Heat Load Calculator (Manual J Estimate)

Estimates residential heating and cooling loads using the Manual J simplified methodology. Enter your home's characteristics to calculate BTU/hr requirements for properly sizing HVAC equipment.

Climate & Design Conditions

99% design temperature for your location
1% design temperature for your location

Building Envelope

Insulation & Construction

Typical: R-13 (2×4), R-20 (2×6)
Typical: R-30 to R-60
Double-pane ≈ 0.30; Single-pane ≈ 0.80
Low-e ≈ 0.25; Clear glass ≈ 0.60
Slab edge insulation or floor insulation
Tight=0.25, Average=0.35, Leaky=0.60+

Internal & Solar Gains (Cooling)

Each person ≈ 250 BTU/hr sensible
Typical residential: 1.0–2.0 W/sq ft

Formulas Used

Conduction Heat Loss/Gain:
Q = A × U × ΔT   where U = 1/R, ΔT = design temperature difference (°F)

Infiltration Load:
Qinf = 0.018 × CFM × ΔT   where CFM = (ACH × Volume) / 60
0.018 BTU/ft³·°F = density × specific heat of air

Solar Heat Gain (Cooling):
Qsolar = Awindow × SHGC × Ipeak × SF
where Ipeak = peak solar intensity by orientation (BTU/hr·ft²), SF = shading factor

Cooling Load Temperature Difference (CLTD):
Walls: CLTD = ΔT × 1.2 (solar absorption correction)
Ceiling: CLTD = ΔT × 1.5 (attic amplification)

Latent Cooling Load:
Qlatent = Qsensible × 0.25 (25% latent fraction, typical residential)

Slab-on-Grade Heat Loss:
Qslab = P × F2 × ΔT   where P = perimeter (ft), F2 = edge loss factor (BTU/hr·ft·°F)

Tons of Refrigeration:
1 ton = 12,000 BTU/hr

Assumptions & References

  • Based on ACCA Manual J (8th Edition) simplified residential load calculation methodology.
  • Outdoor design temperatures should be obtained from ASHRAE Fundamentals Handbook or local weather data (99% heating / 1% cooling design conditions).
  • Air properties: density = 0.075 lb/ft³, specific heat = 0.24 BTU/lb·°F → combined factor 0.018 BTU/ft³·°F.
  • Occupant sensible heat gain: 250 BTU/hr per person (ASHRAE 62.2 residential assumption).
  • Latent load estimated at 25% of sensible — actual value depends on local humidity; humid climates may reach 30–40%.
  • Peak solar intensities (BTU/hr·ft²): North=30, East=150, South=100, West=175 — representative summer peak values for mid-latitudes.
  • Slab F2 edge loss factors: insulated (R≥5) = 0.73, uninsulated = 1.10 BTU/hr·ft·°F (ASHRAE).
  • Ground temperature for conditioned basement assumed 55°F (average U.S. deep ground temperature).
  • CLTD corrections (1.2× walls, 1.5× ceiling) account for solar radiation absorbed by opaque surfaces.
  • This tool provides a simplified estimate only. A full Manual J calculation by a licensed HVAC engineer is required for equipment selection and permit purposes.
  • Does not account for duct losses, ventilation (ASHRAE 62.2), or multi-zone systems.

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