneous output and control restrictions. The model labels on the indoor units do not show how much capacity the shared outdoor unit can supply to each room.
The system schedule should identify:
- exact indoor and outdoor model codes
- permitted manufacturer combination
- cooling and heating capacity at the relevant conditions
- minimum, nominal and maximum capacity where published
- control and simultaneous-operation restrictions
This keeps the product combination and the room-load calculation as separate parts of the design record.
Indoor-unit position and air distribution
The indoor-unit format affects the building work and the way air reaches the room.
A high-wall unit needs wall space, clear discharge and access to the filters. A floor console needs a low-level position that remains clear of furniture and curtains. A ceiling cassette needs enough void depth, a visible grille and service access.
A concealed ducted unit also needs a designed supply and return-air path. Fan duty, duct resistance, grille positions, acoustic treatment and access affect delivered airflow and sound. Concealing the main unit does not remove its service requirements.
An indoor unit does not provide a calculated output to a closed adjacent room. Whole-home coverage needs an indoor unit or designed duct path serving each included space.
Outdoor-unit position and sound
The outdoor unit needs manufacturer clearances and an unobstructed path for intake and discharge air. Restricted airflow or recirculation of discharged air can reduce capacity and efficiency.
The site assessment also covers:
- access for service and eventual replacement
- refrigerant-pipe and cable routes
- indoor condensate and outdoor heating or defrost water
- appearance and planning restrictions
- sound at the relevant neighbouring positions
A manufacturer’s sound-pressure figure is measured under stated conditions. It is not a site noise assessment.
MCS 020 a) Issue 1.1 provides the calculation used by the current English permitted-development route. Its 37.0 dB(A) result uses the standard’s specified assessment method and positions. It is not a universal UK sound limit or a substitute for another planning condition.
Planning across the UK
Planning rules are devolved. The correct route depends on the address, property type, designation, proposed position, number of outdoor units and intended use.
In England, Class G of the General Permitted Development Order can cover some domestic ASHP (air source heat pump) installations. Current conditions include compliance with the referenced MCS Planning Standard and state that the equipment must not be used solely for cooling.
Reversible operation is only one Class G condition. The English conditions also address equipment volume, roof positions, highway-facing locations, listed buildings, scheduled monuments, external appearance and amenity. Planning conditions and Article 4 directions can remove permitted-development rights.
Scotland, Wales and Northern Ireland use their own planning frameworks. An English permitted-development assessment should not be applied to another UK nation.
The final planning evidence must relate to the installed model and position. Moving the outdoor unit or changing its model can change the sound result or planning route.
Refrigerant work and F-gas certification
Split AC uses refrigerant pipework between indoor and outdoor equipment. In Great Britain, anyone installing, servicing, maintaining, leak-checking, recovering gas from or decommissioning stationary F-gas equipment needs the appropriate individual qualification for the activity.
A business that installs, repairs, maintains or decommissions equipment operated by other people also needs company certification. This includes a sole trader working on customers’ equipment.
The qualification category and company certificate need to cover the actual work. They do not establish electrical competence, planning status or MCS certification.
Refrigerant pipe connections and pressure or leak testing are technician work, not household maintenance.
Electrical supply and DNO registration
The electrical design uses the equipment’s electrical input and manufacturer data, not its cooling or heating output in kW. It covers the existing supply, consumer unit, protective devices, isolation, cable route and any ancillary loads.
In England, fixed electrical work remains within Part P whether or not it is notifiable. Installing a new circuit is notifiable. The applicable routes are self-certification by a registered competent person, an eligible third-party certification route arranged before work or building-control involvement.
The appropriate BS 7671 certificate and the Building Regulations compliance or completion evidence are separate records. F-gas certification and an MCS certificate do not replace them.
GOV.UK guidance for England, Scotland and Wales requires heat pumps to be registered with the DNO (distribution network operator). Where a reversible air-to-air system is installed as a heat pump, the installer assesses whether the current DNO route allows connection followed by notification or requires approval before connection.
This DNO registration section applies to England, Scotland and Wales.
Pipework, penetrations and condensate
Refrigerant pipework, interconnection cable and the indoor condensate drain often follow a similar route, but each follows its own product and installation requirements.
In cooling and dehumidification modes, water condenses on the indoor heat exchanger. A gravity drain depends on a suitable continuous route to an appropriate discharge point. Where gravity drainage is impractical, a purpose-made condensate pump can lift the water within its stated head and application limits.
The drain arrangement follows the product instructions and is tested during commissioning. Universal hose diameters, falls and trap details should not be transferred from one product or discharge arrangement to another.
Heating and defrost can produce water at the outdoor unit. That outdoor drainage route is separate from indoor cooling condensate and should not create ice, nuisance or damage.
Commissioning and handover records
Commissioning checks the installed system against the design and manufacturer requirements. The records produced depend on the equipment, work completed and regulatory routes that apply.
Keep those records together. Depending on the installation, they can include:
- exact indoor and outdoor models and design duties
- controls and mode restrictions
- manufacturer commissioning information, including applicable refrigerant and condensate checks
- electrical certificate and Building Regulations evidence
- planning or permitted-development evidence
- DNO response where the heat-pump registration route applies
For a whole-home MCS air-to-air heating project, the applicable MCS design and performance documents form part of the same record set.