Oklahoma Climate Zone Heat Load Calculator
Estimates the heating load (BTU/hr) for a residential building in Oklahoma based on climate zone, conditioned floor area, insulation levels, window area, and infiltration. Uses the Manual J simplified heat loss method with Oklahoma-specific design temperatures.
Results will appear here.
Formulas Used
Conduction Heat Loss (each assembly):
Qcond = U × A × ΔT
- U = 1 / Reffective (BTU/hr·ft²·°F)
- A = surface area (ft²)
- ΔT = Tindoor − Toutdoor design (°F)
- Reff,wall = Rrated × 0.85 + 3.5 (framing factor + sheathing/drywall)
- Reff,ceiling = Rrated × 0.90 + 2.5
- Reff,floor = Rrated × 0.90 + 2.0
Infiltration Heat Loss:
Qinf = 0.018 × ACH × V × ΔT
- 0.018 BTU/(ft³·°F) = cp,air × ρair = 0.24 × 0.075
- ACH = air changes per hour
- V = conditioned volume = floor area × ceiling height (ft³)
Total Heating Load:
Qtotal = Qwall + Qceiling + Qfloor + Qwindows + Qdoors + Qinf
Assumptions & References
- Climate Zones: Per IECC 2021 / ASHRAE 169-2020; Oklahoma spans Zones 3A, 3B, 4A, 4B, and 5A.
- Outdoor Design Temperatures: ASHRAE 99% heating design dry-bulb temperatures: Zone 3A (Idabel) 19°F, Zone 3B (Lawton) 17°F, Zone 4A (OKC/Tulsa) 13°F, Zone 4B (Woodward) 8°F, Zone 5A (Guymon) 0°F.
- Method: Manual J Simplified (ACCA Manual J, 8th Edition) — suitable for preliminary sizing; a full Manual J calculation by a licensed HVAC engineer is recommended for final equipment selection.
- Framing Factors: 15% for walls (2×4/2×6 stud framing), 10% for ceilings and floors, per ASHRAE Handbook of Fundamentals.
- Single-story assumed for perimeter/wall area estimation (perimeter = 4√A).
- Door area: Standard 3 ft × 6 ft 8 in = 20 sq ft per door.
- Infiltration: Sensible heat factor 0.018 BTU/(ft³·°F) per ASHRAE; ACH values per ACCA Manual J Table 5A.
- Equipment Sizing: ACCA Manual J recommends heating equipment sized within 100–115% of calculated load (no more than 15% oversizing).
- Not included: Duct losses, solar gain credits, internal gains, humidity loads, or multi-story corrections — consult a licensed HVAC professional for full load analysis.
- References: ACCA Manual J (8th Ed.), ASHRAE Handbook of Fundamentals 2021, IECC 2021, ASHRAE 169-2020, Oklahoma Uniform Building Code.