Where the water goes: condensate drainage

Every air conditioner that cools also takes moisture out of the air, and that water has to leave the house. A well-planned drain goes unnoticed for years. A poorly planned one shows up as drips down the wall or a green stain on the patio.

Why the units make water

In cooling and dry modes the indoor unit's coil is colder than the dew point of the room air, so moisture condenses on it, the way a cold glass mists up. The water runs into a tray under the coil and out through the drain hose. On a warm, humid day this can add up to several litres. The process is explained in how air conditioning removes humidity.

In heating mode the roles swap. The indoor coil is warm and stays dry, while the outdoor coil gets cold enough to collect moisture from the outside air. In cold, damp weather that moisture freezes, and the unit melts it off in a defrost cycle, releasing water from its base.

Draining by gravity

Where possible, the drain hose simply falls from the indoor unit to the outside. It needs a steady downward slope all the way, with no dips where water can sit and no rises it has to climb. During installation it's taped in at the bottom of the pipe bundle, so the water can flow freely.

Common places for the water to end up:

• An outside drain or gully
• A waste pipe in the house, through a suitable trap
• A soakaway or gravel bed

Places to avoid include paths, steps and patios, where the water causes staining, algae and, in winter, ice. It also shouldn't run down the face of a wall or drip onto a neighbour's property.

Connecting into a waste pipe needs a fitting that stops drain smells coming back up the hose. A trap that relies on standing water can dry out in months when the system isn't cooling, so a waterless trap is often used instead.

Inside the house, the drain hose is usually insulated where it runs through warm spaces. Otherwise the cold water inside can make the outside of the hose sweat.

When a pump is needed

If the water can't fall to an outlet, a condensate pump lifts it to a point where it can drain by gravity. Typical reasons are an indoor unit on an inside wall, pipes that go up into a loft, or a room below ground level.

Small pumps can sit inside the pipe cover beside the indoor unit, or in a ceiling void or cupboard nearby. They need a power supply, and you may hear a quiet hum while they run, which matters most in bedrooms. Many have a safety switch that can stop the unit cooling if the water can't get away, so it shuts down rather than overflowing.

A pump is one more part that needs occasional cleaning and can eventually fail. That's why a good installer uses gravity wherever the route allows.

Water from the outdoor unit

Defrost water pours out through holes in the base of the outdoor unit and falls to the ground beneath. Gravel, a drain or a flower bed are better places for it than a path, because it can freeze in cold weather.

Some outdoor units can take a drain fitting and hose, but a hose can freeze in a cold spell, and some manufacturers advise against one in cold or exposed spots. Raising the unit clear of the ground helps the water get away instead of freezing into the base, as described in wall brackets and ground stands.

Keeping the drain clear

Dust and slime can build up in the drip tray and hose over time and slow the flow. Signs include water dripping from the indoor unit and a musty smell when the unit starts. Water dripping from the indoor unit covers the usual causes.

A routine service normally includes checking that the drain runs freely. At installation, a careful engineer pours water into the drip tray and watches it come out at the other end before handing over.