Air Conditioner Leaking Water?

Air conditioning removes moisture from the air as it cools, and that water normally drains away out of sight. When water starts dripping from the indoor unit, or the system blows warm air or starts to smell, there is usually a simple cause. This guide explains the common ones, what you can safely check yourself and when to call an engineer.

Switch It Off First If

• water is dripping onto electrics, sockets or a ceiling below

• you notice a burning or electrical smell

• the unit is making a new grinding, rattling or buzzing noise

Switch the system off at the controller and, if there is any risk to the electrics, at its isolator switch, then call us on 01772 288651.

Why Is My Air Conditioner Leaking Water?

A blocked condensate drain

This is one of the most common causes. Dust, mould and slime build up in the drain tray or pipe until water backs up and overflows from the indoor unit. An engineer can clear and clean the drain.

Dirty filters or a frozen coil

Blocked filters reduce the airflow over the indoor coil, which can then get cold enough to ice up. When the ice melts, there can be more water than the drain tray can handle. Low refrigerant, caused by a leak, can lead to the same icing.

A failed condensate pump

Where water cannot drain away by gravity, a small pump lifts it. If the pump fails or its float switch sticks, water overflows from the unit.

A drain pipe problem

A drain pipe without enough fall, a kinked or disconnected pipe, or an outside end that is blocked or frozen can all stop water draining away.

The unit is not level

If the indoor unit was not fitted level, or the bracket has moved, water can run to the wrong end of the tray and drip out.

Water from the outdoor unit

Water dripping from the outdoor unit in heating mode is normal. It comes from the defrost cycle, which melts ice off the outdoor coil.

What You Can Safely Check Yourself

1. Clean the filters, following your user manual. On most wall units they lift out from behind the front panel and can be rinsed and dried.

2. Check that the outside end of the drain pipe is not blocked, crushed or frozen.

3. Make sure the outdoor unit is clear of leaves, plants and anything stacked against it.

4. Check the mode and set temperature on the controller.

Do not open the casing, pour chemicals into the drain or touch the refrigerant connections. By law, work on the refrigerant circuit must be carried out by an F-Gas certified engineer.

Why Is My Air Conditioner Blowing Warm Air?

• The wrong mode or setting. Check the unit is on cool rather than fan, dry or heat, and that the set temperature is below the room temperature.

• Dirty filters or a blocked outdoor unit. Restricted airflow reduces cooling.

• Defrost in winter. In heating mode, the indoor fan may stop or blow cooler air for a few minutes while the outdoor unit defrosts. This is normal.

• Low refrigerant. Refrigerant does not get used up, so low refrigerant means there is a leak. An engineer needs to find and repair it.

• A fault with the compressor, the outdoor fan or a sensor. Many units show a flashing light or error code, so note it down before you call us.

Why Does My Air Conditioner Smell?

• A musty or damp smell usually means mould or bacteria in the coil, drain tray or fan. A clean and sanitise during a service usually solves it.

• A drain or sewage smell can come from a condensate drain connected to a waste pipe without a suitable trap.

• A burning or electrical smell needs attention straight away. Switch the system off at the isolator and call us.

Prevent Problems With Regular Servicing

Most of these problems are caught early by a regular service. Every service we carry out includes cleaning the condensate tray and line, testing the drain pump, cleaning and sanitising the indoor coil, cleaning the filters and checking for refrigerant leaks. See our air conditioning servicing page and our guide to the top signs your AC system needs repair.

If your system is old and the problems keep coming back, our guide to replacing old air conditioning may help.

Frequently Asked Questions

Is a leaking air conditioner dangerous?

The water itself is just condensate, but it can damage ceilings, walls and electrics, so switch the system off and get it looked at.

Can I keep using it while it leaks?

It is best not to. Running it will keep producing water. Switch it off until the cause is fixed.

If your system is leaking, blowing warm air or smells, call 01772 288651 or use the form below and we will arrange a visit.

Last updated: November 2026.