Metered history
Twelve months of utility bills, plus the Manual J design temperatures for your area. Estimates end where measurement begins.
Methodology
How HVAC Calculator calculates.
Explicit-input models with visible formulas, dated baselines, machine-checked test cases, and a stated verification boundary. Screening-grade arithmetic for HVAC work, not a Manual J load study.
The principle
Every result must be reproducible from the inputs on the page.
Each calculator is a fixed formula over labeled inputs. There are no hidden scores, no black-box adjustments, and no location lookups: the numbers you type, plus the stated defaults, fully determine the outputs. Every calculator page shows its formula, and every calculator ships with automated test cases that fixed inputs must reproduce before publication.
Units and conventions
One consistent system across all twenty tools.
Power vs energy
kW measures rate of use; kWh measures accumulated use. Cost always derives from kWh multiplied by a rate, never from kW alone. Heating fuel derives from delivered BTU divided by equipment efficiency (AFUE, COP, or HSPF).
Money handling
Rates are entered in cents per kWh and converted once, at a single documented step, to dollars. Displayed money rounds to cents; intermediate values keep full precision so chained steps do not drift.
Duty cycle
Cycling equipment (compressors, furnaces, thermostats) is modeled as rated capacity times effective run hours. Where duty is uncertain, the input stays editable and the assumption stays labeled, rather than being buried in a fixed factor.
Precision statement
Outputs are screening-grade: good enough to rank options, size a first pass, or check a quote. They are not ACCA Manual J, Manual D, or Manual S calculations and do not replace a room-by-room load study or a licensed contractor design.
Method by category
What each tool family models, and what it assumes.
Sizing and load
Rule-of-thumb sizing from floor area, volume, or design temperature difference. These are block-load screens; final equipment size follows an ACCA Manual J room-by-room calculation and Manual S selection.
Airflow and ducts
Airflow from the sensible-heat relation (CFM = BTUH / (1.08 x delta-T)) and duct area from the continuity relation (area = CFM / velocity). Final layouts follow ACCA Manual D friction-rate and fitting-loss practice.
Efficiency ratings
SEER2, EER2, HSPF2, COP, and AFUE are used as defined by DOE test procedure: seasonal ratings price seasonal energy, steady-state ratings price peak draw. EER converts to COP exactly (COP = EER / 3.412); EER to SEER has no exact conversion and is labeled as a rough guess.
Heating fuels and costs
Delivered heat divides fuel energy by equipment efficiency: gas and oil by AFUE, heat pumps by COP or HSPF, electric resistance at COP 1. Fuel energy uses EIA heat content (heating oil about 138,500 BTU per gallon, propane about 91,500).
Refrigerant and diagnostics
Superheat and subcooling compare measured line temperatures against saturation temperatures from the refrigerant pressure-temperature relationship. Target superheat follows the standard return-wet-bulb and outdoor-dry-bulb relation for fixed-metering cooling operation.
Replacement economics
Payback divides net installed cost after rebates by first-year savings. The repair-versus-replace screen multiplies the quoted repair by equipment age against the industry 5,000 rule, bounded by a 15-year expected life.
Sources and baselines
Dated numbers, named sources, annual review.
Usage and rate averages
National figures (for example, fuel heat content and average energy prices) come from U.S. Energy Information Administration tables and carry their data year. A yearly site review checks defaults against the newest release.
Code references
Where practice is discussed, ACCA Manual J, Manual D, and Manual S are cited as context for why a rule exists, never as design instruction. The locally adopted code and the authority having jurisdiction always govern.
Equipment figures
Typical capacities, COP ranges, and equipment lifespans are labeled as starting points. Nameplate data and manufacturer sheets replace them during verification.
Verification protocol
Replace these five before relying on any result.
Current tariff sheet
Rates, tiers, windows, fixed fees, demand charges, ratchets, and riders exactly as billed this month.
Nameplate facts
Actual nameplate capacity, efficiency ratings (SEER2, AFUE, COP), and run behavior of the installed equipment.
Site conditions
Shading, orientation, service capacity, and coincident loads that no generic model can see.
Qualified review
A licensed HVAC contractor confirms the conclusion before money is spent.
Fuel-price baselines are dated starting points, not live utility feeds. Calculator results are screening outputs, not utility bills, quotes, permits, or guarantees.
Known limits
What these tools deliberately do not do.
No code compliance
Nothing here determines whether an installation meets code. Sizing, protection, wiring methods, and clearances are field decisions under the AHJ.
No coincident-peak modeling
Sizing tools estimate block loads. Which rooms peak together, and duct-by-duct losses, require a room-by-room Manual J the tools do not perform.
No forecasts
Escalation and degradation inputs project arithmetic, not the future. They show sensitivity to assumptions; they do not predict rates, weather, or performance.
No account data
Public baselines never describe your account. Two neighbors on different tariffs can have legitimately different bills for identical usage.