Do you need a drain for air conditioning?
Yes, every cooling system produces condensate, litres of water a day in humid weather, and it must go somewhere deliberate: ideally a gravity fall through a small pipe to outside, otherwise a condensate pump. The drain is a design decision made at survey, because positions that force a pump add cost, a little noise and a service item.
The water is the by-product of dehumidification: moisture from the room's air condenses on the cold indoor coil, collects in a tray and leaves through a 16 to 21mm pipe. On a muggy London day a working unit fills bottles, not thimbles, so 'it can just drip somewhere' is not a plan; it is a future stain.
Gravity is the gold standard: a continuous gentle fall from the tray, through the wall, to a discharge outside, into a drain, a gully or clear ground. It is silent, unpowered and unblockable short of real neglect. Good indoor-unit positioning often amounts to choosing the wall that makes this fall possible, one of those quiet survey decisions that pays off for fifteen years.
Where gravity cannot work, an internal wall, a basement room, a long horizontal run in a mansion-block flat, a condensate pump lifts the water to a viable route. Modern mini-pumps are small and quiet, a soft purr on humid days, but they are a moving part: they add £150 to £250, they need their float and line checked at the annual service, and a failed pump announces itself as a drip from the indoor unit.
Aftercare is minimal but real: the drain line and trap get cleared and flow-tested at the annual service, because the same biofilm that makes coils smell will eventually block a neglected line. A dripping indoor unit almost always means a blocked drain or failed pump, one of the cheapest faults we fix and one of the most damaging to ignore.
More in installation & aftercare, or browse all 40 questions on the FAQ index.
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