Commercial heat-load calculations
For a commercial building, working out the right size of air conditioning isn’t a rule of thumb — it’s an engineering calculation. A heat-load (or cooling-load) calculation adds up every source of heat in a building to find the real demand the system must meet, in kW. It’s the foundation of a system that’s comfortable, efficient and correctly sized.
Why a rule of thumb doesn’t work at scale
In a home, a rough “kW per square metre” gets you close. In a commercial building it can be wildly off, because the heat gains are large and vary hugely from space to space — a glazed, south-facing meeting room full of people and laptops is a different world from a north-facing store room. Get the sizing wrong and you either waste capital on oversized plant that runs inefficiently, or you’re left short on the hottest days.
What a heat-load calculation accounts for
- People — every occupant gives off heat; a busy space adds up fast.
- Lighting — especially older or feature lighting.
- Equipment — IT, kitchen gear, machinery and AV all contribute.
- Solar gain — heat through glazing, varying by orientation and shading.
- Building fabric — how much heat passes through walls, roof and floor.
- Fresh air and infiltration — conditioning incoming outside air is part of the load.
- How the space is used — occupancy patterns and hours of operation.
Diversity — sizing to the real peak
A key part of doing this properly is diversity: the recognition that not every space hits its maximum load at the same moment. Sizing the plant to the sum of every room’s individual peak would oversize the system — and waste money — because the simultaneous peak across the building is lower in reality. A good calculation sizes the system to that realistic combined peak, which is where experience and proper method earn their keep.
How it's done, and by whom
A heat-load calculation is carried out by a qualified building-services engineer, usually with software, following recognised UK industry methods (such as CIBSE guidance). They model the building zone by zone, apply the gains above, account for diversity, and produce the cooling and heating loads that drive plant selection — the VRF, chiller or packaged system, and how it’s zoned and controlled.
Why it matters: oversized plant costs more to buy, takes more space, and runs inefficiently because it short-cycles. Undersized plant can’t cope on peak days and leaves the building uncomfortable. The heat-load calculation is what gets it right — and everything else (system choice, zoning, running cost, the business case) follows from it.
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