Room Heat Loss & Gain

Room Geometry

Winter Design Conditions

Design outdoor temp (99.6% percentile)
On vertical surfaces (100-300 typical)

Summer Design Conditions

Design outdoor temp (1% percentile)
On vertical surfaces (500-1000 typical)

Walls (by orientation)

U-Value: lower = better insulation. Leave area at 0 for orientations with no exposed wall.

Windows (by orientation)

SHGC: 0.6-0.8 clear glass, 0.3-0.5 low-e coated. South glazing has the highest winter solar benefit; west has the highest summer cooling impact.

Door, Roof & Floor

Infiltration & Air Properties

Tight: 0.3-0.5, Average: 0.5-1.0, Leaky: 1.5+

Internal Gains & Shading (Summer)

1.0=no shading, 0=fully shaded

Latent (Moisture) Load - Summer Cooling

Light/office activity: ~55W typical
Drives moisture load from occupants and infiltrating outdoor air - determines the cooling coil's Sensible Heat Ratio (SHR)
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Room Heat Load Analysis

This calculator determines the peak heating and cooling loads for a room using the ASHRAE heat balance method. It analyzes heat transfer through the building envelope (walls, windows, roof, floor, door), infiltration air exchange, solar radiation through glazing, and internal heat sources (occupants, lighting, equipment). The governing design load is the maximum of winter heating or summer cooling requirements, ensuring the HVAC system is sized for the worst-case scenario.

Key Points:
  • Winter load includes transmission losses, infiltration, and solar gain offsets with 15% safety margin
  • Summer load includes envelope gains, solar radiation, internal gains, latent (moisture) load, and infiltration with 10% safety margin
  • Latent load (occupant + infiltration moisture) is now included for summer cooling sizing - SHR shows the sensible/total split
  • U-Value (Thermal Transmittance): Lower values indicate better insulation performance
  • SHGC (Solar Heat Gain Coefficient): Fraction of solar radiation admitted through glazing (0-1 scale)
  • ACH (Air Changes per Hour): Measures building airtightness. Lower ACH = better energy efficiency
  • Design load determines HVAC equipment capacity - oversizing wastes energy, undersizing causes discomfort
Calculation Steps:
  1. Enter all required parameters
  2. Click "Update Calculation"
  3. Review results and analysis
Formula Variables:
Q Heat Flow Rate (W)
U Thermal Transmittance (W/m²K)
A Surface Area (m²)
ΔT Temperature Difference (K)
SHGC Solar Heat Gain Coefficient (-)
ACH Air Changes per Hour (1/h)
Q_solar Solar Heat Gain (W)
Q_internal Internal Heat Gain (W)
W Humidity Ratio (kg/kg)
SHR Sensible Heat Ratio (-)