Put the outdoor unit where air moves freely: manufacturers typically require 300 mm behind, 150–300 mm at the sides, 500 mm above and 1–2 m clear in front of the fan. Raise it 200–300 mm off the ground with a soakaway for condensate, and aim the discharge away from bedroom windows. Recirculated cold air costs roughly 2–3% of COP per degree of intake depression.
How much clearance does an outdoor unit need?
Enough that it never breathes its own exhaust. An air source heat pump moves 2,000–5,000 m³ of air an hour and cools it by 4–8 °C, so the discharge is a large, cold plume. If a wall, fence or recess bounces that plume back into the intake, the machine behaves as though the whole neighbourhood got colder — capacity falls, defrost cycles start earlier and the COP drops by roughly 2–3% for every degree of intake depression.
Typical manufacturer minima, which you should always confirm in the specific unit's manual:
| Face | Typical minimum clearance |
|---|---|
| Rear (air intake) | 300 mm |
| Sides | 150 – 300 mm |
| Above | 500 mm, more under a solid canopy |
| Front (fan discharge) | 1,000 – 2,000 mm |
| Service side | 500 mm for access to the electrical panel |
| Below (base) | 150 – 300 mm above finished ground level |
Two units side by side need their combined discharge accounted for, not simply the same clearance twice.
Which wall — sheltered, sunny or exposed?
Prioritise free airflow over compass direction. The intake air temperature difference between a north and a south wall is a fraction of a degree in winter, while a badly ventilated position can cost several degrees. Avoid:
- Internal corners formed by two walls, which trap the discharge plume and add up to 6 dB of reflected noise.
- Narrow side alleys between houses, the single most common cause of recirculation and neighbour noise complaints, because sound reflects off both walls.
- Facing the prevailing wind head-on, which loads the fan and reduces airflow; a side-on or sheltered orientation is better.
- Under a low balcony or a solid canopy, unless the vertical clearance is generous. Warm air cannot escape and cold air recirculates.
Good positions are an open external wall with clear ground in front, a gable end away from bedrooms, or a free-standing plinth in the garden with a short, well-insulated pipe run back to the house.
Can the unit be boxed in or screened?
Screened yes, boxed in no. A slatted acoustic fence that breaks the line of sight to a neighbour's window while leaving the intake and discharge paths open is the standard mitigation, and it can shave several decibels at the assessment position without measurable performance loss. A sealed enclosure, a dense hedge grown right up to the casing, or a "decorative cover" over the fan grille all restrict airflow and will show up as reduced output and premature defrosting.
Remember that noise falls about 6 dB for each doubling of distance in free field, while a hard wall directly behind the unit adds up to 3 dB. Moving the unit three metres further from a boundary is usually worth more than any acoustic product. See how loud is a heat pump? and do I need planning permission for a heat pump?.
How high off the ground, and where does the condensate go?
Mount the unit 200–300 mm clear of finished ground level — higher where snow lies — on a level concrete plinth or ground-anchored frame with anti-vibration mounts. Wall brackets look tidy but transmit structure-borne vibration into the building, and are best avoided on bedroom walls and lightweight construction.
Defrost water is not a detail. In cold damp weather an outdoor unit can shed 5–20 litres a day, and if that lands on a path it becomes a sheet of ice. Route the drain into a gravel-filled soakaway at least 300 mm deep directly under the unit, or into a surface water drain; in freezing climates use a trace-heated drain pan. Never discharge onto a public footway.
How far from the neighbours, and from your own windows?
Aim the discharge across your own garden, not at a boundary, a seating area or a bedroom window — yours or theirs. In cooling mode the same face blows hot air, so a position that works in January can be unpleasant in July. Keep the unit clear of drains, gullies, light wells and openings if it contains R290 propane, which is heavier than air and has a defined exclusion zone in the manufacturer's instructions.
As a manufacturer, Solimpeks would far rather see one of its heat pumps on a plinth in an open garden with ten metres of insulated pipe than tucked neatly into a side alley beside the plant room.
Finally, think about the pipe run. A monoblock carries water outdoors and needs short, well-insulated, freeze-protected pipework; a split carries refrigerant with a maximum line length, typically 15–30 m. Either way, every extra metre of external run is heat lost before it reaches the house.
Frequently asked questions
Can a heat pump outdoor unit be enclosed in a cabinet?
Not in a sealed cabinet. A slatted acoustic screen that keeps intake and discharge paths clear is fine and is the standard noise mitigation. Anything that restricts airflow lowers the effective intake temperature, cuts capacity and triggers earlier defrost cycles.
Does the outdoor unit have to be against a wall?
No. A free-standing plinth in the garden often gives better airflow and lower noise at the house, at the cost of a longer pipe run. Whether wall-mounted or ground-mounted, the priority is unobstructed air in front of the fan and behind the coil.
Which direction should a heat pump face?
Whichever direction gives clear, unobstructed airflow. Compass orientation makes almost no measurable difference to winter performance, while recirculation in a corner or alley can cost several degrees of intake temperature and several per cent of COP.
How much water does a heat pump drip in winter?
Between about 5 and 20 litres a day in cold, damp conditions, all from defrost cycles. It must drain to a gravel soakaway or a surface water drain, never across a path or driveway where it will freeze.