Cooling Load Estimate

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A room's sensible and latent heat gain from walls, roof and glass (U × A × ΔT), people, lights, equipment and outside air — in W, BTU/h and tons

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Enter the values

Walls: area (0 to skip)

Walls: U-value

W/m²K

Example value — use your textbook/material data

Walls: temperature difference (ΔT)

Roof: area (0 to skip)

Roof: U-value

W/m²K

Example value — use your textbook/material data

Roof: temperature difference (ΔT)

Glass (windows): area (0 to skip)

Glass (windows): U-value

W/m²K

Example value — use your textbook/material data

Glass (windows): temperature difference (ΔT)

Number of occupants

Sensible heat per person

ASHRAE: moderately active office work

Latent heat per person

ASHRAE: moderately active office work

Lighting (total)

Equipment (total)

Outside air (ventilation/infiltration; 0 to skip)

Outdoor − indoor temperature difference

Humidity ratio difference (outdoor − indoor; 0 to skip)

g/kg

Read from a psychrometric chart

Safety factor (allowance)

Result

Total cooling load

1,959.69W

Total cooling load

0.5572TR

Total cooling load: 6,686.73 BTU/h
Sensible heat (total): 1,671.53 W
Latent heat (total): 110 W
Sensible heat ratio (SHR): 0.94
Through walls: 480 W
Through roof: 225 W
Through glass (conduction): 182.4 W
Occupants: 260 W
Lighting and equipment: 500 W
Outside air: 134.13 W

This is a simplified estimate for study, not a full CLTD or RTS calculation. A real design accounts for orientation, time of day, shading and the thermal mass of the structure.

Direct solar gain through glass is not included — on a sunlit window it can be much larger than the conduction part.

Working

Q_wall = U × A × ΔT = 2 × 30 × 8 = 480 W
Q_roof = U × A × ΔT = 1.5 × 15 × 10 = 225 W
Q_glass = U × A × ΔT = 5.7 × 4 × 8 = 182.4 W
Q_people = 2 × (75 + 55) = 150 + 110 W
Q_lights + Q_equipment = 200 + 300 = 500 W
V̇ = 50 ÷ 3600 = 0.01389 m³/s
Q_air,s = ρ × cp × V̇ × ΔT = 1.2 × 1006 × 0.01389 × 8 = 134.1 W
Q = (1671.5 + 110) × (1 + 0.1) = 1959.7 W
TR = 1959.7 × 3.412 ÷ 12000 = 0.557 TR

Formulas

Q_surface = U × A × ΔT
Q_people = n × (q_s + q_l)
Q_air,s = ρ × cp × V̇ × ΔT
Q_air,l = ρ × h_fg × V̇ × Δω
Q_total = (ΣQ_s + ΣQ_l) × (1 + SF)
TR = Q (W) × 3.412 ÷ 12 000

U: overall heat-transfer coefficient (W/m²K)
A: area (m²)
ΔT: outdoor − indoor temperature difference (K)
V̇: outside-air flow (m³/s)
Δω: humidity ratio difference (kg/kg dry air)
SF: safety factor

Assumptions

Air density ρ = 1.2 kg/m³, specific heat cp = 1.006 kJ/kgK, latent heat of vaporisation of water h_fg = 2501 kJ/kg.
75 W sensible and 55 W latent per person — ASHRAE Handbook — Fundamentals, Chapter 18, Table 1 ('moderately active office work'). Change them from the table for other activities.
The U-values (wall 2.0, roof 1.5, glass 5.7 W/m²K) are only examples — use your material's values.
All the electrical power of lights and equipment is taken to become heat in the room.
1 TR = 12 000 BTU/h = 3.517 kW; 1 W = 3.412 BTU/h; 1 kcal/h = 1.163 W.

Sources: ASHRAE Handbook — Fundamentals (load calculation and psychrometrics chapters) · C. P. Arora, Refrigeration and Air Conditioning · W. F. Stoecker and J. W. Jones, Refrigeration and Air Conditioning

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