Why fan coil units leak, and how to stop it before the ceiling stains

September 22, 2026 · By the Shax HVAC team

1. A blocked condensate drain

In cooling season the coil removes moisture from the air. That water falls into the drain pan and leaves through a small drain line. Dust, slime and bits of insulation block the line, the pan fills, and water overflows into the wall.

Prevention: clean the pan and flush the line at every service. Drain pan tablets help between visits.

2. A rusted drain pan

Older pans are galvanized steel. After fifteen to twenty years the coating fails and the pan rusts through. A pan that shows rust flakes should be coated or replaced before the next cooling season.

3. A failed valve or connection

The control valve and the hoses that connect the unit to the building risers are under pressure all year. A weeping valve or a bulging hose is an urgent repair: unlike condensate, this water does not stop on its own.

4. Wet insulation

Once the liner inside the cabinet gets wet it holds moisture against the metal, grows mold and sheds particles into the drain. Replacing the liner is a small job that prevents the other three problems.

What to do when a unit is leaking now

Switch the unit off at the thermostat, place a container under the drip and call the management office. If the water is clear and steady, ask for the riser valves to be closed. Then call us: the emergency line is answered day and night.