Why HVAC sizing decides your fit-out budget
HVAC is 12–18% of a fit-out capital cost and most of its running cost. Sizing it by thumb rule is the most expensive shortcut in the industry.
The common thumb rule, one tonne of refrigeration per 250 sq ft, was written for a different occupancy, a different glazing spec and a different equipment load. Applied to a modern office it can be wrong in either direction, and both directions cost money.
Oversizing
Oversized equipment costs more to buy, needs larger electrical provision, and short-cycles in operation, which means poor humidity control, uneven temperatures and higher energy consumption than the nameplate suggests. It also consumes ceiling void and plant space you have already paid to build.
Undersizing
Undersizing shows up after handover, in complaints. The fixes are all expensive: additional indoor units into a finished ceiling, upgraded outdoor capacity with new pipe runs, or a supplementary system with its own electrical feed. Rework in an occupied office runs at a premium and happens at night.
What a real load calculation includes
- Occupancy by zone, at design headcount rather than day-one headcount.
- Equipment load per workstation, plus server, UPS and kitchen loads listed separately.
- Lighting load from the actual fitting schedule, in W/sq ft.
- Envelope: glazing area, orientation, shading coefficient and U-value from the building drawings.
- Fresh air per ASHRAE 62.1 and the local code, with the treatment path stated.
- Diversity and simultaneity assumptions, written down so they can be argued with.
Sequence matters more than method
The calculation has to happen before the ceiling plan is frozen, not after. Load determines indoor unit count and size; unit size determines void depth; void depth determines ceiling height. Reverse that order and you either lose ceiling height late or accept an equipment layout that fights the design.
What to ask for
Ask for the heat load sheet, zone by zone, with assumptions visible, and ask what happens to the tonnage if headcount rises 20%. If the answer is a number and a page, the design is engineered. If the answer is a rule of thumb, the risk is still on your side of the table.
Have a quote or a BOQ in front of you? Send it across and we will read it line by line.
