Heating Loads
Heating load represents the rate at which heat must be added to maintain indoor design conditions during peak heating conditions.
Calculation Method
Section titled “Calculation Method”HVAKR uses steady-state calculation for heating loads:
- Based on design heating temperature (99.6% condition)
- No solar credit (conservative, assumes nighttime peak)
- No internal gains credit (unoccupied condition)
- Results in maximum required heating capacity
Load Components
Section titled “Load Components”Transmission Loads
Section titled “Transmission Loads”Heat loss through the building envelope:
- Indoor-to-outdoor temperature difference
- Wall U-value and area
- No solar effect (nighttime condition)
- Similar to walls
- Higher U-value increases loss
- Significant for top floors
Windows
Section titled “Windows”- Conductive heat loss
- Higher U-value than walls
- No solar credit
Floors and Slabs
Section titled “Floors and Slabs”- Slab-on-grade uses perimeter method
- Basement uses ground temperature
- Usually smaller than other components
- Conductive heat loss
- Higher U-value than walls
- Include all exterior doors
Infiltration Load
Section titled “Infiltration Load”Heat to warm cold outdoor air leaking in:
- Based on envelope tightness
- Increases with wind speed
- May be significant in older buildings
Ventilation Load
Section titled “Ventilation Load”Heat to warm outdoor ventilation air:
- Based on minimum outdoor air requirement
- Outdoor-to-indoor temperature difference
- No latent component (typically)
No Credits Applied
Section titled “No Credits Applied”For conservative sizing, heating loads exclude:
No Solar Credit
Section titled “No Solar Credit”- Peak heating often occurs before sunrise
- Cloudy conditions are possible
- Results in reliable capacity
No Internal Gains Credit
Section titled “No Internal Gains Credit”- Building may be unoccupied
- Morning warm-up requires extra capacity
- Results in quick recovery
Design Temperature
Section titled “Design Temperature”Outdoor Design Temperature
Section titled “Outdoor Design Temperature”The 99.6% heating design temperature:
- Temperature exceeded 99.6% of hours
- Only 35 hours per year below this value
- Conservative for equipment sizing
Indoor Design Temperature
Section titled “Indoor Design Temperature”Typical heating setpoint:
- 68-72°F (20-22°C) occupied
- Lower for setback periods
- May vary by space type
Peak Load Identification
Section titled “Peak Load Identification”When Heating Peaks
Section titled “When Heating Peaks”Heating loads peak when:
- Outdoor temperature is at design minimum
- No solar gain (night or cloudy)
- Building is unoccupied (no internal gains)
Morning Warm-Up
Section titled “Morning Warm-Up”Additional capacity for morning recovery:
- Building cooled down overnight
- Need to reach setpoint before occupancy
- May increase heating load by 20-30%
Results Display
Section titled “Results Display”Space Loads
Section titled “Space Loads”View heating loads for each space:
- Total heating load
- Load component breakdown
- Heat loss per envelope element
System Loads
Section titled “System Loads”Aggregate heating loads by system:
- Sum of zone loads
- No diversity applied (all zones heat simultaneously)
Verification Tips
Section titled “Verification Tips”- Check envelope areas - Verify wall and window areas
- Review U-values - Compare to specifications
- Consider infiltration - May be significant
- Compare to benchmarks - Typical 15-25 BTU/h per sf for offices
- Verify temperature difference - Design delta-T is correct