Heat load: what a room actually asks an aircon to remove

Heat load is the rate at which heat arrives in a room. It is quoted in the same units as cooling capacity, either BTU per hour or kilowatts, because the two figures are meant to be held against each other. One describes what the room takes on. The other describes what a machine can carry away.

A room holds its temperature only while removal keeps pace with arrival. Clear heat faster than it comes in and the temperature falls until the two rates meet. Let the room gain faster than the machine can clear, and the temperature climbs whatever the remote is set to. That balance sits underneath every complaint about a room that will not cool.

The load is not a fixed property of a room. It moves through the day, it moves with the weather, and it moves when the space is used differently. Sizing is worked against the peak the room has to hold, not the average it sees. A room that gives up every afternoon still has a perfectly reasonable average.

Capacity ratings carry assumptions of their own, since they are measured at stated test conditions. That is workable while both sides of the comparison are honest about what was assumed. It stops being workable when the load side was never worked out at all, which is the more common failure.

Floor area generates no heat. It stands in for the things that do, and it does so loosely. A bigger floor usually brings more wall, more ceiling, more glass and more people, so the correlation is real enough to have survived. It is still only a correlation, and it breaks wherever a room is unusual.

Two bedrooms of identical size in the same block can sit far apart. One faces an internal corridor, has a single small window, and is flanked by other cooled bedrooms. The other looks west over an open car park, carries a wide window, and sits directly under the roof. A floor plan cannot tell them apart. Anyone who has slept in both can.

Any figure expressed per square metre is an average taken over assumed conditions. It was fitted to a room with ordinary glass, ordinary exposure and ordinary use. On that room it lands close. On a top-floor corner unit behind full-height glass it does not, and quoting it with confidence changes nothing about the arithmetic.

The method survives because most rooms are ordinary. It fails on the rooms that were already hard, and those are exactly the rooms the complaint comes from. A contractor who has sized from floor plans for years can hold a good record and still have put the wrong unit into every difficult room they ever quoted.

Sun arriving through glass is the biggest single difference between one Singapore room and another. Glass passes most of the sun's radiation straight inside, where it lands on floors and furniture and turns into heat. A solid wall absorbs the same energy, holds it, and passes only a fraction of it inward. Trading a small window for a wide one moves a room into a different bracket.

Direction sets both the size of that gain and the hour it arrives. Singapore lies almost on the equator, so the sun climbs steeply toward the middle of the day and hangs low in the east and the west at either end. Vertical glass takes its worst dose while the sun is low enough to shine straight through it. Near midday, most of that energy is landing on roofs rather than windows.

West-facing rooms therefore struggle late rather than at noon. The gain arrives while the outdoor air is already near its warmest and the structure has been soaking up heat since morning. An east-facing room of the same shape takes a comparable dose earlier, onto walls that have had all night to shed heat. The window is the same. The problem is not.

What stands opposite the glass matters as much as the compass reading. A west window looking into the flank of the next block sees direct sun for a fraction of the time an unobstructed one does. A deep reveal, the slab overhanging from the floor above, or a service ledge all cut into the beam before it reaches the glass. None of that shows up in a room measurement.

The glass itself is the other half of the answer. Solar film and low-emissivity glazing reduce how much radiation gets through in the first place, which is the right place to stop it. Ordinary clear glazing stops very little, and a wide expanse of it is closer to an opening than to a wall.

Conduction is heat pushed through a surface by the temperature difference across it. The wider that gap, and the more readily the material passes heat, the faster it flows. Every wall, the ceiling, the floor and the door are doing this whenever the room is cooler than whatever sits on the other side.

Internal surfaces count alongside external ones. A bedroom wall shared with an uncooled living room is a load. So is the wall a kitchen sits behind, the door to a warm corridor, and the floor above a flat that runs no aircon at all. Cooling one room inside a warm flat means paying for the boundary the whole way round.

People are a steady source and a predictable one. Each person gives off heat continuously, more when active than at rest, and part of that output is moisture rather than warmth. A bedroom sized around two sleepers behaves differently on a night when four are in it, and a living room during a gathering can be dominated by the people standing in it.

Nearly every watt of electricity used inside the room ends up as heat inside the room. A television, a desktop machine, a router, a charger left on the desk, and the downlights overhead all contribute. Older filament and halogen fittings give off far more heat than the same light from LEDs. A bedroom converted into a home office is a warmer room than it was.

Outdoor air finding its way in is the source most often left out. It arrives under door leaves, around older window frames, through the sleeve where pipework passes out to the ledge, and through any door left open. Each parcel brings the outdoor condition in with it, which here means both heat and a large amount of moisture. A bathroom fan running with everything shut pulls that replacement air in from somewhere.

A cooling load comes in two parts, and temperature is only the first of them. Dry heat is the part a thermometer reports. The other part is the water carried in the air, and stripping it out is work the machine has to do before a room feels right. In this climate that second half is large.

Moisture leaves the air only by condensing. The coil has to run cold enough for water to form on it, and that water then has to be caught and drained away. Energy spent turning airborne moisture into liquid is capacity that never went into lowering the temperature. It is real work, it is metered the same way, and a thermometer cannot see any of it.

Outdoor air here sits close to its limit for much of the year, so anything that lets it in loads the room heavily. Leakage is the main route in, and cooking, showering, wet laundry and the occupants themselves add to what is already there. The same room in a cooler country, pulling in the same volume of outside air, takes on a fraction of the water.

This is why rules of thumb imported from cooler countries undersize systems here. They were fitted where the moisture share of the load is modest, so a figure that behaves there quietly assumes a job the local air will not let a machine skip. The temperature arithmetic still looks right. The unit cannot hold the room, because part of its rated output is being spent on water.

Oversizing is not the way around this. A unit with capacity to spare reaches the set temperature quickly and stops, and a coil that spends its time warming back up condenses very little. The room ends up cold and still damp, which is the complaint that follows almost every generously specified system. Capacity chosen properly runs longer at lower output and dries the air while it does.

A nameplate figure is a total, and how it splits between the two halves depends on the air reaching the coil. Two identical units in rooms with different moisture loads deliver different amounts of temperature drop, and neither one is faulty. The rating is a ceiling on the combined job, not a promise about either half of it.

A sizing survey is mostly questions about the room rather than measurements of it. Window direction and glass area come first, along with whatever stands opposite each window. Then the floor the flat sits on, what lies above the ceiling, and which walls face outdoors. Then ceiling height, door and window sealing, who uses the room, what runs inside it, and whether the space opens onto anything uncooled. Floor area is one line on that list.

The outdoor unit gets looked at as well, because the heat has to go somewhere. A condenser boxed into a tight ledge, breathing its own exhaust, cannot reject heat at its rated rate. The room then behaves like an undersized system while the indoor unit is exactly the right size for it. Placement belongs in the sizing decision rather than being settled afterwards.

On an existing system the survey also has to separate load from condition. A fouled coil, a low charge or a blocked drain produce the same room symptoms as a capacity shortfall. Getting the unit into known-good condition first is what makes a sizing verdict worth anything. An upsize sold on a symptom that a service would have cleared is the most expensive way to be wrong about this.

A capacity quoted over the phone from a floor area is an estimate, and there is nothing dishonest about an estimate described as one. What matters is whether the contractor can say what was assumed. Ask which direction the windows were taken to face, and which floor the flat was assumed to sit on. A quote that cannot answer either question assumed the median room.

Load also changes after a system goes in. A partition comes down, a bedroom becomes a home office, the low block that shaded the west window is rebuilt taller. Sizing that was correct can stop being correct without a single component ageing. That is worth establishing before a system which used to hold the room is written off as failed.

The arithmetic itself belongs elsewhere. A separate guide covers how capacity is worked out from these inputs, and the BTU calculator runs the sum for a given room. What this page supplies is the list of things those two have to be told. Answer them honestly and the figure that comes back is a calculation rather than a guess.