How a heat-load calculation actually works (with a worked example)

Two identically sized rooms can need very different AC capacity. Here's what a proper heat-load calculation accounts for, and why a thumb rule can't.

"How many tons of AC do I need?" is usually answered with a thumb rule — something like one ton per 100–120 square feet. That rule ignores almost everything that actually determines how much heat a room gains, which is why so many systems in Bhopal are either oversized (short-cycling, poor humidity control, wasted cost) or undersized (unable to cope by May).

What goes into the calculation

• Solar gain through glass — area, orientation (west-facing glass in Bhopal gains far more heat than north-facing), and whether it's shaded
• Conduction through walls and roof, especially a top-floor room with direct roof exposure
• Occupancy — number of people and their activity level
• Lighting and equipment load — a server room or a kitchen changes the number substantially
• Fresh air / ventilation requirement for the space type
• Outdoor design conditions for the city and season — we use Bhopal's peak summer figures, not an annual average

A simplified worked example

Take two 200 sq ft rooms in the same building. Room A faces north, has one small window, and seats two people at desks. Room B faces west, has a full glass wall, and is a 10-person meeting room with a projector and a large display screen.

A per-square-foot thumb rule would size both rooms identically. The actual load in Room B is roughly 2.5–3 times higher — and that gap is exactly the kind of error that shows up as "the meeting room is never cool enough" complaints months after installation.

Why this matters for your quotation

When we quote a project, the capacity figure comes from this calculation, room by room, not a blanket rate applied to the total floor area. It's also why two quotations for the "same" building can specify very different total tonnage — one of them is usually wrong.

If you're comparing HVAC quotes and the tonnage was never explained, it's worth asking how it was calculated. Our design and consultancy page has more on the full process, from site survey through to the bill of quantities.