Maryland Load Calculation Estimator (Manual J)
Estimates residential heating and cooling loads for Maryland homes using the ACCA Manual J simplified methodology. Results help size HVAC equipment for energy efficiency and comfort compliance with Maryland energy codes.
Building Envelope
Insulation & Construction
Infiltration & Ventilation
Internal Gains & Occupancy
Formulas Used
Heating Load (Btuh):
Qheat = Σ(U × A × ΔTheat) × Fduct + 1.1 × CFMinf × ΔTheat
where ΔTheat = 70°F − Toutdoor design (99% heating)
Cooling Sensible Load (Btuh):
Qcool,s = [Σ(U × A × ΔTcool) + SHGC × Awin × Isolar × IAC + Qinternal] × Fduct + 1.1 × CFMinf × ΔTcool
where ΔTcool = Toutdoor design (1% cooling) − 75°F
Cooling Latent Load (Btuh):
Qcool,l = (0.68 × CFMinf × ΔW + 200 × Nocc) × Fduct
where ΔW = outdoor − indoor moisture content (grains/lb)
Infiltration CFM:
CFMinf = ACHeff × Volume / 60
ACHeff: Tight = 0.15, Average = 0.35, Loose = 0.60
Assembly U-Values:
Uwall = 1 / (Rinsul × 0.85 + 3.5) |
Uceil = 1 / (Rinsul + 2.5) |
Ufloor = 1 / (Rinsul + 2.0)
Assumptions & References
- Standard: ACCA Manual J 8th Edition (Residential Load Calculation), Abridged Method
- Design temperatures: ASHRAE 2021 Fundamentals Handbook, Chapter 14 — Baltimore (99%/1%), Leonardtown, Cumberland
- Climate zones: IECC 2021 / Maryland Energy Code — Zone 4A (southern counties), Zone 5A (central/eastern), Zone 5A-West (Garrett/Allegany)
- Indoor design conditions: 70°F heating, 75°F / 50% RH cooling (ACCA Manual J Table 1)
- Framing factor: 15% for wood-framed walls per Manual J Table 4A
- Solar intensity: ASHRAE peak solar heat gain factors at ~39°N latitude, July 3 PM design day
- Interior attenuation coefficient (IAC): 0.70 assumes medium-colored interior blinds (Manual J Table 3D)
- Occupant gains: 250 Btuh sensible / 200 Btuh latent per person (sedentary, Manual J Table 1A)
- Appliance/lighting gains: 1.5 Btuh/ft² (Manual J Table 1A, residential)
- Duct loss multipliers: Manual J Table 7A — conditioned 1.00, semi-conditioned 1.10, unconditioned attic 1.25
- Moisture differential (ΔW): Zone 4A = 55 gr/lb, Zone 5A = 50 gr/lb, Zone 5A-West = 40 gr/lb (ASHRAE humidity data)
- Equipment sizing: Manual J recommends sizing to calculated load; rounded to nearest 0.5 ton / 10 kBtuh per ACCA Manual S
- Maryland code requirement: COMAR 09.12.52 requires Manual J for all new residential HVAC permits; IECC 2021 adopted statewide effective January 2023
- This tool provides a preliminary estimate only. A certified Manual J by a licensed engineer or ACCA-certified contractor is required for permit submission.