Office Fit-Out: Return Air Path Decided Unit Placement

An office fit-out where the ceiling plan showed spacing and not much else. What actually decided where the units could go.

By Team Snowflake | Updated 24 Aug 2026

Teaching case based on recurring Snowflake service patterns; identifying details are combined.

What gets specified

A reflected ceiling plan with units spaced evenly across the grid, wanted signed off ahead of the partition layout so the ceiling can go on order. Programme pressure to fix the positions early, and no drawing yet showing what will sit underneath them.

Setup: Toshiba CassetteNew installOfficeDowntown Core
Misread as: A spacing problem, because that is all a ceiling plan can show.
Check first: The partition layout and the services void, overlaid on the ceiling plan before any position is fixed

What the drawing does not show

The grid, the lights and the sprinkler heads are what a reflected ceiling plan carries. Partition heights are not on it, and void depth is not either. Both decide whether a position works: a unit needs an open return path on every side, and a continuous fall for its drain. Neither shows up until the partition layout and the services void are laid over the same sheet.

A position sitting directly over a full-height partition head loses its return path on that side, whatever the grid says.
Void depth drops where the sprinkler main crosses, and a drain needs continuous fall rather than an average one.
Positions with open void above and clear floor beneath are the ones that survive the overlay.
Pipe runs crossing a void that stays warm will sweat unless lagging is specified across the full crossing.

Why it goes wrong

The omission is structural rather than careless. Ceiling positions are set on a drawing whose job is the ceiling, at the point in the programme where partitions are still moving and the grid needs ordering. Nobody is asked to check return air, because return air is not on that sheet. The result surfaces after occupation, as two rooms that never cool properly and staining on the tiles below.

What the fix involves

Move the affected positions clear of the partition heads, so each unit keeps open floor beneath it and a free return on all four sides. Take drain lines under the sprinkler main where the void still allows continuous fall. Specify lagging continuous across every void crossing, because a cold pipe in a warm void sweats and the water lands on whatever sits below. All of it before the grid goes on order, which is the only point where it costs nothing.

How to tell it worked

Panels cut once, and air balancing inside tolerance on the first pass. A floor needing a second balancing visit, or tiles cut twice, is a layout that was signed off before the partitions were.

Before you sign off

A ceiling unit needs a path back, not just a place to sit

Ask to see the partition layout and the ceiling plan on one sheet before any position is fixed.
Even spacing across a grid looks correct and can still leave a unit with nowhere to draw air from.
A cold pipe crossing a warm void sweats. Lagging has to run the full crossing, not most of it.

Questions this case answers

Can a ceiling cassette be installed directly above a partition wall?

Not without losing the return path on that side. A ceiling unit draws its air back from the room below, so a full-height partition head sitting directly under it closes off one side of that path. The position looks correct on a ceiling grid and starves the unit once the walls go up.

Why are my office aircon units cooling unevenly after a fit-out?

One common cause is a unit sitting where it cannot draw air back. Positions spaced evenly across a ceiling grid for symmetry with the lights govern how a room looks and say nothing about how air moves through it. Reading the partition layout against the ceiling plan catches it before the grid is ordered.

What causes water stains on ceiling tiles under an office aircon?

Two things, and both are settled at drawing stage. A drain line without clean fall past the sprinkler main, and a cold pipe crossing a void that stays warm with no lagging specified. Condensation forms on bare pipe and drips onto whatever sits below it.

Should the ceiling layout be signed off before the partitions are confirmed?

Better to hold it. Programme pressure usually pushes the other way, because the ceiling grid needs ordering. Neither the return path nor the drain fall is visible on a plan carrying only the ceiling grid, and both are expensive to discover once the tiles are cut.

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