Trace the complaint down the building

Which unit actually serves it

Start by confirming that the unit everyone believes serves that suite is the one that does. In buildings that have been re-partitioned this is wrong surprisingly often. Then the ordinary checks: coil, charge, belt, and the economiser — stuck open in a Georgia summer it pulls 95°F outside air in all day and nothing inside indicates it.

What changed above the ceiling

A flexible branch crushed by a new cable tray or squashed when a ceiling was re-laid. A damper closed during someone else’s fit-out. A diffuser blanked off and forgotten. A return path blocked by a partition that was built to the slab when the rest go to the ceiling grid.

This is where the answer usually is, and it is usually free to find and cheap to fix.

What the tenant put in it

A server cupboard, a display fridge, a bank of monitors, a wall of west-facing glass with the blinds taken down, or twelve people in a room designed for six. None of these are faults — but they are load, and the system was sized before they arrived.

If this is the cause, the honest answer is a supplementary unit or a change to the space, not a bigger rooftop unit for the whole building.

Where the warm air is being put

In a lot of fitted-out suites the diffusers were placed for the original open plan and never moved when offices were built along the glass. Warm air delivered into the middle of a floor does not reach a perimeter that is losing heat, and no amount of extra run time changes that.

Moving or adding a diffuser and a branch is ordinary work. So is finding that the perimeter branch exists and is closed.

The glass and the edge of the slab

Single glazing, failed seals, an uninsulated slab edge, or a curtain-wall detail that was never much good. The complaint is real and the heating system is not the thing that is wrong.

Where this is the cause we will say so, and it is not our work — it is a glazing or building fabric question, and putting more heat at it is the expensive way to lose the same argument every winter.

Extract against make-up air

Find every extract fan on the roof and find what is replacing the air it removes. A restaurant, a salon or a nail bar can extract several thousand cubic feet a minute; if the make-up air unit is absent, undersized or switched off, the building is being emptied.

This is also, by a long way, the most common reason a flue misbehaves in a shared building.

The path between the suites

Demising walls that stop at the ceiling grid instead of going to the slab. A shared return plenum. A duct that crosses from one tenancy into another because the layout changed. Once the building is in depression, every one of these becomes a route.

What the receiving suite is doing

A suite with no extract of its own, doors kept shut, and a system on setback out of hours will sit at the lowest pressure in the building and collect whatever the others are pushing out.

The source, which is usually downstairs

Kitchens, salons and workshops are normally at ground level, and the air they throw out has to come from somewhere. Measuring the pressure difference across the demising line settles who is doing what, and it is the only thing that ends this argument between tenants.

Bearings, wheels and isolators

A fan bearing on its way out, a fan wheel out of balance, or a vibration isolator that has perished and is now transmitting straight into the curb and down into the structure. A rattle is usually a panel or a guard; a hum you feel rather than hear is nearly always a mount.

Ductwork and hangers

Ducts drumming at a joint, a flexible connector that has gone hard, or a hanger that has worked loose and is now letting a run rest on a ceiling grid. All ordinary, all cheap, and all much easier to find with somebody standing in the suite while somebody else is in the void.

Pumps, risers and other trades

Worth ruling out before anybody spends money on the roof: a circulating pump, a booster set, a lift motor room or a compressor belonging to a tenant can all transmit through a structure and arrive several floors away sounding like an air conditioning fault.

Two of those are not ours, and we would rather identify that than replace a bearing that was never the problem.

The curb, the flashing and the membrane

If the stain follows rain, and it is near or downhill of a unit, the curb or the flashing round the penetration is the first suspect. Standing water beside a unit makes this far more likely and is itself a roof problem, not a plant one.

The repair belongs to whoever looks after the roof. We will lift and reset a unit so the work can be done properly, and doing it in the wrong order means paying twice.

Condensate, and it usually is

If the stain appears in the cooling season regardless of weather, it is almost certainly condensate: a blocked drain, a dry or missing trap, a corroded pan, or a secondary pan that has been quietly catching water for a year and has now filled.

This is ours, it is a small job, and it gets considerably larger if it is left through a season.

Tag it, once

Every unit numbered and labelled, matched to the suite or area it serves and to the plate data recorded off it. Half a day on most buildings, and it is the cheapest thing we ever do.

It also produces the real age of every unit on the roof, which changes budget conversations immediately and occasionally changes a sale.

Disconnects, meters and who is paying

Which disconnect isolates which unit, which meter it runs through, and whether the way the building is actually wired matches the way the leases say it is billed. We record what we find.

What is due under a particular lease is a question for you and your attorney, not for us — but you cannot answer it without knowing which unit is on which meter, and that part we can settle.