Austin Climate Load Calculator
Estimates residential heating and cooling loads for Austin, TX using the Manual J simplified method with Austin-specific climate design conditions (ASHRAE 99%/1% design temperatures).
Building Envelope
Construction & Infiltration
Formulas Used
Conduction: Q = U × A × ΔT (BTU/hr)
Solar gain: Qsolar = SHGC × Awin × SHGF × IAC
Infiltration (sensible): Q = 1.1 × CFM × ΔT
Infiltration (latent): Q = 0.68 × CFM × Δgrains
CFM: ACH × Volume / 60 | ACH = factor × (ΔT/10)0.4
Slab edge (heating): Q = F2 × Perimeter × ΔT (F2 = 0.73 BTU/hr·ft·°F)
Duct losses: 15% cooling / 20% heating (attic ducts, Austin typical)
Cooling tons: Total BTU/hr ÷ 12,000
Assumptions & References
- Austin design conditions (ASHRAE 2021): Cooling 101°F DB / 76°F WB (99th percentile); Heating 28°F DB (99th percentile)
- Peak solar heat gain factors from ASHRAE Fundamentals Table 15, 32°N latitude, July, 3 PM
- Window orientation assumed 25% each: South, West, East, North
- Interior attenuation coefficient (IAC) = 0.70 (standard horizontal blinds)
- Outdoor humidity ratio at cooling design: 0.0138 lb/lb (101°F/76°F wb); indoor at 75°F/50% RH: 0.0093 lb/lb
- Internal gains: 250 BTU/hr sensible + 200 BTU/hr latent per person (ASHRAE 62.2); lighting 3 W/ft² × 50% diversity; appliances 1,200 BTU/hr
- Duct loss factors: 15% cooling / 20% heating for ducts in unconditioned attic (Manual J default, Austin common)
- Annual cooling hours: ~1,200 equivalent full-load hours (Austin Energy data)
- Building geometry assumed square footprint; wall area estimated from perimeter × ceiling height
- Slab edge factor F2 = 0.73 BTU/hr·ft·°F (ASHRAE, uninsulated slab, Austin soil)
- Equipment sizing per ACCA Manual S: select within 100–115% of calculated cooling load
- Minimum efficiency: SEER2 15 per Texas/DOE 2023 standards; Austin Energy rebates at SEER2 ≥ 16
- References: ASHRAE Handbook of Fundamentals 2021; ACCA Manual J 8th Ed.; ACCA Manual S; Austin Energy Green Building Guidelines; Texas IECC 2021
- This tool provides estimates only. A licensed HVAC engineer should perform a full Manual J calculation for equipment selection.