Cooling Loads
Cooling load represents the rate at which heat must be removed to maintain indoor design conditions during peak cooling conditions.
Calculation Method
Section titled “Calculation Method”HVAKR uses the Radiant Time Series (RTS) method for cooling load calculations, as recommended by ASHRAE. This method:
- Accounts for thermal storage in building mass
- Calculates 24-hour load profiles
- Identifies peak cooling conditions
- Separates sensible and latent components
Load Components
Section titled “Load Components”Transmission Loads
Section titled “Transmission Loads”Heat transfer through the building envelope:
- Outdoor-to-indoor temperature difference
- Wall U-value and area
- Sol-air temperature for solar absorption
- Thermal mass delay effects
- Similar to walls but with higher solar exposure
- Color affects solar absorption
- Significant for single-story buildings
Windows
Section titled “Windows”- Conductive heat transfer (U-value)
- Separate from solar heat gain
Floors
Section titled “Floors”- Ground-coupled heat transfer
- Usually a cooling credit (ground is cooler)
Solar Loads
Section titled “Solar Loads”Heat gain from solar radiation:
Window Solar Gain
Section titled “Window Solar Gain”- Direct solar radiation through glass
- Diffuse solar radiation
- Based on SHGC and window area
- Varies by orientation and time of day
Opaque Surface Solar
Section titled “Opaque Surface Solar”- Solar absorbed by walls and roof
- Delayed by thermal mass
- Included in sol-air temperature
Internal Loads
Section titled “Internal Loads”Heat generated within the space:
People
Section titled “People”- Sensible heat from body temperature
- Latent heat from respiration and perspiration
- Based on activity level and occupancy
Lighting
Section titled “Lighting”- All electrical energy becomes heat
- Based on lighting power density
- Modified by schedule
Equipment
Section titled “Equipment”- Heat from appliances and machinery
- Based on equipment power density
- May include latent component (kitchens)
Ventilation Load
Section titled “Ventilation Load”Heat to condition outdoor air:
Sensible
Section titled “Sensible”- Temperature difference between outdoor and supply
- Based on outdoor air volume
Latent
Section titled “Latent”- Humidity difference between outdoor and indoor
- Significant in humid climates
Infiltration Load
Section titled “Infiltration Load”Uncontrolled air leakage:
- Similar to ventilation but uncontrolled
- Based on envelope tightness
- Wind and stack effects
Peak Load Determination
Section titled “Peak Load Determination”Hourly Analysis
Section titled “Hourly Analysis”HVAKR calculates loads for each hour of the design day to identify the peak:
- Solar gains shift throughout the day
- Internal loads follow schedules
- Peak may not occur at hottest outdoor temperature
Peak Hour
Section titled “Peak Hour”The hour with the maximum cooling load, typically:
- West-facing zones: afternoon (3-5 PM)
- East-facing zones: morning (8-10 AM)
- Interior zones: late afternoon
- System peak: 2-4 PM for most buildings
Results Display
Section titled “Results Display”Space Loads
Section titled “Space Loads”View loads for each space:
- Total cooling load
- Sensible vs. latent breakdown
- Load component breakdown
- Peak hour identification
Component Charts
Section titled “Component Charts”Visualize load components:
- Stacked bar chart by component
- Hourly load profile
- Sensible/latent pie chart
Verification Tips
Section titled “Verification Tips”- Check solar gains - Largest component for perimeter spaces
- Verify internal loads - Match expected density
- Review ventilation - Significant in humid climates
- Compare to benchmarks - Typical 300-500 BTU/h per sf for offices