Illinois Climate Zone Heat Load Calculator

Estimates residential heating load (BTU/hr) for Illinois climate zones using the ASHRAE Manual J simplified method, accounting for design temperature difference, envelope area, insulation levels, infiltration, and internal gains.

IECC 2021 climate zone designations for Illinois
99% heating design dry-bulb temperature (ASHRAE Fundamentals). Auto-filled by zone; override if needed.
Typically 70°F per ASHRAE 55 comfort standard.
Total heated living space square footage.
Used to calculate conditioned volume for infiltration load.
Total wall R-value including insulation, sheathing, drywall. IECC 2021 Zone 5A minimum: R-20 cavity or R-13+5 ci.
IECC 2021 Zone 5A/6A minimum: R-49 attic insulation.
Crawlspace or basement floor insulation. IECC 2021 Zone 5A: R-30 floor or R-15 basement wall.
Sum of all glazing areas. Typical: 15% of floor area.
IECC 2021 Zone 5A maximum: U-0.30; Zone 6A: U-0.28. Triple-pane ≈ 0.15–0.20.
Typical 3×7 ft door = 21 ft². Include all exterior doors.
IECC 2021 Zone 5A/6A maximum: U-0.20 for opaque doors.
Typical existing homes: 0.5–1.0 ACH. New construction IECC 2021: ≤3 ACH50 ≈ 0.15–0.25 natural ACH.
Used to estimate wall area from floor area and ceiling height.

Formulas Used

1. Design Temperature Difference:
ΔT = Tindoor − Toutdoor,design (°F)

2. Conduction Heat Loss (ASHRAE Manual J):
Qconduction = U × A × ΔT (BTU/hr)
where U = 1/R (BTU/hr·ft²·°F), A = component area (ft²)

3. Gross Wall Area Estimation:
Footprint = Floor Area / Stories (ft²)
Perimeter = 4 × √Footprint (ft, square footprint assumed)
Gross Wall Area = Perimeter × Ceiling Height × Stories (ft²)
Net Wall Area = Gross Wall − Window Area − Door Area (ft²)

4. Infiltration Heat Loss:
Volume = Floor Area × Ceiling Height (ft³)
CFMinfiltration = ACH × Volume / 60 (CFM)
Qinfiltration = 1.1 × CFM × ΔT (BTU/hr)
(Factor 1.1 = 0.018 BTU/ft³·°F × 60 min/hr per Manual J)

5. Total Design Load:
Qtotal = Qconduction + Qinfiltration (BTU/hr)
Qdesign = Qtotal × 1.25 (25% safety factor per Manual J)

6. Unit Conversions:
Tons = Qdesign / 12,000 | kW = Qdesign / 3,412.14

Assumptions & References

  • ACH50 to natural ACH conversion: ACHnatural ≈ ACH50 / 20 (Lawrence Berkeley National Laboratory infiltration model).

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