Condensate Drainage: The Small Pipe Behind Most Ceiling Stains

Why it matters on a project

Ask any facilities manager what goes wrong in the first year of a new office and ceiling stains will be near the top of the list.

Almost none of them are roof leaks or plumbing failures. They are condensate drains — a small, cheap pipe, installed above a ceiling, that nobody thinks about until water appears on a tile in a meeting room.

What the drain has to do

Requirement	Why	If missed
Continuous fall	Gravity is the only motive force	Water sits, then backs up
Correctly sized trap	Seals against the unit's internal pressure	Tray overflows or air is drawn in
Primed trap	A dry trap is not a seal	Immediate overflow or odour
No dips in the run	Debris collects at low points	Progressive blockage
Cleaning access	Trays and pipes foul over time	Deterioration cannot be corrected
Adequate pipe size	Flow plus fouling allowance	Blocks sooner than expected

The pressure point people miss

A condensate drain is not simply a downpipe. The tray sits in an airstream at a pressure different from the room, commonly negative on the suction side of the fan.

Without a trap deep enough for that pressure, one of two things happens: the pressure holds water in the tray until it overflows, or air is drawn up through the drain and water never flows freely.

That is why the trap depth relates to the unit's pressure, and why a standard trap fitted across a mixture of equipment types is a recurring source of trouble. Requirements should come from the equipment data and the project's MEP consultant, within the framework of NBC 2016, Part 8 and the plumbing provisions of NBC 2016, Part 9.

The coordination trap in a fit-out

Units get moved during ceiling coordination — to clear a beam, to suit a lighting layout, to avoid a partition.

The unit moves, the drain route changes, and the fall that worked on the original layout may not exist on the new one. Because the whole run is above a ceiling, nobody sees it, and the first evidence is a stain.

Any change to a unit's position should trigger a re-check of its drain route. It takes minutes on a drawing.

Common mistakes

• Moving units without re-checking the drain fall. The most common cause on a fit-out.
• Leaving traps unprimed at commissioning. Simple, and routinely missed.
• A dip in the run. It will collect debris and block.
• No access for cleaning. Fouling is inevitable; inaccessibility is not.
• One generic trap across different equipment. Trap depth follows unit pressure.
• Combining drains from several units carelessly. One unit can push air or water into another's tray.

Standards referenced

Air conditioning provisions in NBC 2016, Part 8; plumbing provisions in NBC 2016, Part 9; drain pan drainage and access addressed in ventilation standards such as ASHRAE 62.1. Trap sizing, pipe sizing and drain routing for a specific installation must be established by the project's MEP consultant and the equipment manufacturer's data.