Why does a dirty back coil make an aircon leak?
The cooling coil in most wall units has a back section, against the wall, that cannot be reached with the unit on the wall. Packed with slime, it chokes the airflow so the room stops cooling, and slime breaks off into the drain, which blocks and overflows. Only a chemical overhaul reaches it.
Key takeaways
- Weak cooling plus a leak: together they often point to a clogged coil, not two separate faults.
- The back is the blind spot: any clean done with the unit on the wall works from the front and top; the back section stays dirty.
- Slime travels: gel from the fin gaps flows into the drain pipe and blocks it, so clearing the drain alone does not last.
- Only an overhaul reaches it: the unit is taken down so every section of the coil is soaked and flushed.
In the video above, two aircons were not cooling and leaking at the same time. Slime had built up inside the tiny gaps between the coil's fins and was flowing into the drain pipe. A PowerJet chemical overhaul cleaned both units back to as-new.
How do you know it is the back coil?
- Check the cooling first. The aircon runs all the time but the room never gets properly cold, even with clean filters.
- Shine a torch into the air outlet. A blower fan coated in dark, furry dirt means the coil around it is coated too.
- Look at where the water comes from. Drips from the bottom of the unit, or water spitting out with the air, point to the coil and drain pan.
- Ask what has been done before. If the unit has only ever been serviced or chemical-washed on the wall, the back section has never been cleaned.
If the first two match, the back coil is likely clogged: see our chemical overhaul. Leaking right now? Message Luna on WhatsApp.
Why does the back section get missed?
The cooling coil is bent around the blower fan to fit inside the unit. The front faces the grille and the top faces the air intake, so both can be cleaned with the unit on the wall. The back section sits against the wall, behind the fan. With the unit mounted, water and chemical cannot reach it, and spraying hard from the front only pushes dirt further in. As Uncle Eric puts it, it is simple logic: if the unit is not taken down, the back is not cleaned.
Over the months, dust that slips past the filter sticks to the wet fins. Microbes grow in it and turn it into a gel that fills the fin gaps. Air cannot pass through, so cooling drops. Water cannot run off, so it drips where it should not, and the gel that breaks away travels into the drain pipe. Read how drain pipe slime forms.
Which clean reaches the whole coil?
Clean | What happens to the coil | When it is enough |
|---|---|---|
Premium servicing | PowerJet deep clean from the front and top; blower fan and drain pan taken out and washed; drain flushed | On a regular schedule, before slime sets |
Coil chemical then PowerJet flush, from the front and top; the back stays out of reach | A front coil coated with dirt and grime | |
Chemical overhaul | Unit taken down and dismantled; every section of the coil, back included, soaked and flushed | A back coil clogged with slime |
For the full comparison, read chemical wash vs chemical overhaul.
Which units clog fastest?
Our humidity means the coil is wet for most of the night, every night, which is exactly what the microbes need. Bedroom units that run 8 hours or more a night, near a kitchen or a busy road, clog fastest. On a System 2, 3 or 4, each fan coil clogs at its own pace, so one room can be leaking while the others are fine.
What it costs
Job | Price | What drives it |
|---|---|---|
Premium servicing with PowerJet | S$80 for 1 unit | Coil, blower fan and drain pan cleaned |
Chemical wash | S$120 per unit | Heavy-duty coil cleaner from S$30 if the coil is very dirty |
Chemical overhaul, wall-mounted | S$180 per unit | Includes a free gas check and top-up (worth S$165) and a deep evacuation (worth S$395) |
Ceiling cassettes get servicing or a chemical overhaul, never a chemical wash.
When to call a technician
Call us when the aircon leaks and cools poorly at the same time, when a leak comes back weeks after a service, or when the blower fan looks dark with dirt. For every other cause of a leak, read why an aircon leaks water.
Frequently asked questions
What is the back coil on an aircon?
The cooling coil in most wall units is bent into sections around the blower fan. The front section faces the grille and is easy to spray. The back section sits behind the fan, near the wall, and is hard to reach with the unit closed.
Can I clean the back coil myself?
No. It sits behind the blower fan, and spraying it with the fan in place pushes dirt deeper into the fins and the drain. Washing the filters every few weeks is the part you can do, and it slows the build-up.
Why does a dirty coil cause a leak?
Slime in the fin gaps holds water and breaks off into the drain pan. It flows into the drain pipe and narrows it, so the water backs up and spills out of the unit instead of draining away.
How often does the back coil get this dirty?
It depends on how many hours the aircon runs and how clean the filters are kept. Units serviced every three months rarely reach this stage; units left for a year or two often do.
Will a chemical overhaul damage my aircon?
No. The unit is taken down and cleaned by a trained technician, and it includes a free gas check and top-up and a deep evacuation before it goes back up.
Eric Lim, aka Uncle EricFounder of iAircon.Repair, now in its 10th year servicing aircons in Singapore. Every job is filmed, so you can watch the work on YouTube.
