Heat Loss Per Room: Calculation Table & Formula
Understanding Heat Loss Per Room
This guide covers the key concepts, formulas, and practical methods for heat loss per room. 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
How do I calculate heat loss for each room?
Calculate heat loss for each room separately: Q_room = (Wall area × U_wall + Window area × U_window) × ΔT + 0.018 × ACH × Room volume × ΔT. Include only exterior walls and windows for that room.
Which room loses the most heat?
Rooms with the most exterior wall and window area lose the most heat. Corner rooms lose 40–60% more than interior rooms. Rooms with large windows can lose 30–50% of their heat through glass alone.
Do interior walls contribute to heat loss?
No, interior walls between heated spaces don't contribute to heat loss. However, walls between heated and unheated spaces (garage, attic, crawl space) do — treat them like exterior walls with appropriate U-values.