Air-Conditioning Units and the Routes They Open
Installing a split air-conditioner means drilling a hole roughly 65 mm across through an external wall, passing refrigerant lines, a power cable and a condensate drain through it, and filling the remainder with whatever the installer has in the van. That is usually a plug of putty pressed in from the inside, and sometimes it is nothing at all.
It is a direct opening from the outside of the building into the wall cavity of your room, positioned above head height where nobody looks.
Three separate routes, not one
- The wall penetration itself, where the gap around the pipe bundle is rarely fully filled and the putty shrinks and cracks within a couple of years.
- The condensate drain pipe, which is an open-ended tube running from inside your room to outside, usually with no trap and no mesh.
- The pipe trunking, the plastic duct covering the lines on the outside wall, which is continuous, sheltered and often runs past several apartments.
The drain pipe is the one people miss
The condensate line is the most direct of the three, because it is a smooth open tube with a permanent moisture source at one end. Ants use it constantly, small cockroaches use it, and in older buildings where the pipe discharges into a shared gully it becomes a two-way route. If you have ants appearing at ceiling height near an indoor unit and nowhere else, this is almost certainly where they are coming from.
The fix is a fine stainless mesh cap at the outlet, or a proprietary drain trap fitted at the discharge end. It costs almost nothing and it is not something installers do unless asked.
Why the indoor unit itself gets colonised
The evaporator inside the indoor unit is wet whenever the unit runs, the drip tray beneath it holds water, and the housing is dark, enclosed and warm when the unit is off. Cockroaches and ants nest inside them, and geckos get in and cannot get out. Once anything nests in the drip tray the water route carries the smell through the room every time the fan starts.
This is the real argument for having units cleaned professionally once or twice a year, and for asking whoever cleans them to report anything they find rather than just washing it away.
What good sealing looks like
- Expanding foam then a hard exterior sealant at the wall penetration, not putty alone — foam by itself is chewable and degrades in sunlight.
- A downward slope on the drain pipe with no low point where water sits and no upturned end.
- Mesh at the drain outlet, fine enough to stop ants rather than just leaves.
- Trunking joints sealed, and the trunking end capped where it terminates.
The outdoor unit and standing water
Outdoor condenser units sit in a bracket or a tray, often in a service void or on a balcony, and both the tray and the base of the unit collect water. In a city where the mosquito that matters breeds in small clean containers, an air-conditioning tray is a breeding site in a location that is checked by nobody and is within a few metres of an open window.
A drainage hole drilled in the tray solves it permanently. Where the unit sits in a shared service void, this is a building management item rather than a tenant one, and it is worth raising because it affects every apartment on the stack.
When you are having a unit installed or moved
The moment of installation is the only cheap opportunity to get this right. Ask the installer to seal the wall penetration from both sides, to fit a mesh cap at the drain outlet, and to route the drain so it discharges away from the wall rather than down it. Vietnamese installers will do all three without argument if asked in advance; none of them will do it as standard, because it is not what the job is priced for.
Related reading
For the routes above your ceiling see false ceilings and ceiling voids, and for building-wide routes, service risers. Ant behaviour is covered in our ant guide and balcony water in balcony planters. The household programme is our apartment programme. Our parent company runs the Vietnamese-language ant control programme.
Global Pest Control is the English-language arm of Khử Trùng Toàn Cầu, based in Thu Duc City.
Common questions
Ants appear from behind the indoor unit every afternoon. What is happening?
They are following a trail along the refrigerant lines through the wall penetration or up the drain pipe, and the afternoon timing usually reflects when the unit stops running and the housing cools. Bait on the trail rather than wiping it, then seal the penetration once the trail has stopped — sealing first simply moves the trail to the next gap.
Can a gecko really get inside the unit?
Regularly, and it is one of the more common call-outs we get in villas. They enter through the wall penetration or the trunking, get into the fan housing, and cannot reverse out. The unit has to be opened by a technician rather than a pest controller, which is why we usually refer these to an air-conditioning engineer.
Does sealing the hole void the installation warranty?
Sealing around the pipes does not, because it does not touch the refrigerant circuit. Anything that involves disconnecting or re-routing pipework should be done by the installer. In practice we seal the wall penetration and fit the drain mesh, and leave the mechanical work alone.
