HVAC Engineering

Cooling Load Estimation

A rigorous walkthrough of the CLTD/CLF method — from heat transfer fundamentals to equipment sizing

What is cooling load?

When you run an air conditioning system, the equipment removes heat from a space. Cooling load is the rate at which that heat must be removed to maintain a set temperature and humidity. It is measured in watts (W) or BTU/hr.

Two terms are frequently confused:

Heat gain is the instantaneous rate at which heat enters a space from all sources at a given moment. Cooling load is the rate at which the cooling system must actually extract heat — it is always less than or equal to instantaneous heat gain, because building materials absorb heat and release it slowly over time. A concrete wall struck by afternoon sun does not dump all that heat into the room instantly; it releases it over several hours. This lag is the thermal storage effect, and it is why a properly calculated cooling load is lower than a naive sum of all heat gains.

Peak cooling load is the maximum cooling load occurring over a 24-hour design day. HVAC equipment is sized to this peak value.

Design conditions

Before calculating anything, fix two sets of boundary conditions:

Outdoor design conditions: ASHRAE climate data gives the 0.4% or 1% design dry-bulb temperature — the temperature exceeded only 0.4% or 1% of hours in a typical year. For Kathmandu, the design dry-bulb is approximately 32 °C.
Indoor design conditions: Typically 24 °C dry-bulb at 50% relative humidity for comfort cooling.

The temperature difference driving heat transfer through the envelope is:

ΔT = T_outdoor − T_room

Heat transfer mechanisms

Heat enters a building through three simultaneous mechanisms:

Conduction — heat flow through solid material (walls, roof, glass, floor), driven by temperature difference across the material thickness.

Convection — heat transfer through fluid movement; warm room air rises and cooler air falls, continuously redistributing heat from warm surfaces.

Radiation — heat transfer via electromagnetic waves, requiring no medium. Solar radiation passes directly through glass; interior surfaces also exchange radiant heat with each other according to their temperatures.

The HVAC engineer accounts for all three in each component calculation.

Thermal resistance and U-value

Thermal resistance (R-value) measures how strongly a material resists heat flow. Higher R means better insulation.

R = L / k

R = thermal resistance (m²·K/W)
L = material thickness (m)
k = thermal conductivity of the material (W/m·K)

Typical k values: concrete 1.7 · brick 0.84 · mineral wool 0.04 · glass 1.05 · plasterboard 0.16 · air gap 0.18 W/m·K.

For a multi-layer wall, resistances add in series:

R_total = 1/h_i + R₁ + R₂ + R₃ + … + 1/h_o

h_i = inside film coefficient ≈ 8.3 W/m²·K (still interior air)
h_o = outside film coefficient ≈ 34 W/m²·K (at 24 km/h wind)

The U-value (overall heat transfer coefficient) is the reciprocal of total thermal resistance — it tells you how many watts flow through 1 m² of construction per 1 °C of temperature difference:

U = 1 / R_total = 1 / (1/h_i + Σ(L_n/k_n) + 1/h_o)

Example — Wall U-value

Wall build-up: 12 mm plaster (k = 0.72) · 200 mm brick (k = 0.84) · 50 mm mineral wool (k = 0.04) · 12 mm plasterboard (k = 0.16)

R_total = 1/8.3 + 0.012/0.72 + 0.200/0.84 + 0.050/0.04 + 0.012/0.16 + 1/34 = 0.120 + 0.017 + 0.238 + 1.250 + 0.075 + 0.029 = 1.729 m²·K/W U = 1 / 1.729 = 0.578 W/m²·K

The CLTD method

Cooling Load Temperature Difference (CLTD) is an effective temperature difference that already accounts for (i) solar radiation absorbed by the outer surface, (ii) thermal storage in the wall or roof mass, and (iii) the time lag between peak solar exposure and peak heat delivery to the room interior.

Using a raw ΔT for opaque walls would overestimate the load because it ignores that the wall stores heat and releases it hours later. CLTD corrects for this; values come from ASHRAE tables indexed by wall/roof construction group, compass orientation, and hour of day.

1 · Conduction through opaque surfaces (walls and roof)

Q = U × A × CLTD_corr

Q = cooling load (W)
U = overall heat transfer coefficient (W/m²·K)
A = surface area (m²)
CLTD_corr = corrected cooling load temperature difference (°C)

The CLTD correction adjusts the table value for your actual indoor set point and local mean outdoor temperature:

CLTD_corr = CLTD_table + (25.5 − T_room) + (T_mean,outdoor − 29.4) T_mean,outdoor = T_design,db − (daily range / 2)

The correction matters when your indoor set point or climate differs from the ASHRAE base conditions of 25.5 °C indoors and 29.4 °C mean outdoor temperature.

2 · Conduction through glazing

Glass has negligible thermal mass, so there is no meaningful time lag. A direct temperature difference replaces CLTD:

Q_glass,cond = U_glass × A_glass × (T_outdoor − T_room)

Typical U-values: single glazing 5.8 · double 2.8 · double low-e 1.6 W/m²·K.

3 · Solar gain through glazing

Q_solar = A_glass × SC × SHGF_max × CLF

SHGF = Solar Heat Gain Factor — tabulated solar radiation through clear single-pane glass (W/m²)
SC = Shading Coefficient — ratio of your glass solar transmission to that clear reference pane (SC = 1.0 for clear single pane; 0.4–0.7 for coated/tinted glass)
CLF = Cooling Load Factor — accounts for thermal mass absorbing and re-radiating solar gain over time (0 to 1, from ASHRAE tables by zone type and hour)

A south-facing window at 28°N in June has SHGF_max ≈ 300 W/m². External shading or a low-SC film can cut this by 40–60%.

4 · Infiltration

Infiltration is uncontrolled air leakage through gaps, cracks, and construction imperfections. It carries both sensible load (temperature) and latent load (moisture).

The mass flow rate of infiltrating air:

ṁ_inf = (ACH × V_room × ρ_air) / 3600

ACH = Air Changes per Hour (tight construction 0.25, average 0.5–1.0, loose 1.5–2.0)
V_room = room volume (m³)
ρ_air ≈ 1.2 kg/m³ at sea level; ≈ 1.05 kg/m³ at Kathmandu (1 400 m elevation)
3 600 converts hours to seconds

Q_inf,sensible = ṁ_inf × c_p × (T_outdoor − T_room) Q_inf,latent = ṁ_inf × h_fg × (W_outdoor − W_room)

c_p = specific heat of air = 1 006 J/kg·K
h_fg = latent heat of vaporisation of water ≈ 2 501 kJ/kg
W = humidity ratio (kg water vapour per kg dry air); from psychrometric tables using dry-bulb and wet-bulb temperatures

5 · Ventilation load

Ventilation is controlled fresh air deliberately supplied to dilute indoor pollutants and CO₂. ASHRAE 62.1 gives minimum ventilation rates:

V_z = R_p × P_z + R_a × A_z

V_z = zone outdoor airflow rate (L/s)
R_p = per-person outdoor air rate: 2.5 L/s/person for offices
P_z = design occupant count
R_a = per-area outdoor air rate: 0.3 L/s/m² for offices
A_z = zone floor area (m²)

ṁ_vent = V_z × ρ_air / 1 000 Q_{vent,sensible} = ṁ_vent × c_p × (T_outdoor − T_supply) Q_vent,latent = ṁ_vent × h_fg × (W_outdoor − W_supply)

6 · Internal gains — people

People emit heat through metabolism. The sensible/latent split depends on activity level: harder work means more sweating, raising the latent fraction. Latent heat goes directly into the air with no thermal storage effect, so CLF_latent = 1.0 always.

Q_{people,sensible} = N × q_s × CLF Q_{people,latent} = N × q_l

N = number of occupants
q_s, q_l = sensible and latent heat emission per person (W)
CLF = from ASHRAE tables by zone type and hours of occupancy

Activity	Sensible (W)	Latent (W)	Total (W)
Seated, office	75	55	130
Light work, standing	75	85	160
Walking	75	105	180
Heavy work	65	315	380

7 · Lighting

All electrical energy consumed by lighting eventually becomes heat in the space (1 W in = 1 W heat out). The relevant factors:

Q_lighting = W_installed × F_ballast × F_space × CLF

F_ballast = ballast factor (≈ 1.0 for LED; >1 for older fluorescent ballasts that generate extra heat)
F_space = space fraction — heat going into the conditioned room vs. the return-air plenum (0.6–0.85 for recessed luminaires; 1.0 for surface-mounted)
CLF = accounts for thermal storage, same concept as for people

8 · Equipment

Q_equipment = W_rated × F_usage × F_load × CLF

F_usage = fraction of time equipment operates during the design hour
F_load = fraction of rated power drawn when operating
Typical values: desktop PC + monitor 175 W · laptop 45–65 W · laser printer 400–900 W peak (low average) · commercial refrigerator 200–400 W continuous

Psychrometrics: Sensible Heat Ratio

Sensible Heat Ratio (SHR) is the fraction of total cooling load that is sensible:

SHR = Q_{sensible,total} / Q_total

SHR governs equipment selection. A cooling coil must handle both sensible and latent loads simultaneously. If SHR is low (humid climate or high occupancy), you need a coil capable of aggressive dehumidification — lower coil surface temperatures and more condensation. A coil selected only for sensible capacity will leave the space humid even if it hits the temperature set point.

Typical SHR ranges: dry climates 0.85–0.95 · humid climates 0.65–0.80 · high-occupancy spaces (gyms, auditoriums) 0.55–0.70.

Related reading on this site Understanding SHR requires a firm grasp of the psychrometric chart — how dry-bulb temperature, wet-bulb temperature, humidity ratio, and enthalpy relate to each other. See Psychrometry in HVAC for a detailed walkthrough.

Total load and equipment sizing

Q_{sensible,total} = Q_walls + Q_roof + Q_glass,cond + Q_solar + Q_inf,s + Q_vent,s + Q_people,s + Q_lighting + Q_equipment Q_latent,total = Q_inf,l + Q_vent,l + Q_people,l Q_total = Q_{sensible,total} + Q_latent,total

Converting to refrigeration tons:

Tons = Q_total (W) / 3 517

One refrigeration ton = 3 517 W = 12 000 BTU/hr. It represents the rate of heat extraction required to freeze one short ton (907 kg) of water at 0 °C in 24 hours.

Add 10–15% to Q_total before specifying equipment to cover duct heat gain, fan motor heat, and safety margin.

Worked example

Design Brief

Room: 6 m × 5 m × 3 m south-facing office · Kathmandu (design dry-bulb 32 °C, wet-bulb 20 °C, mean outdoor 28 °C) · Indoor set point 24 °C · 10 occupants · 6 m² south-facing double-glazed window · 300 W LED lighting. Wet-bulb temperature and humidity ratios are read from the psychrometric chart.

Establish baseline parameters:

ΔT = 32 − 24 = 8 °C ρ_air at 1 400 m elevation ≈ 1.05 kg/m³

Wall conduction

Q_wall = 0.578 × 15 × 10 = 87 W

U = 0.578 W/m²·K · A = 15 m² · CLTD_corr = 10 °C

Roof

Q_roof = 0.45 × 30 × 28 = 378 W

U = 0.45 W/m²·K · A = 30 m² · CLTD_corr = 28 °C (high solar exposure on roof)

Glazing conduction

Q_glass,cond = 2.8 × 6 × 8 = 134 W

Solar gain

Q_solar = 6 × 0.65 × 280 × 0.70 = 765 W

SC = 0.65 · SHGF = 280 W/m² · CLF = 0.70

Infiltration

ṁ_inf = (0.5 × 90 × 1.05) / 3 600 = 0.013 kg/s Q_inf,s = 0.013 × 1 006 × 8 = 105 W Q_inf,l = 0.013 × 2 501 000 × (0.011 − 0.009) = 65 W

ACH = 0.5 · V = 90 m³ · W_outdoor at 32 °C / 20 °C wb ≈ 0.011 · W_room at 24 °C / 50% RH ≈ 0.009

Ventilation

V_z = 2.5 × 10 + 0.3 × 30 = 34 L/s = 0.034 m³/s ṁ_vent = 0.034 × 1.05 = 0.036 kg/s Q_vent,s = 0.036 × 1 006 × 8 = 290 W Q_vent,l = 0.036 × 2 501 000 × 0.002 = 180 W

People

Q_people,s = 10 × 75 × 0.85 = 638 W Q_people,l = 10 × 55 × 1.0 = 550 W

Lighting

Q_lighting = 300 × 1.0 × 0.85 × 0.90 = 230 W

Equipment (3 PCs)

Q_equip = 3 × 175 × 0.8 × 0.7 × 0.90 = 265 W

Component	Sensible (W)	Latent (W)
Wall conduction	87	—
Roof	378	—
Glazing conduction	134	—
Solar gain	765	—
Infiltration	105	65
Ventilation	290	180
People	638	550
Lighting	230	—
Equipment	265	—
Total	2 892	795

Q_total = 2 892 + 795 = 3 687 W SHR = 2 892 / 3 687 = 0.78 Tons = 3 687 / 3 517 = 1.05 tons With 12% safety margin → specify 1.2 ton (4.2 kW) fan coil unit

Method selection

CLTD/CLF is suitable for hand calculations on simple rectangular zones with standard construction. Fast, transparent, and ideal for checking software output.

Radiant Time Series (RTS) is more accurate for complex zones. It separates radiative and convective heat gain fractions and uses 24-hour radiant time factors to convert them to cooling loads. ASHRAE recommends this for detailed design work.

Heat Balance Method (HBM) is the most rigorous approach — it solves energy balances at every surface simultaneously, hour by hour. Software such as HAP, Trace 700, and IDA ICE uses this internally.

Best practice: Hand-calculate at least one representative zone before trusting software output. It confirms that input data is correct and gives you a feel for which components dominate the load — which is precisely where design decisions actually matter.

External resource The methods described in this article — CLTD/CLF, RTS, and HBM — are all documented in the ASHRAE Handbook of Fundamentals, the primary reference for HVAC load calculations worldwide.

Quick reference — key symbols

Δ (delta) — finite difference · ṁ — mass flow rate (kg/s) · ρ (rho) — density (kg/m³) · Σ (sigma) — summation · c_p — specific heat at constant pressure · h_fg — latent heat of vaporisation · W — humidity ratio (see Psychrometry in HVAC)