Placing condensing units on terraces and rooftops

Airflow first

A condensing unit pulls air across its coil and blows the warmed air out. On a roof, the risk is recirculation: the hot discharge air hits a parapet, water tank or wall and comes straight back into the intake. The unit then condenses against air several degrees hotter than the true ambient.

Keep the discharge pointing to open sky or open space, maintain the clearances in the manufacturer's manual and do not crowd several units together facing each other. Leave space between units and other plant such as AC outdoor units, DG set exhausts and kitchen exhaust outlets.

Sun, heat and the roof surface

Indian roofs get extremely hot in summer, and air just above a sunlit concrete slab is hotter still. Raising the unit on a frame lifts it out of that layer. A ventilated shade canopy reduces direct solar gain on the coil and casing, as long as it is high enough and open enough not to trap discharge air.

The north side of a water tank or stair cabin, where the unit is shaded in the afternoon, is usually better than the open south side.

Size the condensing unit for the real rooftop conditions, not the city's average weather. On a May afternoon the air entering a rooftop coil can be hotter than the weather report, and a unit chosen at a mild design ambient will lose capacity just when the cold room needs it most. Tell your designer exactly where the unit will sit.

Structure and waterproofing

• Confirm with a structural engineer that the roof can carry the unit, frame and any future additions.
• Mount on a steel frame or concrete plinths so the unit sits clear of standing water and roof drains.
• Do not drill through roof waterproofing without a proper detail; leaks into the floor below are expensive.
• Use anti-vibration mounts or pads to stop compressor vibration transmitting into the slab.
• Secure the unit against strong winds, especially before the monsoon.

Pipe runs and height

Longer and taller pipe runs between the cold room and the roof affect refrigerant charge, pressure drop and oil return to the compressor. The installer must size and route the pipes for the actual length and height difference, often with oil traps on vertical risers, and insulate the suction line along its whole length. UV-resistant protection over outdoor insulation stops it crumbling in the sun.

Route pipes and cables where they will not be stepped on, used as handholds or damaged by people working on water tanks and dish antennas.

Noise and neighbours

Compressors and fans run through the night. Under India's Noise Pollution (Regulation and Control) Rules, 2000, ambient noise limits are lower at night (10 p.m. to 6 a.m.) and lower in residential areas than commercial ones. A unit beside a neighbour's bedroom window can cause complaints even when the equipment is normal.

Place units away from neighbouring windows, use the parapet or a stair cabin as a barrier where airflow allows, and consider low-noise fan options. Do not wrap units in enclosures that block airflow.

Service access and safety

• Safe access to the roof for technicians: a proper staircase or ladder, not a climb over water tanks.
• Working space around the unit for maintenance and coil cleaning.
• A lockable weatherproof isolator next to the unit, installed by a licensed electrician.
• Correct earthing, and lightning protection for the building where required.
• Guard against tampering and theft of copper pipe and cable.

Monsoon and dust

Outdoor units are designed for weather, but their electrical boxes must stay sealed and cable entries must face downwards. Check before each monsoon that drains around the plinth are clear. Rooftop dust and pollen clog condenser coils; clean them on a schedule, more often in dry, dusty months.