An indoor air conditioner removes moisture from the air while cooling, and that condensate needs a continuous, durable route to a suitable discharge point. Gravity drainage is usually preferred because it has fewer moving parts, but it needs adequate fall throughout the route. A condensate pump can lift water where gravity is impossible, although it adds noise, failure risk and maintenance. Every route should be tested under realistic operation and remain accessible for cleaning.
Where condensate comes from
When warm humid air passes over a cold indoor coil, water can condense and collect in the drain tray. The amount varies with indoor humidity, cooling load, airflow and operating conditions, so a dry test on a mild day may not represent summer use.
Condensate is separate from refrigerant. Water at the indoor unit often indicates a drain, airflow, insulation or cleanliness problem, while refrigerant faults require different diagnosis by qualified personnel.
Designing a gravity drain
A gravity route should maintain the fall required by the equipment instructions without dips, backfalls or crushed pipe. Supports, joints and termination need to remain secure as the building and pipework move with temperature.
The discharge must not stain masonry, drip onto neighbours, wet electrical equipment or create slippery paths. Connections to building drainage need appropriate design so smells, contamination or backflow do not reach the indoor unit.
- Show the complete drain route on the proposal.
- Keep joints and cleaning points accessible.
- Insulate cold surfaces where condensation could form.
- Test the tray, pipe and final discharge before handover.
When a condensate pump is used
A pump may be needed when the indoor unit is below the available drain, the route cannot maintain fall or a concealed installation requires lifting water. Pump capacity, lift, duty cycle and installation orientation must suit the application.
Pumps can be audible in quiet bedrooms and offices. Floats and reservoirs can become dirty, and a blocked or failed pump can cause overflow. Where supported by the design, a high-level alarm or shutdown arrangement can reduce damage risk but does not replace maintenance.
Maintenance and cold-weather details
Drain trays, strainers, pumps and pipework should be checked and cleaned at intervals suited to use and manufacturer instructions. Slime, dust and biological growth can restrict flow even when filters look acceptable.
External routes need protection from damage and freezing where relevant. Reversible systems can also produce water outdoors during heating and defrost, so the outdoor unit's drainage and icy-surface risk need separate consideration.
Frequently asked questions
Can condensate simply drip onto the ground?
Only if the discharge is suitable and creates no nuisance, staining, erosion, slip risk or damage. Dense London sites and shared paths often require a more controlled route.
Are condensate pumps noisy?
They can be noticeable, especially at night. Sound varies by pump, mounting, lift and water delivery, so discuss location and operation before installation.
Can the drain connect to a waste pipe?
Potentially, but the connection needs competent design to prevent smells, backflow and hygiene problems and to satisfy applicable drainage requirements.
How can a pump failure be contained?
Accessible maintenance, correct sizing and any manufacturer-supported alarm or shutdown control can reduce risk. The system should never rely on an inaccessible pump with no overflow strategy.
Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.