Air conditioner [Water dripping from the indoor unit]what to check

This is the most reported air conditioning problem in a British home and it has a published answer that will save you money. Cooling air makes water. The indoor unit is pulling moisture out of the room by design, that water collects in a tray underneath the coil, and it has to get outside. When it appears on your wall or your floor instead, something in that path has stopped working. The instinct, and the thing you will often be told, is that the condensate pump has failed. Mitsubishi address that head on, and their wording is worth having in front of you before anybody quotes for a part: do not assume the drain pump is faulty here, because on their equipment the drain pump runs whenever the unit is in cooling mode, it is a very reliable and robust piece of equipment and does not often fail. The most common cause is a drain pipe or drip tray blockage causing the water to back up, so inspect the drip tray, the drain pipe and the float switch or sensor before ordering parts. That is a manufacturer telling engineers not to sell you a pump.

Before that, though, a question that decides which half of this page applies to you: does your unit have a pump at all? Most wall-mounted domestic splits do not. Daikin’s, Mitsubishi’s and Toshiba’s wall units drain by gravity through a hose that runs out with the pipework — their installation manuals route a drain hose and nothing else — and Mitsubishi’s MSZ wall units have no drain fault in their code table because there is no float switch to raise one. On those the whole question is the hose: blocked at the outside end, kinked, crushed behind the unit, or running uphill somewhere. Pumps and float switches belong to ceiling cassettes, ducted units and wall units where an installer has added a pump to lift the water. Note what has actually happened when a pump-equipped system reports this rather than just leaking.

Mitsubishi publish codes for a drain fault and for a high condensate level in the drip tray, and both are triggered by a float switch detecting water backing up. So the machine is not failing to pump; it is detecting that the water is not going anywhere and stopping to avoid flooding your room. That is protection working. What you can reasonably do yourself is limited to the outside end. The condensate pipe usually exits through the wall behind or below the indoor unit and drips onto a patio or into a drain, and that outlet gets blocked by leaves, algae, insects and nests. Finding it clear or blocked is genuinely useful information, and Daikin give you a test: check that water comes smoothly out of the drain hose while the unit runs in cooling or dry mode, because if none appears, water may be leaking from the indoor unit and their instruction is to stop and consult the service shop. Everything above the outlet — the tray, the float, the pump — is inside a unit with mains voltage in it and belongs to an engineer, and the manufacturers say the same about the leak itself: Daikin list water leaking from the indoor unit among the symptoms for which you turn the breaker off and call the service shop immediately, and Mitsubishi list water leaking or dripping from the indoor unit among the cases in which to stop using the air conditioner and consult the dealer.

1. 1

   Stop the system and turn it off at the breaker or isolator, which is what Daikin and Mitsubishi both tell owners to do when water is leaking from the indoor unit, so nothing is adding water while you look and nothing is live near it.

2. 2

   Find where the condensate pipe comes out of the wall outside — usually behind or just below the indoor unit — and check the end is clear.

3. 3

   Look at whether that pipe runs downhill all the way. A section that has sagged or been knocked upwards holds water and stops the rest draining. Toshiba’s installation instruction is that the hose slopes continuously and is never allowed to go slack, and Daikin’s owner test is that water should come smoothly out of it while the unit cools.

4. 4

   Take the indoor filter out and look at it. A blocked filter ices the coil, and the meltwater arrives faster than the tray can handle it.

5. 5

   Note whether the unit has reported a code and retrieve it if you can. A drain fault and a drain sensor fault look identical from the floor and lead to different repairs.

6. 6

   Call an engineer for anything inside the indoor unit, and mention what you found outside.

   Do not open the indoor unit beyond the filter cover. The tray sits directly under the coil alongside the fan motor and mains wiring, and water and electricity are already in contact in there.