Skip to main content

Heating Load Calculator

Calculate heating loads for HVAC system sizing including transmission, infiltration, and ventilation losses with support for Imperial and SI units

Category: Hvac

Heating Load Calculator Inputs

Enter values to calculate

Choose between Imperial (ft, °F, BTU/h) or SI (m, °C, kW) units

Imperial: ft² | SI: m²

Imperial: ft | SI: m

Imperial: ft² | SI: m²

Imperial: ft² | SI: m²

Imperial: ft² | SI: m²

Imperial: R-19 ≈ 0.05-0.06, R-13 ≈ 0.08-0.09 | SI: 0.3-0.6 W/(m²·K)

Imperial: Double low-e ≈ 0.28-0.35 | SI: 1.5-2.5 W/(m²·K)

Imperial: BTU/(h·ft²·°F) | SI: W/(m²·K)

Imperial: R-30 ≈ 0.033, R-49 ≈ 0.020 | SI: 0.15-0.25 W/(m²·K)

99% winter design temperature from ASHRAE | Imperial: °F | SI: °C

Imperial: °F | SI: °C

Tight: 0.15-0.25, Average: 0.35-0.50, Leaky: 0.70-1.50

Imperial: CFM (15-20 CFM/person) | SI: L/s (7-10 L/s/person)

Enable JavaScript for interactive calculation and step-by-step results.

Heating Load Calculator Formula

Equation

Q_total = Q_transmission + Q_infiltration + Q_ventilation

Excel Formula

=Q_total=Q_transmission+Q_infiltration+Q_ventilation

Variables

  • Unit System — Choose between Imperial (ft, °F, BTU/h) or SI (m, °C, kW) units
  • Floor Area (ft² or m²) — Imperial: ft² | SI: m²
  • Ceiling Height (ft or m) — Imperial: ft | SI: m
  • Exterior Wall Area (ft² or m²) — Imperial: ft² | SI: m²
  • Window Area (ft² or m²) — Imperial: ft² | SI: m²
  • Door Area (ft² or m²) — Imperial: ft² | SI: m²
  • Wall U-Value (BTU/(h·ft²·°F) or W/(m²·K)) — Imperial: R-19 ≈ 0.05-0.06, R-13 ≈ 0.08-0.09 | SI: 0.3-0.6 W/(m²·K)
  • Window U-Value (BTU/(h·ft²·°F) or W/(m²·K)) — Imperial: Double low-e ≈ 0.28-0.35 | SI: 1.5-2.5 W/(m²·K)
  • Door U-Value (BTU/(h·ft²·°F) or W/(m²·K)) — Imperial: BTU/(h·ft²·°F) | SI: W/(m²·K)
  • Ceiling/Roof U-Value (BTU/(h·ft²·°F) or W/(m²·K)) — Imperial: R-30 ≈ 0.033, R-49 ≈ 0.020 | SI: 0.15-0.25 W/(m²·K)
  • Outdoor Design Temperature (°F or °C) — 99% winter design temperature from ASHRAE | Imperial: °F | SI: °C
  • Indoor Design Temperature (°F or °C) — Imperial: °F | SI: °C
  • Infiltration Rate (ACH) — Tight: 0.15-0.25, Average: 0.35-0.50, Leaky: 0.70-1.50
  • Ventilation Airflow (CFM or L/s) — Imperial: CFM (15-20 CFM/person) | SI: L/s (7-10 L/s/person)

How the Heating Load Calculator Works

Heating load calculation determines the rate of heat loss from a building during winter conditions, essential for proper furnace, boiler, or heat pump sizing. Unlike cooling loads which include internal gains, heating loads focus on heat losses through envelope transmission, infiltration, and ventilation air. Proper sizing ensures comfort while avoiding oversizing that reduces efficiency.

The core relationship is Q_total = Q_transmission + Q_infiltration + Q_ventilation. Typical inputs include Unit System, Floor Area (ft² or m²), Ceiling Height (ft or m), Exterior Wall Area (ft² or m²).

Enter your values in the heating load calculator above, review the step-by-step solution, and compare against the worked examples below so you can see how each input changes the result. This free online hvac tool is built for homework, design checks, and professional verification.

Heating Load Calculator Theory & Explanation

Heat Loss Components

Total heating load consists of transmission, infiltration, and ventilation losses:

**Transmission Heat Loss** through building envelope:

Q_trans = U · A · (T_indoor - T_outdoor)

where: - U = overall heat transfer coefficient (BTU/h·ft²·°F or W/m²·K) - A = surface area (ft² or m²) - Δ T = temperature difference

**Infiltration Heat Loss** through cracks and openings:

Q_inf = \rho · c_p · Q_air · Δ T = 1.08 · CFM · Δ T_°F

Or:

Q_inf = 0.018 · ACH · V · Δ T

where: - ACH = air changes per hour - V = volume (ft³)

**Ventilation Heat Loss** for outdoor air:

Q_vent = 1.08 · CFM_OA · Δ T

**Total Heating Load**:

Q_total = Q_trans + Q_inf + Q_vent

Note: Internal gains (people, lights, equipment) are NOT subtracted for design heating load - conservative approach for extreme cold days with minimal occupancy.

Design Conditions

**Outdoor Design Temperature**:

Use 99% or 97.5% winter design temperature from ASHRAE climate data: - 99%: Temperature exceeded 1% of winter hours - 97.5%: Temperature exceeded 2.5% of winter hours

97.5% recommended for residential (slightly less conservative). 99% recommended for commercial (more conservative).

Examples: - Chicago: 99% = -4°F, 97.5% = 1°F - New York: 99% = 12°F, 97.5% = 15°F - Atlanta: 99% = 22°F, 97.5% = 25°F - Seattle: 99% = 28°F, 97.5% = 32°F

**Indoor Design Temperature**: - Residential: 68-72°F (20-22°C) - Commercial: 68-72°F occupied, 55-60°F setback - Industrial: Varies by process

**Temperature Differential**:

Δ T = T_indoor - T_outdoor,design

Chicago example: 70 - (-4) = 74°F

U-Values for Common Assemblies

**Walls** (BTU/h·ft²·°F): - Uninsulated frame: 0.35-0.45 - R-11 insulation: 0.09-0.11 - R-19 insulation: 0.05-0.06 - R-21 insulation: 0.045-0.055 - Concrete block (insulated): 0.10-0.15 - Brick veneer (insulated): 0.08-0.12

**Roofs/Ceilings**: - R-19 attic: 0.05-0.06 - R-30 attic: 0.03-0.035 - R-38 attic: 0.025-0.030 - R-49 attic: 0.020-0.025 - Flat roof R-20: 0.045-0.055

**Windows**: - Single glazing: 0.90-1.10 - Double glazing (air): 0.45-0.55 - Double glazing (argon, low-e): 0.28-0.35 - Triple glazing (argon, low-e): 0.18-0.25

**Doors**: - Solid wood: 0.45-0.55 - Insulated metal: 0.35-0.45 - Insulated fiberglass: 0.15-0.25

**Floors**: - Over unconditioned space (R-19): 0.05-0.06 - Slab on grade (R-10 perimeter): 0.50-0.80 BTU/h·ft - Basement wall (R-10): 0.08-0.12

Infiltration Rates

Infiltration depends on construction quality and climate:

**Air Changes per Hour (ACH)**:

| Construction | ACH @ 50 Pa | Natural ACH | |--------------|-------------|-------------| | Very tight (new, sealed) | <3 | 0.15-0.25 | | Tight (good construction) | 3-6 | 0.25-0.40 | | Average | 6-10 | 0.40-0.70 | | Leaky (old, poor) | 10-20 | 0.70-1.50 |

**Factors Affecting Infiltration**:

1. **Building Height**: Higher buildings have greater stack effect 2. **Wind Exposure**: Exposed sites have higher infiltration 3. **Temperature Difference**: Larger ΔT increases stack effect 4. **Construction**: Caulking, weatherstripping, air barriers

**Blower Door Test**: Measure ACH@50 Pa, then estimate natural:

ACH_natural ≈ \fracACH_50N

where N = 15-25 (depends on height, exposure).

**Infiltration Reduction Strategies**: - Air sealing: Reduces ACH by 30-50% - Weatherstripping: 10-20% reduction - Storm windows: 25-40% reduction on windows - Positive building pressure: Reduces infiltration

Heating Load Calculator Worked Examples

Worked Example

Inputs

  • unitSystem: imperial
  • floorArea: 2000
  • ceilingHeight: 8
  • wallArea: 1200
  • windowArea: 240
  • doorArea: 40
  • wallUValue: 0.08
  • windowUValue: 0.35
  • doorUValue: 0.40
  • ceilingUValue: 0.033
  • outdoorTemp: 0
  • indoorTemp: 70
  • infiltrationACH: 0.35
  • ventilationCFM: 150

Result: Total Heating Load: 63,800 BTU/h (5.3 tons) | 31.9 BTU/h·ft² | Suggest 70,000 BTU/h furnace

Explanation

For a 2000 ft² house with good insulation in cold climate:

**Step 1: Calculate Volume** V = 2000 × 8 = 16,000 ft³

**Step 2: Calculate Temperature Difference** Δ T = 70 - 0 = 70°F (severe cold climate)

**Step 3: Transmission Heat Loss**

Walls: Q_w = 0.08 × 1200 × 70 = 6,720 BTU/h

Windows: Q_win = 0.35 × 240 × 70 = 5,880 BTU/h

Doors: Q_d = 0.40 × 40 × 70 = 1,120 BTU/h

Ceiling: Q_c = 0.033 × 2000 × 70 = 4,620 BTU/h

Total transmission: Q_trans = 6,720 + 5,880 + 1,120 + 4,620 = 18,340 BTU/h

**Step 4: Infiltration Heat Loss** Infiltration airflow: Q_inf,air = ACH × V / 60 Q_inf,air = 0.35 × 16,000 / 60 = 93.3 CFM

Heat loss: Q_inf = 1.08 × 93.3 × 70 = 7,054 BTU/h

**Step 5: Ventilation Heat Loss** Q_vent = 1.08 × 150 × 70 = 11,340 BTU/h

**Step 6: Total Design Heating Load** Q_total = 18,340 + 7,054 + 11,340 = 36,734 BTU/h

**Step 7: Add Safety Factor (10%)** Q_design = 36,734 × 1.10 = 40,407 BTU/h

**Step 8: Equipment Selection** Select furnace: 40,000-50,000 BTU/h Standard size: 40,000 or 50,000 BTU/h Recommendation: 50,000 BTU/h (allows for pickup load)

**Step 9: Heat Loss per Square Foot** q = 40,407 / 2000 = 20.2 BTU/h·ft²

**Step 10: Component Breakdown** - Transmission: 50.0% (18,340 BTU/h) - Infiltration: 19.2% (7,054 BTU/h) - Ventilation: 30.8% (11,340 BTU/h)

**Energy Cost Estimate** (6-month heating season, 3000 hours):

If 80% efficient natural gas furnace, 1.00/therm: Average load: ~50% = 20,000 BTU/h Seasonal energy: 20,000 × 3000 / (100,000 × 0.80) = 750 therms Heating cost: 750 × \1.00 = \750$/season

**Improvement Opportunities**: 1. Upgrade windows to U=0.25: Save 560 BTU/h (1.5%) 2. Add air sealing (reduce ACH to 0.25): Save 2,000 BTU/h (5%) 3. Increase ceiling insulation to R-49: Save 400 BTU/h (1%)

Second Scenario

Inputs

  • unitSystem: imperial
  • floorArea: 2501
  • ceilingHeight: 8
  • wallArea: 1200
  • windowArea: 240
  • doorArea: 40
  • wallUValue: 0.08
  • windowUValue: 0.35
  • doorUValue: 0.40
  • ceilingUValue: 0.033
  • outdoorTemp: 0
  • indoorTemp: 70
  • infiltrationACH: 0.35
  • ventilationCFM: 150

Result: Total Heating Load: 63,800 BTU/h (5.3 tons) | 31.9 BTU/h·ft² | Suggest 70,000 BTU/h furnace

Explanation

This scenario uses different inputs (unitSystem = imperial, floorArea = 2501, ceilingHeight = 8, wallArea = 1200, windowArea = 240, doorArea = 40, wallUValue = 0.08, windowUValue = 0.35, doorUValue = 0.40, ceilingUValue = 0.033, outdoorTemp = 0, indoorTemp = 70, infiltrationACH = 0.35, ventilationCFM = 150) to show how changing one variable affects the heating load result. Run the calculator above with these values to get the exact updated output with step-by-step work.

Common Heating Load Calculator Use Cases

  • HVAC load and equipment sizing
  • Comfort and indoor air quality analysis
  • Energy audit support
  • Heating Load homework and study
  • Heating Load design and analysis

Heating Load Calculator FAQs

Why do heating and cooling loads differ for the same building?

Heating and cooling loads fundamentally differ: HEATING LOAD (winter): Only heat LOSSES: transmission through envelope, infiltration, ventilation. Internal gains (people, lights, equipment) ignored - conservative for extreme cold with minimal occupancy. Solar gains minimal in winter. Design for PEAK LOSS on coldest day. COOLING LOAD (summer): Heat GAINS: solar (major!), transmission, internal gains (people, lights, equipment significant), infiltration, ventilation. Design for PEAK GAIN on hottest, sunniest day with full occupancy. Typical result: Same building may need 60,000 BTU/h heating but 36,000 BTU/h (3 ton) cooling because: Summer has huge solar gains, but also internal gains offset part of envelope transmission. Winter has all losses, no offsetting gains. Rule: Heating load typically 1.5-2.5× cooling load per square foot in cold climates, but equal or less in mild climates.

Should I oversize my heating system for safety?

NO - slight oversizing okay (10-20%), but excessive oversizing (>25%) causes problems: OVERSIZING ISSUES: Short cycling (frequent on/off), reduces efficiency 10-30%. Poor comfort (rapid temperature swings). Increased wear (more starts/stops). Cold spots (doesn't run long enough to distribute heat). Higher initial cost. Modern systems less tolerant of oversizing than old equipment. PROPER SIZING: Use 99% or 97.5% winter design temperature. Add 10% safety factor for pickup load and uncertainties. For multi-stage/modulating equipment, can size closer to calculated load. Consider setback recovery if deep setbacks used. EXCEPTION - Heat Pumps: Size for COOLING load (summer), accept supplemental heat needed for extreme cold. Heat pump capacity drops as outdoor temp decreases. Balance point: Temperature where heat pump capacity equals building load. Below balance point: Use auxiliary/backup heat. Properly sized system provides comfort, efficiency, and longevity. If in doubt, size smaller and add more if truly needed (rare).

What does the Heating Load Calculator calculate?

It applies the formula on this page to your inputs and returns the primary result plus any supporting values shown in the output panel.

How many decimal places should I trust?

Match precision to your input accuracy. Extra digits from the tool are not evidence of higher measurement quality.

Which units should I enter?

Use the units labeled beside each field. Convert all quantities to that system before calculating to avoid silent scale errors.