About
Methodology and Sources
The conventions every calculator follows, and the sources behind the numbers it starts with.
Conventions
Your inputs come first. A calculator never substitutes its own figure for one you have entered. Defaults exist so the page shows a meaningful example; each is labelled and sourced in the assumption table.
Every component is a separate line. Allowances, losses, fees, taxes and rounding appear as their own rows in the breakdown, never folded into a single figure. If a line surprises you, you can see exactly what caused it.
Units are converted, not approximated. Conversions use exact factors, and the result of a calculation is the same whichever unit system you enter it in.
Rounding happens at the end. Intermediate values are kept at full precision; only the displayed figures are rounded, and quantities that are bought in whole units are rounded up.
Averages are averages. Where a default is a national or typical average, it is a starting point, not a claim about your situation.
Sources for default figures
| Figure | Source | Used by |
|---|---|---|
| Climate zones 1 to 7 and example cities | US DOE and IECC climate zone map | All sizing and season-cost calculators |
| Cooling and heating BTU per square foot by zone | Compiled from published climate-zone sizing charts; set out on the BTU per square foot table | AC, furnace, heat pump, boiler, mini-split, baseboard, radiator, space heater, size check |
| Room AC chart, sun, occupant and kitchen allowances | ENERGY STAR room air conditioner sizing guidance | Window, portable and central AC size |
| Portable AC real capacity (SACC) | US DOE portable air conditioner test procedure (2017) | Portable AC size |
| Equipment sizing limits (115% cooling, 125% heat pump, 140% furnace) | ACCA Manual S | Size check, AC, furnace and heat pump size |
| Winter design temperatures and degree days | NOAA climate normals, rounded for representative cities | Heat pump size, degree-day fuel estimate, heating cost by fuel, heat pump vs furnace |
| Degree-day correction factor 0.65 | ASHRAE degree-day method (range 0.6 to 0.8) | Season energy and cost calculators |
| Heat pump capacity at low temperatures | Straight-line fit of typical manufacturer capacity tables at 47, 17 and 5 °F | Heat pump size, heat pump vs furnace |
| Fuel energy content and average prices | US Energy Information Administration, 2025 residential data | All energy-cost calculators |
| SEER2, HSPF2, EER2, AFUE minimums and the 2023 test change | US DOE appliance standards and test procedures | Efficiency, running-cost and payback calculators |
| Duct friction and equivalent diameter | ASHRAE Fundamentals friction relation for galvanized duct | Duct size, duct velocity, equivalent length |
| Fitting equivalent lengths and velocity limits | Typical values in the range of ACCA Manual D | Equivalent length, duct size, return and register size |
| Whole-house ventilation rate | ASHRAE Standard 62.2-2016 | Whole-house ventilation, ERV/HRV size |
| Bathroom and kitchen exhaust minimums, makeup air | International Residential Code M1507 and M1503.6 | Bathroom fan, range hood |
| Bathroom fan and range hood sizing rules | Home Ventilating Institute | Bathroom fan, range hood |
| Air purifier sizing (⅔ rule) | AHAM | Air purifier CADR |
| Dehumidifier ratings | US DOE 2019 test procedure; ENERGY STAR | Dehumidifier size |
| Dew point and humidity ratio | Magnus formula (Alduchov and Eskridge coefficients) | Dew point, humidifier size |
| Heat index | US National Weather Service (Rothfusz regression) | Heat index |
| Hot water per use and first-hour rating method | US DOE water heater sizing worksheet | Water heater size, hot water use |
| Water heater UEF defaults | DOE and ENERGY STAR typical values | Water heater running cost |
| Equipment service lives | ASHRAE service-life data; DOE and ENERGY STAR guidance | Lifespan planner, repair cost |
| Installed cost ranges | HVAC.com, Bob Vila and HomeServe, EnergySage, PickHVAC, HVAC ProSales, Portlandia Electric Supply, Fixr, This Old House, HomeAdvisor, 2025 and 2026 | Every installation and repair cost calculator |
Where a figure is this site’s own judgement, such as the 12% price change per ton, the regional cost multipliers or the share of leaked duct energy lost in a crawl space, the assumption table says so.
What the calculators do not model
They are not a Manual J. Loads come from floor area, climate zone and a handful of factors. A room-by-room load calculation from the actual walls, windows, orientation and air leakage can differ by 20% or more, especially for unusual houses, and it is what equipment should be bought from.
They do not model equipment curves. Real air conditioners and heat pumps change capacity and efficiency with outdoor temperature, indoor humidity and airflow. The calculators use nominal capacity, seasonal ratings and a straight-line approximation for heat pumps in cold weather.
They do not design duct systems. The duct calculators size one duct at a time with the equal-friction method. A Manual D design balances every run, accounts for each fitting and checks the blower’s real performance.
They do not include local codes, rebates or prices. Code editions, permit fees, utility rebates and labour rates vary by jurisdiction. Costs are national ranges with simple regional multipliers.
They do not judge safety. Gas, refrigerant, combustion venting and electrical work have safety rules the calculators do not check.
Testing
Every calculator carries a set of test cases with hand-worked answers in both unit systems. The tests run automatically before each update to the site, and a calculator that fails any of them is not published. When an error is reported and confirmed, a test case for it is added so that it cannot come back.