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Calculation Methodology

Strict ACCA Compliance

Technical transparency for inspectors, contractors, and building officials verifying Manual J & S compliance.

Load Calc Guru automates the rigorous engineering required for HVAC sizing. Our platform strictly adheres to the procedures outlined in ACCA Manual J (8th Edition) for load calculations and ACCA Manual S (3rd Edition) for equipment selection. This document serves as a technical reference for inspectors and building officials verifying our compliance.

Scope & Limitations

  • Residential Only: Algorithms are strictly for single-family detached dwellings (Manual J Section 1). Commercial or high-rise multi-family buildings are outside of scope.
  • Standard Conditions: Calculations assume standard atmospheric pressure unless the site elevation is explicitly set, in which case air density corrections are applied per Manual S Equation N3-1.
  • Authority Disclaimer: Reports are calculation tools, not stamped engineering documents, unless otherwise stated.

Manual J: Residential Load Calculation

1. Data Sources & Design Conditions

Source Authority: Climatic data is sourced directly from ASHRAE Fundamentals (Stations 2017/2021).

  • Weather Selection: Inverse-distance weighted average of the three nearest ASHRAE weather stations to determine 99% Heating DB and 1% Cooling DB/WB.
  • User Overrides: Default design grains are calculated from station averages. User overrides are explicitly flagged in reports as "User Specified".
2. Envelope & Solar Modeling

We do not use simplified "average load" shortcuts. All loads are calculated element-by-element using official ACCA Manual J data.

  • Official Table Selection: U-Values are selected directly from scientifically derived ACCA Manual J tables (e.g., Code 12B-3w) rather than approximated. This ensures strict adherence to approved component physics.
  • Geometric Shading: Overhangs are modeled geometrically to calculate exact shaded fractions for diffuse-only solar gain factors.
  • Framing Fractions: We strictly use the weighted-average U-values from the tables which account for framing factors (e.g., 25% wood / 75% cavity) specific to the selected assembly.
3. Infiltration & Ventilation
  • Blower Door (CFM50): Converted to natural infiltration (ACHn) using standard LBL correlation (N-factor) based on shielding class and height.
  • Table Method: Defaults to "Effective Leakage Area" (ELA) method based on construction tightness if no test data is available.

Manual S: Equipment Selection

Manual S compliance requires more than just matching total capacity. We strictly verify sensible/latent splits and manufacturer operating limits.

1. Interpolation & Curves

Bilinear Interpolation

Cooling capacity and power interpolated based on both Outdoor Dry Bulb and Entering Wet Bulb (EWB).

Defrost Knee Heuristic

Capacity penalty curve applied in 30°F–40°F range for heat pumps missing OEM data to avoid over-prediction (§N3-3).

2. Sizing Verification

Single-Speed
115%
Max Total Capacity
Variable-Speed
130%
Max Total Capacity (per N2)

3. Auxiliary Heat Sizing

Aux_Required = Heating_Load_Design - HP_Capacity_Design

Visualized via Thermal Balance Point intersection plot.

Traceability Audit

To defend our calculations against skepticism, every Project Report includes a full input dump. Inspectors can trace:

Element-Level Details

Exact dimensions, orientation, and Assembly Code (e.g., "12b-2ws") for every segment.

Component U-Values

Specific U-values printed next to component areas for easy math checking.

Latent Check

Equipment SHR vs Building Required SHR explicitly reported for moisture compliance.

Inspector FAQ