Outdoor unit siting

An air-source heat pump needs a site where it can move a large volume of air without drawing its own cold exhaust back through the coil. The same position must also work for neighbours, defrost water, maintenance, pipe routes, structural support and refrigerant safety.

The nearest empty patch of wall is not automatically the right answer. A good survey tests several constraints together before fixing the system design.

Airflow and recirculation

The manufacturer specifies minimum clearances at the back, sides, front, top and below the exact outdoor unit. These are not interchangeable between models. A high-output side-discharge unit, a top-discharge unit and a compact fan can need different spaces.

Clearance is only the starting point. The outgoing cold air must escape rather than bounce from a wall, fence, hedge or another unit and return to the inlet. Recirculation lowers the entering-air temperature, which can reduce capacity, increase defrosting and make the fan or compressor work harder.

Courtyards, narrow passages, enclosed balconies, light wells and three-sided recesses need particular care. A decorative screen can create the same problem after handover even if the unit initially met its dimensional clearances.

The design should account for:

• the manufacturer’s free-air and obstruction requirements
• prevailing exposure without treating wind as guaranteed ventilation
• snow, leaves, vegetation and stored items
• exhaust direction relative to paths and doors
• more than one outdoor unit operating together
• the final, not temporary, fence and landscaping layout

MIS 3005-I specifically asks installers to consider chilled-air recirculation and to locate the unit in accordance with its instructions.

Noise and vibration

Sound-power data from the product describes the source. The sound at a neighbour depends on distance, reflections, barriers and operating conditions.

As at 22 July 2026, MCS 020 a) uses a permitted-development noise limit of 37 dB LAeq,5mins at each relevant assessment position, ignoring the effect of the receiving facade. An assessment position is one metre outside the centre of a neighbouring habitable-room door or window. Several floors and neighbouring properties may need checking; the closest opening is not always the loudest result.

The calculation uses the unit’s declared A-weighted sound power, distance, the number of reflecting surfaces and qualifying barriers. An open fence, hedge or incomplete wall is not treated as an acoustic barrier under the method. Placing a unit in a corner can increase the calculated level because more surfaces reflect sound.

Passing the planning calculation does not guarantee that vibration will be inaudible inside the owner’s home. MIS 3005-I advises against positions next to sleeping areas or on floors that transmit vibration, and calls for appropriate anti-vibration mounts and flexible connections. The base or brackets still need enough stiffness to support the unit without movement.

Ground, wall and roof mounting

A ground-mounted unit normally sits on a stable, level structure above surrounding ground and expected water or snow. The support should distribute load, resist settlement and leave the appliance’s drainage openings clear.

Wall brackets can help where ground space is limited, but they transfer mass and compressor vibration into the building. The wall, fixings and bracket need a structural assessment appropriate to the load and substrate. Service access and defrost water remain necessary.

A flat-roof position adds wind, structural, waterproofing, edge-protection and long-term maintenance issues. In England, permitted development also requires every part of a flat-roof unit to be at least one metre from the external roof edge, and a pitched-roof installation is outside the domestic permitted-development right. Other UK nations have their own planning rules.

The fact that a roof position is visually discreet does not prove it is accessible or quiet.

Defrost water

Cold, damp weather can produce substantial short bursts of water during defrost. The base and drain route must carry it to a suitable drain or soakaway without crossing a path, wetting the building or forming ice beneath the unit.

The siting decision should therefore be made with the condensate route, not before it. Long exposed pipe runs, low points and an inaccessible soakaway can turn an otherwise good acoustic position into a winter fault.

R290 protection areas

R290 propane is a higher-flammability refrigerant used by many outdoor units. The Heat Pump Association says that manufacturers can use different safety measures or protection areas, and that their instructions must be followed. There is no one national rectangle that can safely be copied between models.

Within a manufacturer-defined protection area, restrictions can include:

• windows, doors, vents and other building openings
• sockets, switches, lamps and other ignition sources
• drains, gullies, ducts and low-lying recesses
• property boundaries, neighbouring property and public access
• walkways and driveways

The purpose is to prevent leaked refrigerant entering the building, finding an ignition source or collecting in a low point. The on-site assessment should verify the final height and geometry, not only a plan-view distance.

Electrical isolation, condensate drainage and pipe penetrations must all be coordinated with this area.

Pipe and cable routes

The position affects heat loss, pressure drop, refrigerant charge on a split system, frost exposure and appearance. The design should keep routes practical without compromising safe clearances or creating inaccessible joints.

External water pipework on a monobloc needs continuous, weather-resistant insulation and the specified freeze-protection strategy. Cables need mechanical and weather protection. Penetrations should fall outwards or be detailed so water cannot track into the wall.

Short pipework is useful, but it does not override a serious noise, airflow or refrigerant-safety problem.

Maintenance access

An engineer needs safe access to panels, valves, electrical isolation, filters and the fan or coil areas specified by the manufacturer. Maintenance should not require leaning over a drop, entering a neighbour’s land without agreement or dismantling permanent landscaping.

Allow room to remove major components where the manual requires it. A unit can meet running clearances yet be impossible to repair because a wall or fence blocks the service panel.

The coil needs protection from accidental damage without a guard that obstructs airflow. Bin storage, bicycles and garden growth should not be expected to remain clear through good luck.

How the constraints interact

Clearance, noise, vibration, condensate, refrigerant safety, pipe and cable routes, maintenance access and planning status all apply to the same physical position. The applicable evidence may include manufacturer clearance data, an MCS 020 a) calculation, base or bracket details, the condensate route and an R290 protection area.

Moving a unit can solve a noise problem while creating unsafe drainage or an impossible pipe route. No single clearance or noise figure establishes that the complete position is suitable.

Related entries

• Planning constraints for ASHP
• MCS 020 heat pump noise calculation
• Condensate drainage
• Monobloc ASHP units
• Electrical supply for heat pumps