Aircon indoor unit: what it holds and what it gives away
The outdoor unit does the heavy work, but the indoor unit is where you notice the problem. Noise, drip, smell and weak airflow all start or surface behind that front cover, which makes it the one part worth learning to read.
By Team Snowflake | Updated 5 Aug 2026
One air path, and every part sits on it
An indoor unit is a fan pulling room air across a cold surface. Air enters through the grille on the top face, passes the filter, then crosses the coil behind it. The blower barrel sits downstream of the coil and throws that air back into the room through the slot along the bottom front.
The louvre only steers what has already been moved. It sets how far the stream is thrown and where in the room it lands. A flap left flat changes how a bedroom feels without changing anything the machine is doing.
That path is a single line, so a restriction anywhere on it reads the same from the sofa. A loaded filter, a caked coil face and a dust-packed blower all cut the same stream. The air arrives weaker, and the weakness by itself does not say which of the three caused it.
The cold arrives from the other half of the system. Refrigerant reaches the coil cold, picks up heat from the passing air, and hands that heat to the condensing unit outside. The indoor unit makes no cold of its own. It moves room air over a surface that something else is keeping cold.
Judge the stream and the temperature separately
Two complaints separate cleanly on this line, and blending them is where money goes missing. Weak air that is properly cold points inside the casing. Strong air that is barely cool points across the wall, at the refrigerant charge and the outdoor half.
Hold a hand at the vent and take the two readings one at a time. Strength first, then temperature. Most people report a single blended impression of not cold enough, and a blended impression sends the first test to the wrong half of the system.
What lives behind the front cover
Eight things share the space inside. Each has its own page on this site covering how it fails and how it is tested. What is worth carrying away here is narrower: where each one sits on the air path, and what it depends on.
- The filter is a coarse washable mesh at the intake. It guards the coil rather than the air, and it is the first restriction on the path.
- The evaporator coil is the cold fin stack the air crosses. Cooling happens here, and so does the moisture removal that makes a cooled room feel lighter.
- The blower wheel is the long barrel below the coil. It is the only thing generating airflow, and it collects whatever dust gets past the mesh.
- The drain pan sits under the coil and catches what condenses on it. The drain pipe carries that water out through the wall.
- The indoor PCB reads the sensors, drives the fan and the flap, and talks to the outdoor unit over the interconnect wire.
- Two thermistors report temperature. One sits at the return air intake and reads the room, the other clips to the coil and reads the metal.
- The louvre swing motor turns the horizontal flap. It is a small motor carrying a large share of the noise complaints.
- The IR receiver and display sit behind the front panel. Everything the remote asks for arrives through that one window.
The wet side and the dry side
The casing holds two zones that are not supposed to meet. The coil, the pan and the drain path run wet whenever the unit is on. The board, the receiver, the motors and their connectors sit beside them and have to stay dry.
Most of what goes badly wrong indoors is water finding the dry side. A pan that overflows, a drain that backs up, or condensation forming on a cold pipe where the insulation has perished all end at the same place. Water reaches something electrical, and a leak complaint becomes a board complaint.
This is why a persistent drip is not something to live with while deciding. The visible cost is a stained wall. The expensive version is what the water touched on its way down.
Cassette, ducted and floor-standing are the same machine
Format changes the case, not the family of parts. A ceiling cassette draws air through a central grille and throws it out at four edges. A ducted unit hides the assembly above a false ceiling and sends the air away through ductwork. A floor-standing unit stands the same case upright at skirting level.
Access, install and service differ enough that each format gets its own treatment elsewhere. The filter, coil, blower, pan, board and sensors are the same set in all of them.
Where the water comes from
Every drop of water an aircon produces is made at the indoor coil. Room air here carries a heavy load of moisture. When it touches a surface cold enough, that moisture leaves the air and lands on the fins as liquid.
That is not a side effect of cooling. It is half of what makes a cooled room comfortable. The lightness people describe after the unit has been on for a while is moisture that has been taken out of the air and sent down the drain.
Singapore makes the volume large. Warm, humid air crossing a cold coil puts a real quantity of water through a small plastic tray, every run, every day. A system that handles it quietly for years is not doing anything clever. It is draining properly.
So the whole water-leak family starts in this box. Water is created at the coil, collected in the pan, and carried away by the drain pipe. A drip means water was made and did not leave along that route. Where it appears narrows which step failed, and a separate guide covers how the drain line has to be laid to work at all.
What the drip location narrows
Water at the bottom front edge of the casing is the common one. Either the pan filled and spilled forward, or the air is carrying droplets off the coil face before they can run down the fins.
Water tracking down the wall behind the unit points somewhere else. The pan connection or a pipe joint is the usual answer, not the pan itself. Water that only shows up in wet weather, or only when the unit above runs, points at the outdoor end of the pipe.
Ice is the third source and the one most often missed. A coil that freezes sheds far more water when it thaws than the pan was built to hold. A leak that shows up only after a long run, then stops while the unit rests, is worth reporting as a possible ice question rather than a drain question.
Push back if a drip is answered with a gas top-up before anyone has looked at the drain path. Low refrigerant does ice a coil, so it is a real candidate. It is the second candidate, and it is the costly one to guess at.
Why the smell starts here
The indoor coil is the dampest and darkest surface in the system, and it is fed a steady supply of room dust. Those three conditions together are what grows a smell. Nothing on the ledge outside offers the same combination.
The coil is wet the whole time the unit runs and stays damp after it stops. The pan below holds a film of standing water between runs. Fine dust that slips past the mesh settles on the fins and on the blower blades, and damp dust feeds growth.
This is why a smell survives a filter wash. The mesh is the one part a homeowner can reach, so it is the part that gets cleaned, and it is not where the growth lives. A clean filter and a returning smell is not a contradiction. It is the ordinary result.
Timing separates the two versions worth reporting. A musty note at startup that clears once the unit has settled is growth disturbed by the first movement of air. A smell that holds steady through a run, or builds as it goes, is a different conversation and usually a wetter one.
The one smell that is not a cleaning question
A hot, sharp or burning plastic smell is electrical and does not belong in this category at all. Switch the unit off at the isolator and stop using it. That smell comes from a connector, a motor winding or the board, and running it longer is how a small fault turns into a replacement.
Everything else is a surface problem. Musty, sour, or the smell of a damp cloth all point at the wet side of the casing. Those are answered by cleaning the surfaces the air actually touches, which means the coil, the blower and the pan rather than the mesh.
Reading the unit from across the room
Five observations are available without tools, and each one shortens the list before anyone arrives. Not one of them settles a fault by itself.
What they do is sort faults living inside the casing from faults living on the far side of the wall. That sorting decides whether a first visit opens with a cleaning kit or with a set of gauges.
| What you notice | What it points at | What it argues against |
|---|---|---|
| What you noticeAir is properly cold but the stream is weak | What it points atA restriction on the indoor air path | What it argues againstRefrigerant charge, which weakens temperature before flow |
| What you noticeStream is strong but the air is barely cool | What it points atThe refrigerant side and the outdoor half | What it argues againstA loaded filter or a dirty blower barrel |
| What you noticeWater at the bottom front edge of the casing | What it points atThe pan, the drain path, or a coil that has iced | What it argues againstAnything happening outside on the ledge |
| What you noticeMusty on startup, fading as the run goes on | What it points atGrowth on the coil and in the pan below it | What it argues againstThe filter mesh, which does not hold the smell |
| What you noticeFlap sits still while the fan keeps blowing | What it points atThe swing motor or its drive on the board | What it argues againstA flat remote battery, since the fan took the command |
Airflow: compare the unit against its own past
Judge the stream against how it felt before, not against another room or a number. Same fan speed, same setting, same distance from the vent. Rooms differ too much for one unit to be a fair reference for another.
A fade that arrived slowly points at buildup, because dust accumulates slowly. A drop that arrived overnight points at something that changed state, and that is a much shorter list. Timeline is the most useful thing a homeowner can supply, and the thing most often left out of the first message.
Noise: the character matters more than the volume
Indoor noise sorts by what it tracks. A hum or whine that rises and falls with fan speed belongs to the blower or its motor. A tick at a steady rhythm usually means something is brushing the barrel as it turns.
A click at start and at stop is normal, because the flap parks itself. A rattle coming from the casing rather than from inside it is often a clip or a panel that was not reseated after the last service. Gurgling is its own signal and belongs to the drain path, not to the machine.
The flap is a free test of the control chain
A louvre that sweeps on command proves three things in one movement. The receiver saw the remote, the board acted on what it saw, and a motor did what the board asked.
That makes a working flap useful even when the complaint is about cooling. It takes the whole control chain off the list. A flap that stays shut while the fan still blows is a narrow fault, not a broken aircon. Reporting it that way keeps the visit pointed at the right part.
What none of this settles
Charge, compressor and board verdicts cannot be reached from the room. Refrigerant quantity is the clearest example. It is read on gauges, outdoors, while the machine is working. No amount of watching the vent substitutes for that reading.
Treat any diagnosis that jumps from weak airflow straight to a top-up as premature. Restricted airflow and low charge both land in the room as not cold enough, and the airflow half is checked at the indoor unit in the first few minutes of a visit. It is quicker to rule out, so it goes first.
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