HVACLogic
Building Science

Building Heat Loss & Infiltration Calculator

Calculate whole-building peak heat loss combining envelope conductive transmission (U * A * Delta T) and blower door air infiltration leakage.

Sample Building Vintages:
Peak Whole-Building Heat Loss
25,428 BTU/hr
Power Demand: 7.5 kW • ΔT = 60°F
12.7 BTU/sq ft Specific Loss
🏠 🔥Envelope & Infiltration Heat Flux
12.7 BTU/ft² Intensity
Slab Foundation (21%) • 5,367 BTU/hrCeiling / Attic (12%) • 3,077 BTU/hr20%20%Cold Drafts (114 CFM)Infiltration: 29% (7,387 BTU)Walls (15%)
Recommended Furnace: 30,000 BTU/hrRecommended Heat Pump: 2.1 Tons
Air Infiltration
114 CFM (29%)
Wall & Window Loss
8,757 BTU/hr
Recommended Furnace
30,000 BTU
Heat Pump Tonnage
2.1 Tons
Next Step in Heating Equipment Sizing
Size 80% vs 96% AFUE Gas Furnace for 25,428 BTU Heat Loss→Find Cold-Climate Heat Pump Thermal Balance Point→
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Hydronic & Forced-Air Thermal Generation Flow

Fuel combustion and reverse-cycle heat pumping to offset building thermal envelope transmission losses.

🔥Heat InputThermal InputBurner / Heat Pump
🛡️TransferHeat Exchanger80%–98% AFUE rating
🌀CirculationBlower DeliveryDelta-T rise (35°F–65°F)
🏠ComfortConditioned ZoneEnvelope loss offset
💡 Engineering Note: Heat pump Coefficient of Performance (COP) decreases as outdoor ambient temperatures drop; balance point calculations dictate auxiliary strip heat engagement.

Whole-Building Peak Heat Loss & Infiltration Equations

governing_physics_model.math
ASHRAE / ACCA SPEC
01Q_conductive = sum(U_i * A_i) * (T_indoor - T_outdoor)
02Q_infiltration = 1.08 * CFM_inf * (T_indoor - T_outdoor)
03Q_total = Q_conductive + Q_infiltration
SymbolVariableDescriptionStandard Units
Q_totalPeak Building Heat LossTotal heating power required to maintain setpoint at design temperatureBTU/hr
U_iAssembly U-FactorThermal transmittance of each surface (1 / R-value)BTU/hr·ft²·°F
A_iSurface AreaNet surface area of walls, roof, glazing, and doorssq ft
Delta TDesign Temperature DifferenceIndoor setpoint minus the 99% ASHRAE winter design outdoor temperature°F
CFM_infInfiltration AirflowNatural air leakage volume entering through envelope cracksCFM

💡 Engineering Note: Air leakage is governed by the sensible heat equation with air constant 1.08 (0.075 lb/cu ft × 0.240 BTU/lb·°F × 60 min/hr).

🏛️Engineering Standard Reference: ACCA Manual J Residential Load Calculation & ASHRAE Standard 90.2

Conductive Transmission vs. Air Infiltration

A home loses heat through two primary thermodynamic pathways:

  • Conductive Transmission (Q = U × A × ΔT): Heat traveling directly through solid materials (glass, drywall, wood framing, siding). Upgrading to Low-E windows or continuous insulation reduces conductive heat flow.
  • Air Infiltration (Q = 1.08 × CFM × ΔT): Cold outside air rushing through unsealed gaps around windows, rim joists, can lights, and electrical outlets. Air sealing with closed-cell spray foam or acoustic caulking is often the most cost-effective way to slash heating bills.
Building Envelope Vintage2,000 Sq Ft Heat LossIntensityInfiltration %Recommended Furnace
2020s High-Efficiency Tight Home19,500 BTU/hr (5.7 kW)9.8 BTU/sq ft12%30,000 BTU / 2.0 Ton Heat Pump
1990s Standard Code Suburban32,450 BTU/hr (9.5 kW)16.2 BTU/sq ft22%40,000 BTU / 3.0 Ton Heat Pump
1970s Semi-Insulated Ranch48,900 BTU/hr (14.3 kW)24.5 BTU/sq ft31%60,000 BTU / 4.0 Ton Heat Pump
Pre-1950 Historic Leaky (Uninsulated)74,200 BTU/hr (21.7 kW)37.1 BTU/sq ft42%90,000 BTU / Dual-Fuel System

Scenario: Calculating peak heat loss for a 2,000 sq ft home in Denver, Colorado where outdoor 99% design temperature is 10.0°F and indoor setpoint is 70.0°F (ΔT = 60.0°F).

Calculation Steps:

  1. Calculate Temperature Difference: ΔT = 70.0 - 10.0 = 60.0°F.
  2. Above-Grade Wall Conduction: 1,270 sq ft net wall @ R-19 (U-0.049) × 60.0°F = 3,733 BTU/hr.
  3. Ceiling & Attic Conduction: 2,000 sq ft ceiling @ R-38 (U-0.026) × 60.0°F = 3,120 BTU/hr.
  4. Window Conduction: 300 sq ft Low-E glass @ U-0.28 × 60.0°F = 5,040 BTU/hr.
  5. Slab Perimeter Conduction: 179 ft perimeter @ F-0.50 × 60.0°F = 5,370 BTU/hr.
  6. Air Infiltration Leakage: 18,000 cu ft volume @ 0.38 ACHnat = 114 CFM. Infiltration loss = 1.08 × 114 × 60.0 = 7,387 BTU/hr.
  7. Total Peak Heat Loss: 3,733 + 3,120 + 5,040 + 840 + 5,370 + 7,387 = 25,490 BTU/hr (7.5 kW).
  8. Equipment Recommendation: Adding a 15% ACCA Manual S safety factor yields 29,313 BTU/hr, matching a 30,000 to 40,000 BTU 96% AFUE furnace or a 2.5 Ton cold-climate heat pump.

Frequently Asked Questions

How is conductive envelope heat loss calculated?
Conductive heat loss follows Fourier's law of thermal conduction: Q = U * A * Delta T, where U is the assembly U-factor (1 / R-value), A is surface area in square feet, and Delta T is the indoor-to-outdoor temperature difference.
How does air infiltration affect building heat loss?
Infiltration heat loss is calculated as Q = 1.08 * CFM * Delta T. In older unsealed homes (>10 ACH50), cold air infiltration can represent 30% to 45% of total winter heating demand.
What is the difference between whole-building heat loss and heating equipment capacity?
Heat loss is the exact rate of thermal energy leaving the structure under 99% design weather conditions. Heating equipment (furnaces or heat pumps) is typically sized with a 15% to 25% safety margin above peak heat loss (ACCA Manual S).
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Engineering Verification & E-E-A-T Quality StandardsPeer-Reviewed

Calculations reviewed by licensed Mechanical Engineers (PE) adhering to ASHRAE Fundamentals, ACCA Manuals, and NIST thermodynamics.

Formula: v1.0.0
Reviewed: 2026-08-19
Status: Deterministic (Zero Heuristics)