Dripping Water or a Musty Smell from Your Air Conditioning

Why an indoor unit drips

When air conditioning cools a room, it dries it as well. Warm room air passes over a cold coil inside the indoor unit, and the moisture in it condenses on the coil, as it does on a cold glass. That water, called condensate, runs down into a tray under the coil and away through a small drain pipe, usually to the outside or into a waste pipe.

On a humid day there is far more of it, which is why a unit that copes for most of the year can start to drip in a muggy week. It drips indoors when the water cannot get away:

• The drain is blocked. Dust that gets past the filters mixes with the water into a slime that grows in the tray and the pipe. This is the usual cause.
• The pipe is kinked or squashed, or no longer runs downhill. Water sits in it and backs up into the tray.
• The end of the pipe is blocked outside. Leaves or a nest can stop it.
• The drain pump has stopped. Where the pipe cannot run downhill, as in most ceiling cassettes, a small pump lifts the water. If it fails or blocks, the tray fills.
• Ice inside the unit is melting. Blocked filters or a system short of refrigerant can freeze the coil, and the ice drips as it thaws.

In heating it is the outdoor unit that makes the water. A puddle under the outdoor unit in cold weather is frost melting off in a defrost, and normal: see defrost in our winter heating guide.

What to do about a drip

A unit that is dripping now needs looking at before it is used again, so call us on 0161 987 7786 or message us on WhatsApp on 07766 395693, and we will tell you what it needs.

If there is no drip and no smell, but the system is not cooling or heating as it should, see what to check before you call.

Where the musty smell comes from

The coil is wet whenever the unit is cooling. Just behind it, in the dark, sits the barrel fan: the long, drum-shaped fan that pushes the air out into the room, also called the blower wheel or blower barrel. Fine dust that the filters let through sticks to both.

Wet dust feeds mould and bacteria, and the smell is theirs: musty or sour, and strongest in the first minutes after you switch on. You can often see it. While the flap is open, shine a torch up into the slot the air comes out of, from the floor, without reaching in or pushing the flap: black speckles on the flap or on the blades of the fan are usually mould.

What helps

• Rinse the filters every two weeks to a month while the system is in regular use, heating or cooling, and let them dry before they go back. Your manual gives the interval for your model. Switch the power off at the isolator before you open the front panel.
• Run the fan after cooling. Half an hour or so in fan-only mode dries the coil before the unit is switched off. Some units do this by themselves, and your manual will say if yours does.

Neither reaches the barrel fan. Once mould has taken hold there it has to be cleaned off, and an air freshener or a disinfectant spray will not do it: a spray covers the smell without removing what causes it, and liquid sprayed into the unit can reach its electrics.

The same happens in a ceiling cassette or a ducted unit. Only the shape of the fan and the route of the drain are different. If you are not sure which you have, see the types of air conditioning unit.

What an annual service and a deep clean each do

We offer two levels of service, and both deal with the drain and the dirt. This is what each one includes:

Both services include an inspection of the outdoor unit, not a clean. Cleaning it is optional: a full strip-down, where we take the panels off, clean the coil and fan, and refit it.

For a drip, the part that matters is the drain flush, which both levels include. It clears the slime out of the tray and the pipe, which is the usual cause, and the test run shows the water getting away. If the cause turns out to be the pump, the run of the pipe or a shortage of refrigerant, that is a repair, and we tell you what it involves and what it will cost before we do it.

For a smell, the annual service cleans the coil and the barrel fan where they sit, which is usually enough for a unit that is serviced every year. But a barrel fan has dozens of narrow, curved blades, and cleaning it in place only reaches so far. Where a unit has gone years without a service, the fan has to come out. That is the deep clean: the fan is taken out and cleaned, and everything in the annual service is done as well.

Our price page shows the price of each for your system, and you can ask for a visit there. If you are not sure which one you need, message us a photo of the fan, taken while the flap is open and without pushing it, and we will tell you. If the unit is dripping now, call or message us first: we will tell you whether it needs a service or a repair visit.

A burning or fishy smell is different

A smell of burning, of hot plastic or of fish is not dirt. Electrical parts give off these smells as they overheat. Switch the system off at its isolator, leave it off, and call us on 0161 987 7786 or message us on WhatsApp on 07766 395693. Do not switch it back on to see whether the smell has gone.

If you see smoke or flames, or a burning smell carries on with the power off, leave the room, close the door and call 999. If you cannot reach the isolator safely, switch the system’s breaker off at the fuse box instead.

A sweet or chemical smell can be refrigerant: switch the system off at its isolator, open a window and call us. A smell of gas is never the air conditioning, which uses none: call the National Gas Emergency Service on 0800 111 999.

Frequently asked questions

Related Articles