Cooling Load – Sensible and Latent Heat Explained

Educational explanation, not professional design advice. Last reviewed 11 November 2026. Disclaimer

In plain English

A room gets warmer and more humid because heat keeps coming in: through the walls and roof, as sunshine through the windows, from people, lights and machines, and with the warm outside air that leaks or is blown in. The cooling load is the rate at which an air-conditioning system has to remove that heat to keep the room at the temperature and humidity you want.

Part of the load raises the air temperature — this is sensible heat, because you can sense it on a thermometer. The other part adds moisture to the air, from people breathing and sweating, cooking, wet processes and humid outside air — this is latent heat. The air conditioner removes latent heat by condensing water on its cold coil, which is why it drips. A good system has to handle both parts.

Technical definition

The cooling load is the rate of heat removal, in watts (W) or kilowatts (kW), needed to hold a space at its design conditions. It is the sum of the sensible load Q_s, which changes the dry-bulb temperature, and the latent load Q_L, which changes the moisture content (humidity ratio W, kg of water per kg of dry air). The sensible heat ratio SHR = Q_s ÷ (Q_s + Q_L) describes the mix. In trade units, cooling capacity is often given in Btu/h or tons of refrigeration (TR), where 1 TR = 12 000 Btu/h = 3.517 kW.

How it works and where it is used

A cooling load estimate adds up the separate heat gains at the design condition — usually a hot afternoon in the warmest month:

Heat gain	Type	How it is estimated
Walls and roof	Sensible	U-value × area × temperature difference (larger for sunlit surfaces)
Windows	Sensible	Conduction (U × A × ΔT) plus solar gain (area × SHGC × solar radiation)
People	Sensible and latent	Number of people × heat per person; depends on activity
Lights and equipment	Mostly sensible	Power actually in use; kettles and cooking add latent heat
Outside air (ventilation, infiltration)	Sensible and latent	Airflow × air properties × temperature and humidity difference

In hot, humid climates the outside air is often the largest single latent load, and the solar gain through glass the largest sensible one. That is why shading, smaller west-facing windows and controlled ventilation make such a difference to the size of the equipment.

Formula

Q_s = ρ × V̇ × c_p × ΔT Q_L = ρ × V̇ × h_fg × ΔW

Q_total = Q_s + Q_L SHR = Q_s ÷ Q_total

ρ = air density (about 1.2 kg/m³), V̇ = airflow (m³/s), c_p = specific heat of air (about 1.006 kJ/(kg·K)), ΔT = temperature difference (K), h_fg = latent heat of water vapour (about 2501 kJ/kg), ΔW = humidity ratio difference (kg/kg). With these values, ρ × c_p ≈ 1.21 kJ/(m³·K), so 1 m³/s of air cooled by 1 K carries about 1.21 kW of sensible heat. The inch-pound version of the sensible formula is the familiar Q (Btu/h) = 1.08 × cfm × ΔT (°F).

Worked example

Example 1 — air carrying heat. A supply of 0.5 m³/s of air leaves the coil 10 K colder than the room. Sensible cooling = 1.2 × 0.5 × 1.006 × 10 = 6.04 kW (about 1.7 TR).

Example 2 — a small office. Design conditions 24 °C inside and 40 °C outside (ΔT = 16 K); humidity ratio 9.3 g/kg inside and 14 g/kg outside.

Walls 12 m² × U 0.6 × 16 K = 115 W; roof 20 m² × U 0.45 × 16 K = 144 W
Windows 3 m²: conduction 3 × 2.8 × 16 = 134 W; solar 3 × 0.6 × 400 W/m² = 720 W
4 people: 4 × 75 W = 300 W sensible and 4 × 55 W = 220 W latent
Lights 200 W and equipment 600 W (sensible)
Outside air 40 L/s: sensible 1.2 × 0.040 × 1.006 × 16 = 773 W; latent 1.2 × 0.040 × 2501 × 0.0047 = 564 W

Sensible total = 2.99 kW, latent total = 0.78 kW, total = 3.77 kW (1.07 TR), SHR = 0.79. With a 10% margin the design load is 4.15 kW = 1.18 TR, so a nominal 1.5 TR unit would be chosen. Notice that the window sun gain alone is almost a fifth of the total.

In the workshop and in practice

A cooling load estimate is used to choose the size of a split unit, packaged unit or chiller, to size air ducts and chilled-water pipes, and to compare the effect of improvements such as roof insulation, window film or shading. The SHR matters when selecting the unit: equipment made for dry climates may remove too little moisture in a humid building, leaving people cold but clammy. Rules of thumb such as “one ton per so many square metres” give only a first guess, because they ignore the windows, the people and the ventilation, which can double or halve the real load.

Common mistakes and misconceptions

Ignoring latent heat. Sizing only on temperature leaves the room humid; people, kitchens and outside air add a lot of moisture.
Oversizing “to be safe”. A unit that is far too large cools quickly, switches off before it has removed the moisture, and wastes energy. Use a modest, stated margin.
Using the outdoor air temperature for sunlit roofs. A dark roof in the sun is much hotter than the air; use an equivalent temperature difference.
Mixing kW of cooling with kW of electricity. A 3.5 kW (1 TR) air conditioner may draw only about 1 kW of electricity; the ratio is its efficiency (COP or EER).

Related calculators

AC Cooling Load Estimator — Preliminary room cooling load from its separate heat gains — sensible and latent — in kW, Btu/h and tons, with a suggested AC size.
Air Sensible & Latent Heat Calculator — Sensible, latent and total heat of an air stream — or the airflow or temperature change for a given load.
Psychrometric Properties Calculator — Humidity ratio, dew point, wet bulb, enthalpy and specific volume of moist air from dry-bulb temperature and relative humidity.

References

ASHRAE Handbook — Fundamentals: Nonresidential Cooling and Heating Load Calculations; Psychrometrics
CIBSE Guide A: Environmental Design
ISO 52016-1: Energy performance of buildings — Calculation of the energy needs for heating and cooling

Cooling Load: Sensible and Latent Heat