Calculate HVAC Load

Methodology

The core load and airflow formulas, calculator-specific assumptions, and limits of the site's planning estimates.

What we use

  • ACCA Manual J 8th Edition heat-transfer relationships for residential cooling and heating loads
  • ACCA Manual N as a reference for commercial load calculations; our commercial tool uses area-based presets rather than the complete Manual N procedure
  • ASHRAE 62.1 outdoor air ventilation rates (Rp + Ra) for commercial occupancies
  • Representative design temperatures assigned to IECC climate zones (1–8), not site-specific ASHRAE station lookups
  • Standard residential air constants: 1.08 sensible air constant at standard sea-level conditions, 12,000 BTU/hr per ton, and selectable 400 or 350 CFM/ton planning assumptions. Equipment efficiency alone does not determine required airflow.

Last updated October 13, 2026. Danish Javaid maintains this methodology. This site uses simplified implementations inspired by these standards; it does not reproduce or certify compliance with the complete standards.

Core conversions and their limits

  1. Load → tonnage: tons = BTU/hr ÷ 12,000. The shared conversion reports exact tons to two decimals and a nominal size rounded to the nearest 0.5 ton unless that would fall below 95% of the estimated load, in which case it rounds up to the next half ton. This is not final equipment selection under Manual S.
  2. Tonnage → airflow: CFM = tons × the selected 400 or 350 CFM/ton assumption. Check manufacturer airflow requirements.
  3. Sensible load → airflow: CFM = sensible BTU/h ÷ (1.08 × ΔT), with ΔT in °F and greater than zero. At 21,600 BTU/h and a 20°F difference, airflow is 1,000 CFM. This excludes latent load.
  4. Air changes → airflow: CFM = volume in ft³ × ACH ÷ 60. ACH-based circulation is not automatically a fresh-air requirement.

The duct tool calculates straight galvanized-duct friction using Darcy–Weisbach with a Haaland turbulent-friction approximation at standard air conditions. It selects the first stock diameter meeting both a 0.08 in. w.g. per 100 ft friction target and a 900 FPM velocity limit. Velocity is CFM ÷ circular area in ft². Rectangular alternatives use equivalent round diameter = 1.30 × (width × height)^0.625 ÷ (width + height)^0.25, with dimensions in inches, and a velocity check. A full duct design needs actual available static pressure, fittings, equivalent lengths, and material properties.

Furnace input capacity requires an efficiency adjustment: input BTU/h ≈ required heating output ÷ (AFUE percent ÷ 100). The dedicated furnace calculator uses this relationship and selects from its standard input sizes between 30,000 and 120,000 BTU/h, accepting a size only when it is at least the required input and no more than 140% of that input. It reports no matching size when none qualifies. The homepage instead multiplies its estimated heating load by 1.15 and rounds up to 5,000 BTU/h; that display does not include AFUE and should not be treated as an input-rating calculation.

Climate zone resolution

US ZIP codes resolve to an IECC climate zone via a 3-digit prefix lookup table, with first-digit fallback where a prefix is not mapped. Prefixes can span climates, counties, and elevations; this is approximate. The resolved zone sets the cooling and heating BTU/sq ft baseline rates and the design temperatures used for ΔT calculations.

Envelope adjusters

From the climate-zone baseline, results are adjusted by:

  • Insulation level (poor 1.25× / average 1.0× / good 0.85× / excellent 0.7×)
  • Sun exposure (shaded 0.9× / average 1.0× / sunny 1.1× / very sunny 1.2×) — cooling only
  • Ceiling height (max(1, height ÷ 8 ft); ceilings below 8 ft do not reduce this factor)
  • Home type (single-story 1.0× / two-story 0.95× / apartment 0.85×)
  • Window count (above the typical 1 window per 100 sqft baseline)
  • Occupants (each occupant beyond the 2-person baseline adds 600 BTU/hr as a combined cooling allowance)

Room-by-room sensible and latent estimates

The simplified Manual J tool calculates room total cooling load from area, zone cooling rate, insulation, sun, ceiling, wall, and window factors. Each exterior wall beyond one adds 8%; each window beyond one adds 2%. Each occupant beyond two adds 600 BTU/h to total cooling load. Latent load equals 30% of total cooling load in zones 1–3 or 20% elsewhere; sensible load is the remainder. These are heuristic ratios, not humidity-based calculations; an IECC zone number alone does not identify humidity. Room airflow uses sensible load ÷ (1.08 × 20°F). Heating uses the area, zone heating rate, insulation, ceiling, and wall factors only. The homepage uses a combined cooling estimate without a separate latent calculation and adds 600 BTU/h per occupant beyond two before applying home-type and window multipliers.

Commercial loads

The commercial calculator adds three components residential calculators ignore:

  • Lighting load: watts/sq ft × floor area in sq ft × 3.412 BTU/h per watt
  • Plug / equipment load: same conversion at building-type-specific power densities
  • Outdoor air load: ASHRAE 62.1 Vbz = Rp × occupants + Ra × area, then 1.08 × OA × ΔT for the sensible component plus a climate-tuned latent component

Commercial people gains are 250 BTU/h sensible and 200 BTU/h latent per occupant. Outdoor-air latent load uses 4.5 BTU/h per CFM in zones 1–3 or 3.0 elsewhere, rather than measured humidity. Cooling ventilation ΔT is max(0, zone cooling design °F − 75°F); heating ventilation ΔT is max(0, 70°F − zone heating design °F). Supply airflow is the greater of sensible load ÷ (1.08 × 20°F) and outdoor airflow. Commercial heating subtracts a 25% credit for lighting, equipment, and sensible people gains, with a minimum of zero. The outdoor-air formula is a breathing-zone estimate; it omits full system ventilation-effectiveness calculations.

Heating and residential ventilation

The heating-load tool adds an area-based envelope estimate to infiltration heat loss. Envelope BTU/h = area × zone heating rate × insulation factor × max(1, ceiling height ÷ 8) × ΔT ÷ (70°F − zone heating design °F), where ΔT = max(0, indoor °F − zone heating design °F). Infiltration BTU/h = 1.08 × (volume × ACH ÷ 60) × ΔT. Changing indoor temperature scales both terms, but the envelope estimate still uses a zone rate rather than measured surface U-values and areas.

The residential ventilation tool uses CFM = 0.03 × floor area in sq ft + 7.5 × (bedrooms + 1), rounded to a whole CFM. This simplified ASHRAE 62.2 estimate omits infiltration credits and other compliance adjustments. Actual required outdoor airflow must be checked for the applicable edition and local requirements.

Where we're less precise than full Manual J

  • We don't take a glazing schedule (orientation × area × U + SHGC). Solar gain is approximated by a single "sun exposure" multiplier.
  • The heating-load tool uses an assumed natural ACH. The homepage and simplified room-by-room tool do not explicitly calculate infiltration. Blower-door ACH50 is not interchangeable with natural ACH.
  • We don't account for duct losses to unconditioned spaces, so actual duct conditions require a separate assessment.
  • We don't do orientation-specific solar gain (south + west glass dominate cooling load in real Manual J).

When to upgrade to a full Manual J

For permits, rebates, code compliance, or unusual buildings (cathedral ceilings, lots of glass, large additions, mixed-use, historic retrofits), obtain a full calculation from a qualified professional. Check software capabilities and applicable requirements before choosing a tool. Examples of load-calculation software include:

  • Wrightsoft Right-J — desktop, professional Manual J
  • Cool Calc — web, draws floor plan first, fast mobile entry
  • HVAC-Calc — desktop, residential-focused
  • Elite Software — desktop, commercial Manual N and beyond

Disclaimer

Estimates only. Consult a licensed HVAC contractor or mechanical engineer for final system design and equipment selection.