How BTU Planner calculates
This page lists every formula and constant the calculators use. All of them live in one JavaScript file. The browser runs it on each page, and our automated tests run the same file before every release.
Room cooling
We take the ENERGY STAR room air conditioner chart (100 to 2,500 ft², 8 ft ceiling) as published and apply its rules: −10% for heavy shade, +10% for a very sunny room, +600 BTU/h for each regular occupant beyond two, and +4,000 BTU/h for a kitchen. One addition is ours: we scale by ceiling height ÷ 8. Where a range boundary appears in two rows of the chart, we treat each range as including its lower bound, so 150 ft² reads as 6,000 BTU/h. Above 2,500 ft², we scale the last row by area (13.6 BTU/h per ft²) and point you to the whole-house tool.
Heating load
The design temperatures are ASHRAE 2009 Fundamentals 99% heating dry-bulb values for one airport per zone, converted from °C and rounded:
| Zone | Station | 99% heating | Cooling BTU/ft² (avg.) |
|---|---|---|---|
| 1 | Miami, FL | 52 °F | 30 |
| 2 | Houston, TX | 33 °F | 28 |
| 3 | Atlanta, GA | 26 °F | 25 |
| 4 | Baltimore, MD | 17 °F | 22 |
| 5 | Chicago, IL | 2 °F | 20 |
| 6 | Minneapolis, MN | -8 °F | 18 |
| 7 | Duluth, MN | -14 °F | 16 |
| 8 | Fairbanks, AK | -38 °F | 16 |
UA is the whole-house heat loss coefficient per square foot of floor. We derived the "average" and "good" levels from a 2,000 ft² two-story reference house:
| Part | Average house | Good (2010s code) |
|---|---|---|
| Walls (net of windows) | 1,300 ft² × U 0.09 (R-11 cavity) | 1,600 ft² × U 0.0625 (R-20 class) |
| Attic ceiling | 1,000 ft² × U 0.05 (R-19) | 1,000 ft² × U 0.026 (R-38) |
| Windows | 300 ft² × U 0.50 (double-pane) | 300 ft² × U 0.30 |
| Floor / foundation | ≈ 70 BTU/h·°F | ≈ 50 BTU/h·°F |
| Air leakage | 0.018 × 16,000 ft³ × 0.7 ACH | 0.018 × 16,000 ft³ × 0.35 ACH |
| Total UA ÷ 2,000 ft² | ≈ 587 → 0.29, we use 0.30 | ≈ 366 → 0.18, we use 0.20 |
The poor level (0.60) adds uninsulated walls, single-pane glass and about 1.2 ACH of leakage. The high-performance level (0.12) assumes R-49+ attic, R-20+5ci walls, U-0.27 windows and tight air sealing. The result is a design-day load with no safety margin. The page notes that ACCA Manual S lets furnace output run up to about 140% of it.
Whole-house cooling and tonnage
The load is floor area × the zone's cooling BTU per ft² × an envelope factor (1.20 / 1.00 / 0.85 / 0.70) × ceiling ÷ 8 × a sun factor (±10%). The per-ft² values are planning figures, not a Manual J. They put an average house at about 400 ft² per ton in zone 1 and 750 ft² per ton in zone 7, the span usually quoted by contractors. We then list every nominal size from 1.5 to 5 tons that is within ACCA Manual S's 90–115% of the load, and pick the one closest to 100%. A tie goes to the smaller unit.
Ducts
Round duct diameter comes from the fitted friction-chart equation for galvanized duct, Δp = 0.109136 Q1.9 / D5.02. Rectangular equivalents use the Huebscher equation, De = 1.30 (ab)0.625 / (a+b)0.25. Airflow defaults to 400 CFM per ton. For flex duct we recommend the next standard size up rather than pretend to model compression.
Insulation
R per inch uses one typical value per material, inside the published range, and the page always prints the range too. Wall assemblies use the ASHRAE parallel-path method with 25% framing by default, softwood at R-1.25 per inch, and standard surface films. Code tables are copied from 2021 IECC Table R402.1.3 and the ENERGY STAR retrofit table.
How we test
- Every calculator has a written specification and hand-worked test cases, such as the ENERGY STAR chart at each boundary and the friction equation against the Engineering ToolBox head-loss table.
- Property tests check that more area never gives a smaller unit, that feet and inches agree, and that duct size never shrinks as airflow grows.
- Each tool is compared with at least two outside sources on at least three inputs before release. The results are written up with the differences explained.
- The release script refuses to publish if a test fails.
Found a wrong number?
Email [email protected] with the page, your inputs and what you expected. If we confirm the error, we fix it, add your case to the test suite and note the change on the page with a new date.
Last full source check: November 14, 2026.