Reference
Methodology
Last updated
BuildSolver runs HVAC engineering calculations as deterministic code following the procedures published in ACCA Manual J 8th Edition, ACCA Manual S, ACCA Manual D, ASHRAE 62.2, and the ASHRAE Handbook of Fundamentals. The AI layer orchestrates the conversation and chooses which tool to call - the math itself is not LLM inference. Every result names the standard behind it.
01 · Standards used
What we follow
| Standard | What we use it for |
|---|---|
Air Conditioning Contractors of America - Residential Load Calculation, 8th Edition | Whole-house cooling and heating loads on the free pages; room-by-room loads in the Pro plan. |
ACCA - Residential Equipment Selection | Picks the smallest nominal size whose cooling capacity, derated to the outdoor design temperature, fits the sizing limits, or the smallest that covers the load with a warning when none fits, then checks total, sensible and latent capacity against the loads. An AHRI rating you enter replaces the default capacity; there is no equipment database. Ceilings follow the first edition; the 100% floor and the 115% sensible cap are BuildSolver rules. ANSI/ACCA 3 Manual S-2023 is the current edition. |
ACCA - Residential Duct Systems | Friction-rate sizing for residential branch runs; equal-friction balancing. |
ANSI/ASHRAE Standard 62.2 - Ventilation and Acceptable Indoor Air Quality in Residential Buildings | Whole-dwelling mechanical ventilation rate on the free ventilation calculator. |
ASHRAE Handbook - Fundamentals: Chapter 1, Psychrometrics (2021 edition); Chapter 14, Climatic Design Information (2021 and 2017 editions); Chapter 21, Duct Design (2017 edition) | Psychrometric relations and enthalpy; the definition of the 99% heating and 1% cooling design conditions; duct friction loss. Design values: ASHRAE 2021 Climatic Design Conditions (99% heating / 1% cooling) station values when the site is one of 10 metro cities; an approximate IECC climate zone estimate, not location-specific, for a ZIP code or any other location. |
International Energy Conservation Code: 2021 climate zones; 2021 and 2024 air and duct leakage limits | Climate zone defaults: U-values, infiltration rates, design temperatures. Leakage limits on the blower door and duct test pages. |
International Fuel Gas Code, 2021 edition: Section 304 and Chapter 4 | Combustion air openings and gas pipe sizing on the free fuel gas pages. |
National Electrical Code, 2023 edition | The branch circuit for HVAC equipment on the free breaker and wire pages (Article 440, Table 310.16). Not the primary scope. |
02 · Per-tool methodology
How each calculation runs
Cooling load
- ACCA Manual J 8th Ed. §6 (sensible)
- ACCA Manual J 8th Ed. §8 (latent)
Sensible gains summed from envelope (UA × CLTD), internal loads (people, lights, appliances), infiltration, duct losses. Latent gains from infiltration moisture, occupant respiration, and ventilation. Design conditions: ASHRAE 2021 station values (99% heating / 1% cooling) when the site is one of 10 metro cities; for a ZIP code or any other location, an approximate estimate for the IECC climate zone (ZIP → county → zone), not location-specific.
The code lives in lib/manual-j/load.ts.
Heating load
- ACCA Manual J 8th Ed. §5
Envelope UA × ΔT (indoor design − 99% outdoor design), infiltration via the crack/ach method. No credit taken for solar or internal gains, per Manual J convention.
The code lives in lib/manual-j/heating-load.ts.
Equipment selection
- ACCA Manual S (first-edition limits)
- AHRI 210/240 rating
- NREL/Cutler capacity model
Picks the smallest nominal size whose capacity at the design temperature covers the Manual J load. Capacity at design is the AHRI 210/240 rating derated with the NREL/Cutler biquadratic model, and total, sensible and latent capacity are checked against the loads. Ceilings are the Manual S first-edition limits. The 100% floor (stricter than the 95% of Manual S 1st ed) and the 115% sensible cap are BuildSolver rules. Golden test fixtures pin output against published Manual S worked examples.
The code lives in lib/manual-s/equipment-selection.ts.
Duct sizing (simplified)
- ACCA Manual D
Friction-rate method for residential branch runs with equal-friction balancing. This is preliminary sizing - not a full pressure-drop / TEL calculation. For permit-grade duct designs, use a full Manual D package.
The code lives in lib/manual-d/duct-size.ts.
Psychrometric state
- ASHRAE Handbook of Fundamentals Ch. 1
Saturation vapor pressure via the Magnus-Tetens approximation. Wet-bulb, dew point, enthalpy, and humidity ratio derived from dry-bulb + one secondary variable.
The code lives in lib/psychrometrics/state.ts.
03 · Every calculator
Which standard each page follows
Each calculator page lists its full sources and limitations. The line under each name is the standard or data the math comes from.
The calculations
| Calculator | Based on |
|---|---|
| Manual J Load Calculator | ACCA Manual J 8th Edition |
| Manual S Equipment Selection | ACCA Manual S |
| How Manual S Equipment Selection Works | ACCA Manual S Table 1 sizing limitations |
| Heat Pump Sizing Calculator | AHRI 210/240 rating points, BuildSolver derate averages |
Free on-page calculators
| Calculator | Based on |
|---|---|
| Furnace Size Calculator | ACCA Manual J 8th Edition |
| Duct Sizing Calculator | ACCA Manual D (equal friction) |
| HVAC Design Temperatures by ZIP | IECC 2021 climate zones |
| Dew Point Calculator | ASHRAE Fundamentals Ch. 1 |
| Psychrometric Calculator | ASHRAE Fundamentals Ch. 1 |
Field and sizing formulas
| Calculator | Based on |
|---|---|
| What Size AC by Square Footage | ACCA Manual J 8th Edition |
| BTU per Square Foot Calculator | ACCA Manual J 8th Edition |
| HVAC CFM Calculator | ASHRAE sensible heat equation |
| Superheat and Subcooling Calculator | CoolProp PT data, manufacturer charging charts |
| Refrigerant PT Chart | CoolProp PT data, checked against Daikin, Honeywell and Chemours charts |
| ASHRAE 62.2 Ventilation Calculator | ANSI/ASHRAE Standard 62.2 |
| Bathroom Exhaust Fan Calculator | HVI guidance, IRC M1505.4.4 |
| SEER2 Energy Cost Calculator | DOE 10 CFR 430 Appendix M1 |
| BTU to Tons Converter | NIST SP 811 conversion factors |
| AC Temperature Split Calculator | ASHRAE Fundamentals psychrometrics |
| Condensate Drain Calculator | 2021 IMC 307.2, IRC M1411.3 |
| Gas Pipe Sizing Calculator | 2021 IFGC Tables 402.4(2) and 402.4(28) |
| Combustion Air Calculator | 2021 IFGC Section 304 |
| Duct Leakage Test Calculator | 2021 IECC R403.3.6, 2024 IECC Table R403.3.8 |
| Line Set Charge Calculator | Manufacturer's rate, Goodman TP-107M example |
| Evaporative Cooler Size Calculator | ASHRAE HVAC Systems and Equipment Ch. 41 |
| AC Breaker Size Calculator | NEC 2023 Article 440 and Article 424 |
| AC Wire Size Calculator | NEC 2023 Table 310.16, 110.14(C) and 310.15 |
| AC Amps and Watts Calculator | 10 CFR 430 Appendix M1 (EER2), DOE power equations |
| Voltage Drop Calculator | K method (Mike Holt), NEC 2023 Chapter 9 Table 8 |
| Tankless Water Heater Size Calculator | DOE Energy Saver sizing method, 10 CFR 430.32 |
| Static Pressure Calculator | Manufacturer blower data, ACCA Manual D method |
| Fan Law Calculator | Fan laws (HVAC School), Aerovent FE-1600 density correction |
| Rectangular to Round Duct Calculator | ASHRAE Fundamentals Ch. 21, Huebscher equivalent diameter |
| Air Changes per Hour Calculator | IECC 2021 and 2024 air leakage limits |
| Range Hood CFM Calculator | HVI guidance, IRC M1505.4.4 and M1503.6 |
| Dehumidifier Size Calculator | ASHRAE psychrometrics, DOE 10 CFR 430 Appendix X1 |
| Garage Heater Size Calculator | U x A x dT heat loss, BuildSolver engine defaults |
| Sunroom BTU Calculator | U x A x dT and solar gain, BuildSolver engine defaults |
| Basement BTU Calculator | Seattle Energy Code Table A104.1 below-grade defaults |
| Room Addition BTU Calculator | U x A x dT and solar gain, BuildSolver engine defaults |
| Mini Split Sizing Calculator | U x A x dT and solar gain, BuildSolver engine defaults |
| Duct Velocity and Friction Loss Calculator | ASHRAE Fundamentals Ch. 21 (Darcy, Colebrook), ACCA Manual D Table 3-1 |
| Boiler Size Calculator | ACCA Manual S (boilers 100-140%), Net AHRI rating with the 1.15 piping and pickup allowance |
| Baseboard Heater Breaker Size Calculator | NEC 2023 Article 424 (424.4), Table 240.6(A) |
| Furnace Temperature Rise Calculator | Furnace manufacturers' instructions, ASHRAE Fundamentals Ch. 1 (1.08) |
| Garage AC Sizing Calculator | Conduction, sun and infiltration heat gain, BuildSolver engine defaults |
| Water Heater Breaker Size Calculator | NEC Article 422 (422.13, 422.11) |
By job type
| Calculator | Based on |
|---|---|
| Replacement AC Sizing Calculator | ACCA Manual J 8th Edition |
| Retrofit Sizing Without a Blueprint | ACCA Manual J 8th Edition |
| Manual J for New Construction | ACCA Manual J 8th Edition |
| When a Permit Needs a Stamped Manual J | IECC R403 / local AHJ practice |
04 · Transparency commitments
What you can verify
- Every BuildSolver PDF names the standard its numbers come from, with a section reference where the tool has one.
- Each generated report lists the inputs it ran on, such as climate zone, design conditions and the values the user supplied.
- Calculations execute as deterministic code, not LLM inference. The AI chooses which tool to call and how to phrase the explanation; the numbers themselves come from the formula modules listed above.
- The test suite includes golden fixtures that pin output against published Manual J/S worked examples - regressions show up before any change ships.
05 · Scope and limits
Where to use BuildSolver - and where not to
Use BuildSolver for the sales phase
Preliminary sizing, client quotes, retrofit estimates, equipment shortlists, ballpark psychrometric checks, and branch-run duct sizing. The target is the call where you need a defensible number in 90 seconds, not a stamped permit submittal.
Use a licensed engineer for the permit phase
AHJs in most US jurisdictions require an ACCA-approved package (Wrightsoft, Cool Calc, Kwik Model) with a licensed mechanical engineer's seal. BuildSolver output is preliminary and clearly stamped as such - it is not a substitute for a PE review and final design.
The chat covers HVAC only
The chat's calculation tools are HVAC tools. It will refuse off-scope questions rather than guess across disciplines we have not validated. The free calculator pages go a little wider, to the electrical circuit for HVAC equipment, fuel gas for furnaces and tankless water heater sizing. They run public formulas in your browser, cite the standard on each page and sit outside the chat.
06 · Disclaimer