Cooling Load Calculation: Sensible, Latent and Solar Gains

Heat gain is energy entering or generated in a space. Cooling load is the rate at which the cooling system must remove heat at that time. Convective gains become load quickly, while radiant gains can be absorbed by surfaces and released later.

That delay is why adding the maximum value of every component can exaggerate the real peak. Methods such as radiant time series or heat balance account for time and storage more explicitly than a purely steady-state sum.

Sensible load changes dry-bulb temperature. Latent load changes moisture content. Lighting, equipment and envelope conduction are primarily sensible. People and outdoor air can add both sensible heat and water vapour.

Supply airflow based only on total tons can hide a moisture problem. Review sensible heat ratio and the equipment's sensible and latent capacity at the actual entering conditions.

Window solar gain depends on glazing area, solar heat gain coefficient, orientation, shading and the sun's position. East glazing tends to be important earlier in the day; west glazing commonly drives a later afternoon peak. North-facing glazing can still have conductive and diffuse-solar effects.

A floor-plan model needs a north arrow and windows assigned to the correct exterior wall. A glazing total without direction cannot represent the timing of solar exposure.

A west room and an east room may each have a 5 kW maximum, but not at the same hour. The sum of their independent maxima is 10 kW; the coincident combined maximum may be lower. Terminal units and room airflow use room-level peaks, while central plant selection should consider the coincident system peak under the adopted method.

Ventilation conditioning may be handled at room level, at an air-handling unit or as a separate outdoor-air system block. State the treatment clearly so it is not omitted or counted twice.

Use component and timing checks before accepting an automatically calculated result.

A cooling calculation becomes more useful when the major gains are evaluated over a design-day sequence. Outdoor conduction usually changes gradually. Direct solar exposure moves from east to south to west façades. People, lights and equipment follow schedules. The room's fabric delays part of the radiant gain before it becomes a space cooling load. Evaluating a series of hours reveals when the room peaks and prevents every component from being combined at an impossible worst-case instant.

Consider two identical meeting rooms, one facing east and one west. Both may have the same window area, U-value, SHGC, occupancy and lighting. The east room can peak during the morning solar period; the west room can peak late in the afternoon when outdoor temperature is also high. Their independent room peaks help size terminal airflow, but the system block peak must compare both rooms at common hours. Adding their separate maxima can overstate the air-handler or chiller load.

Window calculations require more than cardinal direction. Exterior shading, overhangs, adjacent buildings, glazing tilt, interior blinds and frame fraction all change transmitted solar gain. In early design, simplified orientation factors can support option comparisons, provided the assumptions remain visible. For final design, use the solar and heat-balance method required by the project and document any shading model that materially affects equipment size.

Room sensible load sets a first-pass supply-airflow requirement when the supply temperature is known. In IP units, CFM ≈ sensible Btu/h ÷ (1.08 × room-to-supply ΔT). A room with 12,000 Btu/h sensible load and 55°F supply air at a 75°F room condition needs roughly 556 CFM. In SI, the same relationship uses air density and specific heat: volumetric flow = watts ÷ (ρ × cp × ΔT). This airflow must also satisfy ventilation, distribution and equipment minimums.

Latent load cannot be converted with the sensible equation. It determines the moisture-removal rate and affects the required coil leaving-air humidity ratio. Outdoor air can dominate latent capacity in humid weather even when room sensible load is modest. The coil selection therefore needs entering dry-bulb, entering wet-bulb or humidity ratio, airflow, bypass behaviour and leaving condition—not only a total tonnage number.

A useful report separates room sensible load, room latent load, outdoor-air sensible load and outdoor-air latent load. It then states whether outdoor air is mixed at the central unit, conditioned by a dedicated outdoor-air system or delivered untreated to local equipment. This allocation prevents both omission and double counting and makes the sensible heat ratio available for coil and terminal review.