Engineering Calculation Methodology
HVACLogic implements reproducible browser-based calculation engines based on recognized engineering equations, ASHRAE and ACCA technical references, and published thermophysical property data.
Core Engineering Principles
1. Deterministic & Reproducible
Given identical aerodynamic, thermal, and dimensional parameters, the calculation engines evaluate mathematical formulations deterministically directly in your browser with transparent input-output relationships.
2. Engineering References & Standards
Formulas cite recognized industry references (such as the ASHRAE Handbook of Fundamentals, ACCA technical manuals, and SMACNA guidelines) for specific physical equations and reference tables.
3. Documented Assumptions & Limits
Empirical deratings, physical property bases, boundary limits, and screening assumptions (such as flexible duct sag models and zeotropic glide calculations) are explicitly defined.
1. Airflow & Duct Sizing Fluid Mechanics
The Digital Ductulator implements the standard ASHRAE equal-friction exponential formulation for galvanized sheet metal air ducts (absolute roughness ε ≈ 0.0003 ft under standard air density ρ = 0.075 lb/ft³) and Huebscher's formula for equivalent rectangular dimensions:
Where Q is volumetric airflow (CFM), hf is design friction loss rate (in. wg per 100 ft), D is equivalent round diameter (inches), and a and b are rectangular duct cross-sectional dimensions (inches). Flexible duct calculations incorporate empirical compression and sag derating factors referenced to ASHRAE Research Project RP-1333 laboratory data (Culp et al., Texas A&M ESL) across modeled 0%, 4%, 15%, and 30% compression scenarios.
2. Building Thermal Loads & Equipment Sizing Concepts
Building load screening models steady-state envelope conductive transmission (q = U × A × ΔT), fenestration solar heat gain (SHGC), air infiltration sensible losses, and internal occupant gains (modeled using screening baselines of 230 BTU/hr sensible and 200 BTU/hr latent per person, derived from ASHRAE Fundamentals Chapter 18 moderately active residential seated/standing benchmarks):
Equipment capacity comparisons reference ACCA Manual S selection principles, illustrating recommended sizing bands (such as 90% to 115% of design total load for standard cooling systems and up to 125%–130% for heat pumps to moderate low-ambient heating deficits) to support adequate humidity control and avoid short-cycling.
3. Refrigeration Thermodynamics & A2L Temperature Glide
Refrigerant diagnostics reference thermophysical saturation property formulations based on NIST REFPROP models. For zeotropic blends such as R-454B (68.9% R-32 / 31.1% R-1234yf by mass) and R-407C, discrete bubble-point (liquid) and dew-point (vapor) states are evaluated to account for temperature glide (~1.5°F to 2.7°F):
In field diagnostic workflows, suction vapor superheat is evaluated against the saturated dew-point temperature at suction pressure, while liquid-line subcooling is evaluated against the saturated bubble-point temperature at liquid pressure. An initial 15-minute operating stabilization window is recommended as an HVACLogic diagnostic procedure to allow evaporator and condenser thermal equilibrium before recording service manifold pressures.
📐 Model & Standards Traceability Matrix
Summary of mathematical formulations, reference publications, implemented calculation scopes, and baseline engineering assumptions across the HVACLogic suite:
| Model / Domain | Reference Publication | Reference Basis | Implemented Scope | HVACLogic Assumptions |
|---|---|---|---|---|
| Duct Sizing (Round & Rectangular) | ASHRAE Handbook of Fundamentals | Chapter 21 / Huebscher | Equal-friction diameter and rectangular aspect-ratio equivalence | Standard air (ρ = 0.075 lb/ft³), galvanized metal roughness (ε = 0.0003 ft) |
| Duct Friction & TEL | ACCA Manual D | Appendix 3 Fitting Tables | Fitting equivalent length accumulation & available static pressure (ASP) | Representative equivalent lengths for standard fitting archetypes |
| Flexible Duct Derating | ASHRAE RP-1333 / SMACNA | Texas A&M ESL Lab Data | 0%, 4%, 15%, and 30% sag and compression multiplier curves | Modeled friction multipliers (1.15× to 2.20×) under constant static pressure |
| Conductive Heat Loss | ASHRAE Fundamentals / IECC | Chapter 26 & 27 / IECC Tables | 1-D series thermal resistance (R = ∑Rlayer) & parallel path | Standard boundary air film resistances; nominal insulation R-values |
| Thermal Bridging | ASHRAE 90.1 Appendix A | Table A9.2-1 & A9.2-2 | Cold-formed steel and wood stud framing effective cavity resistance | Prescriptive framing factors (16" and 24" on-center spacing) |
| Residential Load Sizing | ACCA Manual J / Manual S | 8th Edition Principles | Screening estimates for sensible/latent cooling and heating loads | Regional climate factors; default internal gains (230 sensible / 200 latent BTU/hr/person) |
| Refrigerant Saturation | NIST REFPROP / ASHRAE 34 | Thermophysical Database | Pressure-temperature saturation curves, bubble/dew points for zeotropic blends | Thermodynamic equilibrium; standard atmospheric pressure (14.696 psia) |
| Superheat & Subcooling | Field Diagnostic Protocols | Industry Service Practice | Target superheat formula (fixed orifice) and TXV target subcooling evaluation | 15-minute operating stabilization baseline before pressure acquisition |
| Psychrometric Properties | ASHRAE Fundamentals | Hyland-Wexler Formulations | Moist air enthalpy, wet bulb, dew point, humidity ratio, specific volume | Standard barometric pressure elevation compensation (-1,000 to 15,000 ft) |
Technical Documentation & Open Monograph
The mathematical models and numerical algorithms powering HVACLogic are documented in an open technical preprint and engineering monograph:
Technical Feedback & Calculation Errata Policy
We maintain continuous formula validation against active ASHRAE, ACCA, and AHRI references. If you discover a calculation discrepancy, boundary condition error, or standard update in any tool, please submit a detailed issue report directly to our open-source tracking repository: