Texas Climate Zone Heat Load Calculator

Estimates the design heating load (BTU/hr) for a residential building in Texas based on IECC climate zone, conditioned floor area, envelope characteristics, and ASHRAE 99% design temperatures.

Total heated living space (200–20,000 sq ft)
Typical residential: 8–10 ft
ASHRAE recommended: 70°F
ASHRAE 99% heating design dry-bulb temperature
IECC Zone 4B minimum: R-13+R-3ci. Typical range: R-13 to R-21
IECC Zone 4B minimum: R-38. Zone 2A/3A minimum: R-30
Slab-on-grade: R-10 perimeter (Zone 4B). Enter 0 for uninsulated slab.
IECC Zone 4B max: U-0.30. Double-pane low-e: ~0.25–0.30
Typical residential: 12–18%. Energy code max: 15–20%
Standard 3×6.8 door ≈ 20 sq ft. Include all exterior doors.
Insulated steel door: ~0.17–0.20. Solid wood: ~0.40
ASHRAE 62.2 minimum: 0.35 ACH. Tight new construction: 0.15–0.25 ACH
ACCA Manual J recommends 0–15% oversizing allowance

Formulas Used

Conduction Heat Loss (opaque assemblies):
Q = (1/R) × A × ΔT  [BTU/hr]
where R = assembly R-value (hr·ft²·°F/BTU), A = area (ft²), ΔT = Tindoor − Toutdoor (°F)

Window/Door Conduction:
Q = U × A × ΔT  [BTU/hr]
where U = overall heat transfer coefficient (BTU/hr·ft²·°F)

Slab-on-Grade Floor Loss (ASHRAE 2021 Fundamentals, Ch. 18):
Qslab = F2 × P × ΔT  [BTU/hr]
where F2 = perimeter heat loss factor (BTU/hr·ft·°F), P = exposed perimeter (ft)
F2 values: Zone 2A=0.54, 3A/3B=0.64, 4B/5B=0.84 (uninsulated); reduced ~50% with R-10 perimeter insulation

Infiltration Heat Loss:
Qinf = 1.1 × CFM × ΔT  [BTU/hr]
CFM = (ACH × Volume) / 60
where 1.1 = ρ·Cp·60 = 0.075 lb/ft³ × 0.24 BTU/lb·°F × 60 min/hr ≈ 1.08 ≈ 1.1

Design Load:
Qdesign = Qsensible × (1 + Safety Factor / 100)

Assumptions & References

  • Building footprint assumed square for perimeter estimation: P = 4√(Floor Area)
  • Gross wall area = perimeter × ceiling height; net wall = gross − windows − doors
  • ASHRAE 99% heating design dry-bulb temperatures auto-filled per representative Texas city per climate zone
  • Slab-on-grade perimeter heat loss method per ASHRAE Fundamentals Handbook 2021, Chapter 18, Table 18-2
  • Infiltration calculated using ACH method; ASHRAE 62.2-2022 minimum ventilation = 0.35 ACH or 15 CFM/person
  • No internal heat gains (occupants, lighting, appliances) included — conservative worst-case heating load
  • No duct heat loss included; add 10–15% for unconditioned attic ductwork per ACCA Manual D
  • Climate zone boundaries per 2021 IECC and ASHRAE 90.1-2022
  • Minimum insulation requirements per 2021 IECC Table R402.1.2 for Texas climate zones
  • Equipment sizing per ACCA Manual J — 8th Edition residential load calculation
  • Results are estimates only; a licensed HVAC engineer should perform a full Manual J for permit/design purposes

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