How to drain air-conditioning condensate without it dripping

The condensation water from an air conditioner can be considerable on very humid days, and if the drain isn't properly dealt with, the result is a leak inside the home that seems a mystery until the air-conditioning installation is checked.

Gravity drain: the standard solution

The unit's condensate must drain through a thin PVC pipe, Ø16–20 mm, from the indoor unit to the nearest drain. A 10% margin is added to the measured length.

When a condensate pump is needed

If the nearest drain is higher than the outlet of the indoor unit — the typical case of a ceiling cassette — the water can't drain by gravity alone. This is where the condensate mini-pump comes in, which lifts the water to the drain point.

It is practically mandatory on ceiling cassettes and highly advisable on any installation where a gravity run is not possible. An important safety detail: the pump must include a small tank with a float that cuts off the unit if the pump fails — that way you get a shutdown rather than a leak, instead of not noticing the fault until the stain appears on the ceiling.

Worked example

For 5 metres of condensate drain from the indoor unit:

1. Drain length: 5 m
2. Total drain pipe (with margin): 5.5 m

This is the same calculation the PresupuestoYa calculator applies automatically for the AC Technician trade.

Frequently asked questions

When do I need a condensate pump instead of a gravity drain? When the nearest drain is higher than the outlet of the indoor unit — typical with ceiling cassettes, where the water can't run down on its own.

Why must the condensate pump have a float? Because if the pump fails and there's no float to cut off the unit, the water keeps accumulating and ends up dripping from the ceiling or wall instead of stopping in time.

What pipe diameter is used for the condensate drain? Between 16 and 20 mm, a thin PVC pipe specific to this use, different from the general plumbing drain.

Conclusion

Dealing properly with the condensate drain —with a pump and a safety float when the height difference demands it— avoids the most annoying problem of a badly quoted air-conditioning installation: a leak that appears months after the job is finished. Quoting it as its own item, not as a minor detail, avoids that complaint.