How to Calculate a Building's Cooling Load: Where Heat Sneaks In

First you weigh the heat, then you buy the cooling

Imagine your building is a bucket, and heat is water constantly trickling in. The sun pours some in through the windows. More seeps through the walls and roof. Every time a door opens, hot sticky outdoor air sloshes in. And inside, people, lights and computers all drip warmth of their own. To keep the water level steady — to hold the room at a comfortable temperature — your air conditioning has to bail heat out exactly as fast as it trickles in.

Before you can buy the right-sized bailing bucket, you have to know how fast the water comes in. That hourly rate of heat sneaking into the space is the cooling load, and working it out is the single most important number in the whole design. Get it right and the building is comfortable and cheap to run. Get it wrong and you either roast on hot afternoons or shiver in a cold, clammy room while paying too much.

This part is about where the heat comes from, why the tropical mix is unusually heavy, and why matching your equipment to the real load — not a guess — is the goal.

The four doors heat sneaks through

Cooling load is really just an addition sum. You count every source of heat entering the space in an hour, add them up, and that total is what the cooling system must carry away. There are four main sources.

1. Solar gain through glass

Sunlight passing through a window becomes heat the moment it lands on a floor, a desk or a person inside. Glass is nearly transparent to incoming sunlight but traps the heat that radiates back — the same greenhouse effect that makes a parked car an oven. Solar gain is often the single largest slice of the load in a modern building, because we love big glass facades.

In Malaysia the sun is close to overhead and shines hard all year, so an unshaded window can pour in a great deal of heat every daylight hour. A west-facing glass wall catching the full afternoon sun is, in effect, a heater the size of the window.

2. Conduction through walls and roof

Heat also seeps straight through solid surfaces, flowing from the hot outside to the cooler inside, the way a metal spoon warms up in a hot drink. This is conduction, and it works on every wall, and especially the roof, which bakes under direct sun all day. The rate depends on how well the material insulates: a thin uninsulated metal roof lets heat flood in, while a well-insulated one slows it to a trickle.

3. Hot, humid outdoor air

A sealed box would be easy to cool, but real buildings must let fresh air in so the people inside can breathe — and air also leaks in through gaps around doors and windows. The deliberate, controlled fresh air is ventilation; the uncontrolled leakage is infiltration. Either way, every parcel of outdoor air that comes in arrives hot and, in our climate, soaking wet, and the cooling system has to both chill it and wring the moisture out of it.

This is the tropical sting. In a dry climate you mostly just cool the incoming air. Here, drying it is a huge job in its own right — more on that in a moment.

4. Internal gains: people, lights and machines

Finally, heat is generated inside the space, whether or not the sun is shining. These internal gains come from three main sources:

- People. A human body at rest gives off heat roughly like a 100-watt bulb, and more when active. A packed meeting room heats up fast.
- Lighting. Every watt a light fitting draws ends up as heat in the room. Old fittings run hot; efficient LED lighting cuts both the electricity bill and this slice of the cooling load.
- Equipment. Computers, screens, printers, kitchen gear and motors all turn the electricity they draw into heat. A server closet or a busy kitchen can be a serious hot spot. (Motors are a story in themselves — how electric motors work explains why a hard-working motor is also a small heater, and why the energy it uses shows up as warmth in the room.)

A useful rule of thumb: for most equipment, the heat it dumps into the room roughly equals the electrical power it draws. If it pulls power, it makes heat. That link between watts drawn and watts of heat is worth keeping in mind, and it is the same power-versus-energy idea covered in power vs energy: kW and kWh explained.

Two kinds of heat: sensible and latent

Here is the part people most often get wrong, and it matters enormously in Malaysia. Cooling load splits into two distinct types, and your system has to handle both.

Sensible load is heat you can feel as temperature — the part that makes a thermometer read higher. Sun through the glass, warmth off a laptop, heat conducting through the roof: these push the temperature up.

Latent load is heat locked up in moisture — the humidity in the air. It takes energy to turn liquid water into water vapour, and that energy rides along invisibly in humid air. Your air conditioner only removes it by cooling a surface below the dew point so moisture condenses out, the way a cold glass of iced drink drips with water on the outside. That condensed water is the latent load being carried away, and it does not move the thermometer at all — it just makes the room feel less muggy.

We introduced this split earlier in the course when we looked at what "sensible" and "latent" really mean, and it is worth reaching back to that idea here, because the tropics tilt the balance hard toward latent. The mix of the two is described by the sensible heat ratio — the fraction of the total load that is sensible. A dry-climate office might be mostly sensible. A Malaysian building, fed with warm humid outdoor air and full of people (who breathe out moisture), carries a much larger latent share.

Why does the ratio matter? Because a system sized only for temperature can hit the setpoint on the thermometer while barely touching the humidity — leaving you in a room that is cold but clammy. Sizing has to cover both parts, and the ratio between them decides how much drying, or dehumidification, you actually get. Miss the latent load and you have a comfortable-looking number and an uncomfortable room.

Why the envelope is such a big deal here

Three of those four heat sources — solar gain, conduction, and much of the air leakage — all come through the building's outer skin, its envelope: the walls, roof, windows and doors. A leaky, glassy, sun-facing facade is essentially a funnel dumping outdoor heat into the space, and every watt it lets in is a watt the chiller then has to pull back out and pay for.

Malaysia takes this seriously enough to regulate it. The building energy standard MS1525 sets a cap on the Overall Thermal Transfer Value (OTTV) — a measure of how much solar and conducted heat the envelope lets through per square metre of facade. For buildings with more than 1,000 square metres of air-conditioned space, OTTV must not exceed 50 watts per square metre. In plain terms, the rule limits how much of a heat funnel your facade is allowed to be, by pushing designers toward shading, better glass and insulation. A building that respects OTTV starts life with a smaller cooling load baked in — the cheapest cooling is the heat you never let in.

Adding it all up — and why right-sizing is the goal

Put the pieces together and the recipe for cooling load is:

$$\text{Cooling load} = \text{solar} + \text{envelope conduction} + \text{ventilation/infiltration} + \text{internal gains}$$

each of those split into its sensible and latent parts, and all of it measured as a rate of heat per hour. For a house or a small shop, a structured hand method such as Manual J walks you through exactly this, room by room, so the number is calculated rather than guessed. Bigger buildings use software that does the same sum for every hour of a typical year.

The whole point of this arithmetic is right-sizing: choosing cooling equipment that matches the real peak load, not a padded guess. It is tempting to think "bigger is safer," but oversizing backfires. An oversized unit cools the air so fast that it satisfies the thermostat and switches off before it has run long enough to wring much moisture out — so you get a room that is cold but still damp, plus a machine that spends its life stopping and starting. That stop-start pattern, called short cycling, drives up wear and energy bills; we devote a later part entirely to why it is so damaging, so we will leave the mechanism there.

Undersizing is the opposite failure. A unit too small for the load runs flat out on the hottest, most humid afternoons and simply never reaches the setpoint — the bucket fills faster than it can bail. Right-sizing threads the needle: enough capacity for the genuine peak, not so much that the system spends its life short-cycling.

And notice that every number in this chapter has been in units of heat — watts and kilowatts of heat to be removed. The next step is to convert that pile of heat into the language the cooling industry actually buys and sells in: tons of refrigeration. That translation is where we go next.

The takeaway

A building's cooling load is simply the heat sneaking in every hour, added up from four doors — sun through glass, conduction through walls and roof, hot humid outdoor air, and the warmth of people, lights and machines inside — and split into a sensible (temperature) part and a latent (moisture) part. In Malaysia the solar and latent shares are unusually large, the envelope is a major contributor that OTTV rules deliberately rein in, and the goal is always to right-size: match the equipment to the real load so the room is both cool and dry, without the waste of oversizing or the misery of undersizing.

Next, we turn that heap of watts into the unit cooling is actually sold in — refrigeration tons — and see how a load on paper becomes a machine you can specify.