An undersized or uninsulated fan coil unit condensate drain is a leading cause of ceiling stains and floor damage, so correct pipe sizing and insulation are part of the installation, not an afterthought.
Where the Water Comes From
A cooling FCU can produce a surprising volume of condensate. A typical 5 kW cooling coil at 7ยฐC supply and humid summer air may generate 2โ4 L/h of water per unit, and a bank of dozens of units multiplies that into tens of litres per hour that must leave quietly. The drain must carry this without trap gurgle, blockage, or sweating onto finishes, and a single clogged branch can back up into several units at once because they share a collector.
Because the pipe runs at coil temperature, it is also a condensation risk itself in humid spaces, which is why insulation matters as much as diameter. A cold uninsulated drain in a humid plant room drips as surely as a leaking one, and the stains look identical to the occupant even though the cause is different.
Not every unit can drain by gravity. Where a ceiling void is shallow or the nearest stack sits above the pan, a condensate pump becomes necessary, and it changes the detailing entirely: the pump needs a reservoir with a float switch, a high-level alarm wired to close the cooling coil valve, and a discharge line that can be a 10 to 16 mm tube rising several metres. Gravity remains the first choice because it has no moving parts and no power supply to fail, so the void depth allocated at design stage is worth arguing for. A pump added late becomes a maintenance item that outlives the argument about who should have allowed the fall.
Sizing and Insulation Rules
Drain lines are sized on gravity flow, not pressure, so diameter and slope dominate. A 25 mm (1 inch) line serves a single small FCU; branch lines of 32โ40 mm collect multiple units. Slope should be a minimum of 1:50 (about 20 mm per metre) and ideally 1:30, with accessible cleanouts at changes of direction so a jetting hose can clear sludge without cutting the ceiling.
| Item | Specification | Note |
|---|---|---|
| Coil load | 2โ4 L/h | Per 5 kW cooling FCU |
| Branch pipe | 25โ32 mm | Per unit or pair |
| Main collector | 40โ50 mm | Multiple units |
| Slope | โฅ 1:50 | Toward floor drain |
| Insulation | 9โ13 mm | Closed-cell, vapour-tight |
| Condensate pump lift | Commonly up to 4–6 m | Where gravity fall is unavailable |
| High-level alarm | Interlocked to coil valve | Stops cooling before overflow |
A trap at each FCU prevents air being drawn into the pan and causing gurgling or siphoning. The trap must be primed and, in dry periods, kept from evaporating by a seal holder. Insulate the trap and the horizontal run, not just the vertical drop, because the horizontal run is the long cold surface that drips in humid air.
Blockage is a maintenance problem with a design cause. Condensate is nearly distilled water, but it carries dust washed off the coil, and in a warm void that mixture forms a biofilm that narrows the bore over a season or two. Rigid pipe with solvent-welded joints resists this far better than corrugated flexible hose, whose ridges trap sediment and are almost impossible to clear. Where a flexible connection is unavoidable, keep it under a few hundred millimetres and make it the only such section in the run. Annual flushing before the cooling season keeps the line clear, and the cleanouts specified at design stage are what turn that flush into a ten-minute job.
Installation Checklist
- Size by total load: Sum the FCU condensate rates on a branch before picking the main.
- Keep slope constant: Avoid flat or reverse-pitched runs that hold water and breed biofilm.
- Insulate fully: Closed-cell insulation with sealed joints stops pipe sweating and dust pickup.
- Add cleanouts: Place them at every direction change for jetting without ceiling cuts.
- Prime the traps: Fill each trap at start-up so the seal exists before the first cooling cycle.
- Prefer gravity: Argue for ceiling void depth at design stage rather than adding a pump afterwards.
- Avoid corrugated hose: Use rigid pipe with solvent-welded joints so sediment cannot lodge in the ridges.
Why Detailing Pays Off
A correctly drained and insulated FCU bank runs for years without a stain call, while a careless one generates recurring claims and repaint costs. Keruite supplies fan coil units with defined drain pan outlets and trap guidance so the installer sizes the line to the actual coil condensation rate rather than guessing from the unit footprint.
Condensate management is judged years after handover, when either nothing has happened or a ceiling tile is stained. Fan coil units produced in an 80,000 m² factory by a manufacturer with more than 30 years of experience are supplied with pan outlet size, condensate rate, and trap requirement stated on the data sheet, which lets the installer size the branch from real numbers. ISO 9001:2015 covers pan fabrication and leak testing, CCC certification covers the electrical side, and because units ship to more than 50 countries the drain detail is documented for both pumped and gravity practice.
Our engineers respond to selection enquiries within 24 hours, including FCU condensate load and drain layout advice for multi-unit projects.