Pictured: chemical being worked into the coil of a wall unit, with the water cover fitted underneath so everything runs off into a bag. That coil and the drain tray under it are where an aircon's wet side lives, and a smell that reminds you of a drain almost always comes from somewhere on that wet side or from the drain line that carries its water away.
You switch the aircon on and, for a few minutes or for the whole evening, the room smells like a bathroom, a blocked sink or a roadside drain. It is not the damp-towel smell you may have met before, and it is not the sharp sour smell either. It is a drain smell, and it is unpleasant enough that people stop using the room.
The good news is that this is one of the smells with a short list of causes, and you can narrow it down yourself before anybody visits. Here is how we would do it, what is safe to try, what is not an aircon problem at all, and what to tell us so the first visit is the right one.
First, be sure it is a drain smell
"Aircon smell" covers several different problems that need different fixes. Sort yours by what it reminds you of:
- Drain, toilet or sewage. Flat, stale, a bit sulphurous. This post.
- Musty, like a damp cupboard or a wet towel. Mould on the coil or blower. Read why your aircon smells musty.
- Sharp, sour or like vinegar. Read why your aircon smells like vinegar.
- Hot, electrical or like burning plastic. Switch the unit off at the wall and read aircon burning smell: what to do first. Do not keep running it to see if it clears.
- Rotting, like something has died. Possible if an animal has got into the casing or the trunking. See ants, geckos or birds in your aircon.
- Cooking gas. That is not an aircon smell. See the section further down.
If you are not sure which one you have, do this: switch the unit off for ten minutes, then smell the room. If the room smells the same with the unit off, the aircon is probably not the source. If the smell only appears once the fan is blowing, it is coming through the unit or its drain.
Why a drain smell can reach your room at all
Every indoor unit makes water. The coil is cold, the air in a Singapore flat is humid, and that moisture condenses on the coil, drips into the drain tray and leaves through a drain pipe. On a normal HDB job that pipe ends at a floor trap or similar drainage point inside the flat, as the rules we work to require.
Two things stop that pipe from smelling. First, the water that normally sits in the line and in the floor trap acts as a seal. Second, the water keeps moving, so nothing stagnates. A drain smell in the room means one of those two has failed.
- The seal has dried out. If the unit has been off for weeks, the water in the trap and the line evaporates and the line becomes an open path to the drainage behind it. We described this in what to do with an aircon when the flat is empty for a long trip. The same can happen after plumbing or bathroom work that disturbed the trap, or in a room whose unit is rarely used.
- The drain is partly blocked and the water is sitting. A film builds up in the drain tray and the pipe, water stops flowing freely and goes stale. Blocked drains are the number one cause of aircon water leaks in HDB and condo units, and the same blockage that makes a unit drip can make it smell first. If the smell comes with any dripping, read aircon drain that keeps choking too.
- The dirt is on the wet side of the unit itself. A coil, blower wheel and tray that have not been cleaned for a long time can smell like a drain rather than like mould, particularly in the first few minutes after the fan starts.
Three questions that narrow it down in ten minutes
1. One unit or every unit? If only one room's aircon smells, the cause is in that unit or its own drain line. If every unit smells, and the bathroom or kitchen drains in the flat smell too, the problem is probably the flat's drainage and not the aircon, and the aircon is only moving the smell around. That is a plumber's job, or a conversation with your town council or management office, not something a wash will fix.
2. When does it smell? A smell that is strongest in the first minutes and then fades often points to a dry trap or the drain line. A smell that stays as long as the fan runs points more to the coil, tray and blower. A smell that arrives only after you have been away is the trap again.
3. Where does your drain end? Follow the white or grey drain pipe from the indoor unit if you can see where it goes. In many HDB flats it ends in the service yard floor trap, a bathroom floor trap or the kitchen sink waste. If the floor trap it ends in is dry, or looks like it has not seen water for weeks, that is a strong lead.
Safe checks you can do yourself
None of these involve opening the unit.
- Re-wet the floor trap. If you can see the floor trap that your aircon drain empties into, pour in a jug of plain water. A trap that has dried out refills its seal this way. Do not pour bleach, drain cleaner or any other chemical into it, and never into the aircon drain pipe or the unit.
- Run the unit on cooling for half an hour and smell it again. A smell from a dried-out line often fades once water is flowing through it again. A smell that comes back every time you switch on does not.
- Check the filters. Pull them out and look. Clogged filters hold damp dust that smells. Washing them is a safe job; our guide to washing aircon filters yourself covers where DIY stops.
- Open the window and let the room air out. A room that has been shut for days holds smell in its soft furnishings, and that is not the aircon's fault.
Things not to do: do not spray perfume or air freshener into the vents, because it covers the smell without removing the cause. Do not take the casing off. Do not put anything into the drain pipe.
When it is not the aircon
If the smell is like cooking gas, switch nothing on or off, open the windows and call your gas supplier or the emergency number on your gas bill. That is not an aircon fault and it is not something to test by running the unit.
If every drain in the flat smells, or the smell is strongest at a bathroom floor trap with the aircon off, treat it as a drainage problem first. Mention it when you talk to us, because it changes what we look for, but do not pay for a chemical wash to fix a smell that is coming from the pipes.
What a visit covers, and what it costs
Every service booking starts with the 9-point pre-check, and two of the nine points are exactly this question. Point 4 is a drainage flow check, to confirm the drain pipe is clear. Point 8 is an assessment of how dirty the filter and coil are, so we can tell you honestly whether the unit needs a general service or a chemical wash. The pre-check is included in the booking: the minimum is $55 for 1 unit, which covers the visit, the pre-check and standard servicing.
What happens next depends on what we find, and you decide:
- The service fixes it. Normal servicing dismantles the roller and blower and cleans the filter, coil and drainage, then flushes and checks. It is $55 for 1 unit, $90 for 2, $120 for 3, $160 for 4, and $35 per unit from 5. If the drain was only dirty, the drainage clean and flush in the service may be the whole job.
- The drain itself needs work. Standard repair for drainage or electrical work is $80, quoted after the pre-check and before any work starts. We have no published price for work on the flat's own plumbing, so we will not guess one.
- The coil and tray are heavily fouled. A chemical wash goes further: it dismantles the roller, fits a water cover, puts chemical on the coil and blower, rinses, flushes the drainage and tests. It is $135 for 1 unit, $245 for 2, $345 for 3 and $425 for 4.
- The smell comes back after a wash. The overhaul strips the unit down and chemical washes every part, which is the job for a smell that a wash has not cleared. It is $200 for 1 unit, $370 for 2, $515 for 3 and $640 for 4. What the word overhaul covers sets out what is and is not in that job.
If you decide not to go ahead with the fix we recommend, the $55 covers the visit and the diagnosis and nothing more. If you do go ahead, the $55 is part of the total and is not charged twice. Our work carries a 90-day workmanship warranty.
If you are comparing options, the chemical wash page and the price list show every figure above.
What to tell us on WhatsApp
You will get a better first visit if we know these before we come:
- Which unit smells, or whether every unit does.
- When you notice it: the moment you switch on, after a few minutes, or all the time.
- Whether the flat has been empty or the unit unused in the last month.
- Whether any plumbing, bathroom or renovation work was done recently.
- Whether the room's floor trap or the other drains in the flat smell too.
- When the unit was last serviced or washed.
Drain smell from your aircon? Tell us which unit and when
Send us the unit, when you smell it and when it was last serviced. WhatsApp us on 9107 2601 and we will tell you what a visit would cover before we come.
If the smell comes with water dripping, read what is normal when an aircon drips right after a wash or service as well.