Office HVAC Problems After Handover

Why office HVAC problems start after handover

An office floor that is uncomfortable in its first month costs more than the discomfort. It generates a steady flow of tickets to facilities, a queue of people asking for desks to be moved, and a dispute between the occupier, the fit-out contractor and the HVAC vendor about whose problem it is. Often the equipment is fine.

The common instinct is to conclude that the system is too small and to ask for more tonnage. That is usually the wrong diagnosis. Capacity is how much cooling the plant can produce. Comfort is decided by where that cooling ends up. A floor can have ample capacity and still have a hot corner, a cold desk under a supply grille and a meeting room that never settles.

Capacity and distribution are separate questions

Commissioning tests answer the capacity question well: does the plant run, do flows reach their set points, do the controls respond. They answer the distribution question only partly, because distribution depends on the final furnished floor, which often does not exist when the tests are done.

Symptom	Typical cause	Does more capacity help?
Hot corner or edge	Sun-facing glass, or heat load above the design assumption	Sometimes, but check the assumption first
Hot room, cold neighbour	Zoning does not match the final partitions	No
Draught on a desk	Supply air thrown directly onto seating	No, it can worsen it
Room never reaches set point	Return air path blocked by a closed door or partition	No
Thermostat overshoots or hunts	Sensor in the wrong place or in a different zone	No
Whistling or hiss	Air velocity high through a small terminal	No
Whole floor warm at peak	Genuine capacity shortfall	Yes, but this is the minority case

Only the last row is a capacity problem. Nearly all of the rest sit in the way air is delivered, returned and sensed.

The layout must be frozen before terminals are

Terminal positions, zone boundaries and thermostat locations are drawn against a layout. Terminals for a cooling system are usually placed in a regular pattern related to the ceiling grid and the expected seating. Then the layout changes: a cabin is added, a meeting room is enlarged, a row of desks moves. The terminals do not move with it.

The result is predictable. A supply terminal ends up inside a small room that now has no return. Two rooms with very different occupancy share one thermostat. A partition lands beneath a terminal so that half of its air is directed at a wall.

The commercial consequence is that the point at which the layout is frozen is also the point at which the HVAC design can be finished. If terminals are fixed while the layout is still moving, some of them will be wrong, and correcting a terminal after the ceiling is closed costs far more than correcting it on a drawing. This is the same discipline that governs the wider programme, and it is why layout sign-off should be treated as a dependency for services, not only for joinery.

Partition changes after handover need the same treatment. A change that looks trivial to the occupier, such as a new glass meeting room in the open area, can alter airflow for an entire zone.

Zoning, thermostats and heat loads

Zoning. A zone is a group of spaces controlled by one sensor. The design assumes that spaces in a zone behave alike. That assumption fails when a zone spans a sun-facing edge and an internal core, or a meeting room and an open area. The zoning should follow how spaces will actually be used, not just the ceiling grid. How finely a system can be zoned also depends on the system type, which is one of the trade-offs in VRF vs chiller systems for offices.

Thermostat location. A sensor reads only the air around it. Placed near a door, in direct sun, behind a cabinet or in a spot with poor circulation, it will report conditions that do not represent the space and the system will respond to the wrong signal. Where a sensor sits is a design decision and should be checked against the furnished layout.

Heat loads. Cooling is sized on assumptions about occupants, equipment and solar gain. The final floor may differ: more people per unit area than assumed, a bank of large monitors, a print area, a small server or communications closet, or a run of glazing on the sun-facing side. Each adds heat in one place. Comparing the design assumptions with the fitted-out floor is a simple check that is rarely done. The same logic applies to the fresh-air and ventilation provisions of the design; see air changes per hour for how ventilation is expressed.

Air movement: supply, return and noise

Supply air onto desks. Supply terminals throw air a certain distance in a certain pattern. If that pattern lands on a seated person, they feel a draught even though the room temperature is fine. Choosing terminal type and position relative to the furniture, not just to the ceiling, avoids most of this.

Return air. Air has to leave a space as well as enter it. When a room is closed off by a door or a full-height partition and has no return or transfer path, supply air cannot circulate properly. Rooms that pressurise this way often feel stuffy and fail to reach set point. Where the return goes through the ceiling void, the depth and continuity of that void matter; see plenum depth.

Noise. Noise complaints commonly come from air moving quickly through a small opening, a terminal sitting right above a workstation, equipment not properly isolated from the structure, or fans working harder because a return is restricted. Unit type also matters: a fan coil unit puts a fan close to the occupied space, while a VAV system moves the air handling further away but depends on terminal boxes above the ceiling. Neither is quieter by nature; it depends on selection and installation.

The value of a post-occupancy balancing visit

The floor at commissioning is not the floor that will be used. Once furniture, screens, people and a season's weather are present, the real behaviour of the system can be observed. A post-occupancy visit, some weeks after move-in, lets the engineer:

• re-check flows and balance against the furnished layout
• watch how zones respond to sun and occupancy through the day
• move or adjust a terminal, damper or sensor that does not suit the final arrangement
• record what changed since the design so future layout changes are reviewed

It should be a planned line in the handover, not a reaction to complaints. It is the cheapest point at which most comfort problems get corrected, and it converts a stream of individual complaints into one review.

Common mistakes

• Treating a comfort complaint as a capacity problem and ordering more equipment before anyone has measured airflow at the affected desks.
• Placing terminals before the layout is settled, then leaving them where they were after it changes.
• One thermostat for spaces that behave differently, such as a sun-facing edge and an interior meeting room.
• Closing a room without a return or transfer path, then wondering why it will not cool.
• Sizing to the design headcount and equipment, then filling the floor with more of both.
• Signing off at commissioning as if it were the end, with no visit after occupation.
• Adding partitions later without asking the HVAC designer, because it looks like an interior change.

What to ask

• Was the HVAC design drawn against the final, frozen layout, and what changes since would need a review?
• Do zone boundaries and thermostat positions match how each space will actually be used?
• What occupancy, equipment and solar assumptions was cooling sized on, and how do they compare with the final floor?
• Does every enclosed room have a defined return or transfer path?
• How is noise at terminals checked, and by whom?
• Is a post-occupancy balancing visit included, and when?
• Who reviews a partition or seating change before it goes ahead?

A contractor who can answer these has thought about distribution. One who answers only with plant capacity has answered a different question.

Standards referenced

Building services provisions, including air conditioning and mechanical ventilation, are set out in NBC 2016, Part 8. Thermal comfort concepts used in design are described in ISO 7730. Design conditions, airflow, noise limits and control strategy for a specific building must be established and confirmed by the project's HVAC engineer and consultants; nothing on this page replaces that design.