The AC condenses water at the coil; it collects in a drain pan and is piped away by a condensate line. Problems: the line blocks with dust, algae or debris and the pan overflows; the line has too little fall or a sag that holds water; the pan is cracked or corroded; or the trap dries out and lets sewer gas and pests in. Standing water anywhere in this path grows biofilm and mold, and an overflow wets the surrounding structure.
Key takeaways
- A blocked condensate drain is one of the most common AC-mold causes.
- Standing water in the pan or a sagging line grows biofilm.
- An overflowing pan wets the ceiling or wall around the unit.
- Have the drain and pan checked at every AC service, more often in summer.
The condensate path
Warm humid air over the cold coil condenses into water that drips into a drain pan under the coil. From there a condensate pipe carries it — by gravity or a small pump — to a drain. A trap in the line stops air being pulled back through it. In a Dubai summer a single split unit can produce many litres a day, so every part of this path matters.
How it fails
- Blockage — dust, algae and slime build up and plug the line or the pan outlet; the pan fills and overflows.
- Poor fall or a sag — a section of pipe that does not drain holds water permanently.
- Cracked or corroded pan — leaks straight onto the ceiling below.
- Failed condensate pump — where one is fitted, it stops and the pan backs up.
- Dry trap — on a system that has been off, the trap evaporates and odours and insects enter.
Warning signs
- Water dripping from a diffuser or the edge of an air handler.
- A ceiling stain that grows below or beside the indoor unit.
- Gurgling from the unit, or a sewer-like smell.
- The float switch (a safety cut-out on some units) tripping the AC off.
Keeping it clear
- Have the drain pan and condensate line checked and flushed at every AC service.
- In peak summer, a mid-season check is worthwhile on units that have blocked before.
- Keep the coil and filter clean — a fouled coil sheds debris into the pan.
- If a unit has been off for months, prime the trap before relying on it.
A float switch that keeps tripping your AC off is not a fault to bypass — it is telling you the drain pan is full. Get the drainage cleared.