Core drilling for aircon: siting a hole you cannot undo
Homeowners settle the brand and the unit positions, then leave the holes to the installer. The holes are the one part of the job that cannot be moved afterwards. They decide where a unit can hang, and whether its drain will run.
By Team Snowflake | Updated 6 Aug 2026
Core drilling makes a hole instead of breaking one
Core drilling cuts a clean round opening rather than smashing through the wall. The bit is a hollow tube with diamond segments set around its rim. It grinds a ring through the concrete and lifts out the cylinder left in the middle. That cylinder is the core, and it is where the method gets its name.
A hammer drill works the other way round. It pounds a bit into the material and shatters whatever sits ahead of it. That is fine for a screw anchor. It is the wrong approach for an opening wide enough to pass a pipe bundle, because the shock spreads into the wall and cracks the finishes on both faces.
The difference shows in the edge. A cored opening has a smooth bore, so a sleeve fits it and sealant grips it evenly. A hammered one has a ragged mouth and broken plaster around it. Those are harder to close properly, which is why they so often end up filled with foam and left alone.
One opening usually has to carry the whole bundle. On a standard split system the two copper lines, their insulation, the condensate pipe and the interconnecting cable all cross the wall together. That bundle sets the diameter, not the largest single pipe. Bore for the pipes alone and the drain gets pressed flat against the copper, which is a leak waiting for a humid week.
Nothing about the method makes the hole safe on its own. Core drilling is quieter, tidier and kinder to the surrounding concrete than hammering. None of that says anything about what the bit is about to pass through.
What sits inside the wall decides where the hole can go
Reinforced concrete is not solid material. Steel bar runs through it in a grid, set close to both faces, and that steel is what stops the element cracking under load. A diamond barrel will cut straight through bar without complaint. That is the worry, not the reassurance it sounds like. The opening gets finished, nobody sees the cut ends, and the wall has quietly lost part of what carries it.
Services are buried in there too. Electrical conduit is cast into walls and slabs in most Singapore homes rather than run along the surface. Water pipes are often chased into masonry and plastered over. A bore that clips a live conduit turns an installation into an electrical repair. One that clips a water pipe turns it into a wet ceiling for the flat below.
Newer buildings add a harder case. Some floor slabs are post-tensioned, meaning steel tendons run through the concrete under high tension and hold the floor up by being pulled tight. Cutting one releases that force in an instant. The result is a structural event, not a repair item. Tendon runs cannot be guessed at from the room below, and drawings are treated as a guide to where to survey rather than a licence to drill.
This is why scanning belongs before the bore rather than after it. A concrete scanner reads what sits behind the surface, and a mark can then shift a hand's width to clear it. Moving a mark costs nothing. Moving a hole means patching a wall.
Push back if the answer to what is behind the wall is that the crew has done hundreds of these. Layouts differ between blocks, and a wall on one floor is not the same wall on another. A useful answer names how the position will be checked. A count of past jobs is not that answer.
Not every wall is the homeowner's to open
Some walls belong to the block, not to the home inside it. A party wall shared with the next unit, the outer skin of the block, and anything the drawings treat as load-bearing all sit apart from a plasterboard partition. Structural elements fall under building control in Singapore. Works that alter them run through a qualified person and a permit, not through a judgment call made on site.
In a strata development the external wall is usually common property. Approval from the management corporation is normally the gate on anything crossing it, and estates differ in what they allow and where. That approval process is its own piece of work, and the condo installation checklist covers it properly. HDB flats take a different route, and whether an HDB aircon permit applies at all is set out separately.
The hole has to serve the pipe route
Hole position is decided by where the water has to go. Condensate leaves the indoor unit under gravity alone, so the opening has to sit below the unit's drain outlet. It also has to fall away from the room across the full thickness of the wall. Bore it level, or a shade high on the outside, and the water has to climb before it can leave.
That one constraint fixes more than it appears to. It sets how high the indoor unit can hang, since the unit must sit above the opening. It also rules out some wall positions outright. A hole low enough to fall may land in a skirting, a socket box or a beam. How the fall holds along the rest of the route is covered under drain gradient.
Copper has a say as well. Refrigerant lines bend to a limit and no further, and a kinked line is a restriction that stays in the system for its life. An opening set too close to a corner leaves nothing to bend in. The pipe then gets pulled tighter than it should be, and the damage is sealed inside the trunking before anyone sees it.
The outer face matters as much as the inner one. The bore exits somewhere, and that spot has to let the pipe reach the ledge without crossing a window or a protected facade. An exit chosen for the shortest path through the wall can force a longer and uglier run outside. Where the condenser is allowed to sit is its own constraint, set out under outdoor unit placement.
The order these get decided in is the tell. A job that has been thought through arrives with the indoor position, the hole position and the ledge position already agreed between everyone. A job that has not been thought through arrives with a drill, and settles the question against the wall.
| What the position has to respect | What goes wrong when it does not | What settles it before anything is bored |
|---|---|---|
| What the position has to respectFall from the drain outlet through to the outside face | What goes wrong when it does notWater sits in the line and finds its way back into the room | What settles it before anything is boredFixing the indoor unit height first, then marking below it |
| What the position has to respectSteel and buried services inside the concrete | What goes wrong when it does notA cut bar, a dead circuit, or a punctured water pipe | What settles it before anything is boredScanning the marked area, then moving the mark rather than the date |
| What the position has to respectRoom for the copper to bend on both sides | What goes wrong when it does notA kinked line the system never fully recovers from | What settles it before anything is boredStanding the mark clear of corners, sills and the casing route |
| What the position has to respectWhere the pipe lands on the outside | What goes wrong when it does notA long external run, or a route the building will not allow | What settles it before anything is boredAgreeing the ledge and the external path in the same sitting |
| What the position has to respectWhat the wall itself is | What goes wrong when it does notAn opening cut in an element that was never the owner's to open | What settles it before anything is boredConfirming structural status and ownership before the quote closes |
A bore is not finished until it is sleeved and sealed
The bore is the easy half. What keeps it from becoming a defect is a sleeve through the opening and a proper seal at both faces. The sleeve gives the pipes a smooth liner to pass through and keeps them off raw concrete. It also gives the sealant a consistent surface to grip.
Water is the first thing an open penetration invites. Driven rain hits the outer face of a block hard during a squall, and an unsealed hole hands it a straight path indoors. Inside, that shows up as a stain that appears only in bad weather. It reads as a window or roof fault, and gets chased in the wrong place.
The bore should also fall slightly toward the outside, so anything that does get in runs back out instead of tracking along the pipes. That slope is set at the moment of drilling. It cannot be added afterwards.
Pests find these openings before anyone else does. A gap around the pipes is a door for ants, cockroaches and lizards, and for rodents on lower floors. It is also a route to insulation that gets chewed. That is how a cable fault starts inside a wall nobody has opened.
Expanding foam on its own is the shortcut to watch for. It looks sealed, and it hides the state of the opening underneath. Foam degrades in sunlight on the outer face, holds water against the wall, and gets picked apart by insects. Ask what the seal is made of, and ask to see the outside face once the crew has finished.
The outside face is the part you can inspect
The outer face of a penetration is the one part of an installation a homeowner can check without tools. A neat sleeved hole, sealed flush, with the pipe dressed down toward the ledge, says the crew worked to a standard. A ragged mouth with foam bulging out of it says the opposite. It usually says it about the parts of the job that are now hidden as well.
Look at that face during handover rather than long afterwards. It belongs alongside the other things worth confirming while the crew is still on site, gathered in the installation handover checklist. A penetration finished badly is also one of the clearer signs of poor aircon installation elsewhere in the work.
What to settle before anyone marks the wall
Every decision worth making sits upstream of the drill. Once the opening exists, all of them are fixed. The ways back are patching a hole in a wall that should not have been opened, or living with where it landed.
- Who confirms what is behind the wall, and by what method. The answer should describe a check, not an assurance.
- How many openings the job needs, and where each one lands on both faces. One bundle hole per indoor unit is normal, and multi-split layouts often need more.
- What the wall is. Structural, shared with the neighbouring home, or common property each change who has to agree before anything opens.
- What the sleeve and the seal will be, on the inside face and the outside face.
- What happens to the layout if the position has to move once the wall is scanned.
When the wall is already open
Where pipework already exists, the question changes rather than disappears. An old opening can often be reused, and that removes the argument entirely. It has to be the right size for the new bundle, and in the right place for the new indoor unit. Confirm that before a replacement is priced on the assumption that it fits.
A reused opening carries its own history. It was sized for whatever crossed it last, sealed by whoever left it, and may have sat behind casing for years. Worth asking for the seal to be cut back and remade rather than patched over, since the old one is now buried under the new pipework.
Slow down if the position is being settled on install day with the crew already standing there. That is the moment the convenient mark wins, because everyone in the room wants the job to start. A mark agreed in advance, on a photo or a floor plan, costs nothing to move.
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