Air Conditioner Installation and Drainage Planning: What to Know Before You Install

Principles for Choosing the Indoor Unit Position

The indoor unit should be installed where cool air can cover the whole room evenly — usually on a wall near the centre of the room, avoiding a position directly above a bed or sofa (prolonged direct airflow is less comfortable for occupants). Sufficient return-air clearance is needed around the unit; the top and sides should not be flush against a corner or ceiling, as this restricts air circulation.

Split-type units are typically installed at a height of 1.8–2.2 metres from the floor — high enough for the remote signal and for cool air to descend and cover the activity zone effectively. Installing too high aids air distribution but increases the difficulty of remote reception and future maintenance access.

Drainage: The Most Easily Overlooked Installation Step

Condensation produced during cooling must be drained away smoothly. The condensate pipe must maintain a continuous downward slope throughout its run (generally a drop of at least 10 mm per metre), allowing water to flow by gravity. Any horizontal section or slight reverse slope will cause condensate to pool inside the pipe, leading to dripping problems over time.

Confirm the discharge point of the condensate pipe before installation: it should connect to the bathroom drain, external wall drain pipe or a designated drainage point — not somewhere prone to standing water or likely to affect others. In older buildings where installation positions are restricted, a long pipe run may require an electric condensate pump as a supplement, but the pump's own maintenance requirements and potential failure should be factored into the long-term plan.

After installation, test whether drainage is working correctly: pour water into the condensate tray and observe whether it drains away promptly, rather than discovering a dripping problem only after normal use begins.

Outdoor Unit Position and Ventilation Requirements

The outdoor unit needs sufficient clearance for heat dissipation. There should be at least 300–500 mm of free space in front of the unit (the discharge side) and the intake side at the rear should also be unobstructed. If the outdoor unit is enclosed in a sealed space or a poorly ventilated corner, poor heat dissipation increases the compressor load and accelerates ageing over time.

Many older split units in Hong Kong are installed on external wall brackets, and the outdoor unit position is often constrained by building rules or available space. Before installation, confirm whether building management approval is required and whether the bracket's load capacity meets the unit's weight specification. Some buildings prohibit installation in certain positions — clarify this before committing.

The refrigerant pipe length between the indoor and outdoor unit has an upper limit that varies by model. Exceeding the limit may require additional refrigerant or a redesign; excessively long pipe runs also reduce cooling efficiency. A technician should confirm the actual pipe run distance before installation.

Special Considerations for Window Unit Installation

Window unit installation requires confirming that the window ledge dimensions match the unit, and that the unit is secured with an appropriate bracket with a slight outward tilt (typically 3–5 degrees), so that condensate drains from the rear of the unit to the outside rather than into the room. The bracket's load capacity and fixing method must meet safety requirements; older brackets should be inspected periodically for loosening.

When a window unit is installed, gaps between the unit body and window frame must be properly sealed to prevent warm air infiltration. Also confirm that the window can still open normally after installation — some units prevent the window from opening fully once installed, which requires planning the ventilation arrangement in advance.

Pre-Installation Planning Checklist

• Confirm the indoor unit's installation position, allowing for return-air space and cooling coverage
• Plan the condensate pipe run, ensuring a continuous downward slope with no reverse-slope or horizontal sections
• Confirm the condensate pipe discharge point — avoid areas prone to standing water or affecting others
• Assess whether the outdoor unit's position has adequate ventilation and heat-dissipation clearance
• Check with building management for any installation restrictions or approval requirements
• Confirm the refrigerant pipe run length does not exceed the model's specified maximum
• After installation, test drainage flow and carry out a functional startup test

Frequently asked questions