Cooling Load Calculations for HVAC Engineers

Cooling loads result from heat gains that must be removed to maintain comfort. Unlike heating (instantaneous heat loss), cooling involves thermal storage effects requiring time-lag analysis.

Heat Gain vs. Cooling Load

Heat Gain: Instantaneous rate of heat transfer into space Cooling Load: Rate at which heat must be removed (accounts for thermal mass storage)

Radiant heat gains (solar, lights, equipment): Absorbed by surfaces, released slowly Convective heat gains (infiltration, ventilation): Immediate cooling load

Solar Heat Gain

Through windows:

$$Q_{solar} = A \times SHGC \times SHGF \times CLF$$

Where:

$A$ = window area (ft²)
$SHGC$ = solar heat gain coefficient (0-1)
$SHGF$ = solar heat gain factor from tables (Btu/(h·ft²))
$CLF$ = cooling load factor (accounts for storage)

Peak solar gain times:

East windows: 8-9 AM
South windows: 12-1 PM
West windows: 3-4 PM
Horizontal (skylight): 12-1 PM

Envelope Conduction

Through walls/roofs:

$$Q_{cond} = U \times A \times CLTD$$

Where CLTD = Cooling Load Temperature Difference

CLTD accounts for:

Sol-air temperature (solar radiation heating surface)
Thermal mass effect
Time of day
Wall/roof orientation

Internal Heat Gains

Occupants:

Sensible: 250 Btu/h per person (office work)
Latent: 200 Btu/h per person
Total: 450 Btu/h per person

Lighting:

$$Q = W \times 3.41 \times F_{use} \times F_{ballast} \times F_{SA}$$

Where $F_{SA}$ = return air plenum credit (0.6-0.8 if return air through plenum)

Equipment:

$$Q = W \times 3.41 \times F_{load} \times F_{motor} \times F_{use}$$

Radiant Time Series Method

Modern ASHRAE method replaces CLF/CLTD.

Process:

Calculate instantaneous heat gains
Apply radiative/convective split
Convolve radiative gains with radiant time factors
Sum convective gains and time-delayed radiative loads

Benefits: More accurate for heavyweight construction, varying schedules

Ventilation Load

$$Q_{vent} = 1.08 \times CFM_{OA} \times (T_{out} - T_{in})$$ (sensible)

$$Q_{vent,latent} = 4,840 \times CFM_{OA} \times (W_{out} - W_{in})$$ (latent)

CFM from ASHRAE 62.1:

$$V_{oz} = R_p \times P_z + R_a \times A_z$$

Where:

$R_p$ = people outdoor air rate (typically 5 CFM/person)
$R_a$ = area outdoor air rate (typically 0.06 CFM/ft²)

Cooling Load Example Summary

Typical office space peak load breakdown:

Solar: 25-35%
Lights: 20-30%
Equipment: 15-25%
People: 10-15%
Envelope conduction: 10-15%
Ventilation: 15-25%

Safety Factors

Do NOT add safety factors to calculated loads

Acceptable adjustments:

Duct heat gain: 5-10%
Fan heat: 2,500 Btu/h per ton
Future expansion: Document separately

Practical Applications

Room-by-room: Required for VAV, zone control
Block load: Acceptable for small residential
Peak diversity: Not all zones peak simultaneously (building load < sum of room peaks)

Related Technical Guides:

References:

ASHRAE Handbook of Fundamentals, Chapter 18: Nonresidential Cooling and Heating Load Calculations
ASHRAE Radiant Time Series Method