Heat Loss Calculation for Buildings: Formula & Example
Understanding Heat Loss Calculation for Buildings
This guide covers the key concepts, formulas, and practical methods for heat loss calculation for buildings. All methods referenced are based on ASHRAE Fundamentals and GB 50736 standards.
Key Formula
The primary formula used in heating load calculations is based on the heat balance method described in ASHRAE Fundamentals Chapter 18 (nonresidential) and Chapter 19 (residential). The total heating load is the sum of conduction losses through the building envelope and infiltration/ventilation losses.
Qtotal = Qwalls + Qwindows + Qroof + Qfloor + Qinfiltration + Qventilation
Each component: Q = Area × U-value × ΔT where ΔT = indoor design temp − outdoor design temp (ASHRAE 99% heating value for your location).
Step-by-Step Example
Scenario: 150 sq ft bedroom in New York (outdoor design temp 14°F, indoor 70°F, ΔT=56°F).
Given: 2 exterior walls (total 180 sq ft, U=0.077), 1 window (15 sq ft, U=0.5), ceiling (150 sq ft, U=0.03), infiltration (0.7 ACH).
Walls: 180 × 0.077 × 56 = 777 BTU/h
Window: 15 × 0.5 × 56 = 420 BTU/h
Ceiling: 150 × 0.03 × 56 = 252 BTU/h
Infiltration: 0.018 × 0.7 × (12×12×8) × 56 = 813 BTU/h
Total: 777 + 420 + 252 + 813 = 2,262 BTU/h
Standards Reference
- ASHRAE Fundamentals — Chapter 18 (Nonresidential), Chapter 19 (Residential) cooling and heating load calculations
- ASHRAE Handbook — Chapter 25 (fenestration), Chapter 26 (thermal insulation)
- GB 50736-2012 — Design Code for Heating Ventilation and Air Conditioning of Civil Buildings (China)
- ACCA Manual J — Residential load calculation (8th Edition)
Frequently Asked Questions
What are the main components of heat loss?
The 5 components: 1) Walls (conduction through exterior walls), 2) Windows (conduction + air leakage), 3) Roof/ceiling (conduction upward), 4) Floor (conduction to ground), 5) Infiltration (air leakage through cracks and openings).
How do you calculate infiltration heat loss?
Q = 0.018 × ACH × Volume × ΔT. ACH = air changes per hour (0.5 for tight construction, 1.0 for average, 2.0 for leaky). Volume = length × width × height in cubic feet.
What is the U-value in heat loss calculation?
U-value (thermal transmittance) measures heat flow through a material. Lower = better insulation. R-13 wall → U=0.077. R-19 wall → U=0.053. Double-pane window → U=0.5. Single-pane → U=1.1.