Dehumidifier sizing

Start with the moisture load

Central Florida suggestions: the 76°F outdoor dew point is rounded from Orlando’s 76.2°F historical 2% condition in the 2016 NREL study (ASHRAE 2013 weather data). The 2,000 sq. ft. home, four average occupants, 75°F / 50% RH target, 0.10 natural ACH and 20 operating hours are editable design examples, not measured regional averages. The 90 CFM example follows the 2013 ASHRAE 62.2 gross-flow equation for 2,000 sq. ft. and three bedrooms before any infiltration credit; it is not a current code determination. Height, leakage and other moisture suggestions change with the space served. Verify actual ventilation and remaining leakage so the same air is not counted twice.

The source estimate combines outdoor-air moisture with occupant and additional moisture sources. Outdoor air is compared with your indoor target; its dew point matters more than outdoor relative humidity alone. The known-load path uses your complete latent-load calculation instead.

For each outdoor-air source, latent load in Btu/h = 0.68 × CFM × positive humidity-ratio difference in grains/lb. Daily removal in pints = latent Btu/h × 24 ÷ 1,054; the ventilation input is a 24-hour average. Infiltration uses floor area × height × natural ACH ÷ 60. Occupant moisture uses 200 Btu/h per average occupant for 24 hours. For intermittent ventilation or occupancy, multiply the on-period flow or people by hours per day ÷ 24.

These are conventional sea-level, standard-air approximations; no high-altitude correction is applied. Outdoor design conditions are held for the relevant calculation hours rather than modeled hour by hour. Drier outdoor air contributes zero outdoor-air moisture load; it does not cancel internal moisture sources.

No A/C removal credit is assumed. Required 24-hour equipment capacity = net pints/day × 24 ÷ available operating hours/day. This operating allowance does not replace an equipment performance correction.

Outdoor dew point determines outdoor moisture content; outdoor dry-bulb temperature is not needed for this moisture-load calculation. Equipment airflow and duct static pressure affect installed performance and must be checked separately. This simplified form does not calculate mixed-air inlet conditions or automatically select equipment. The customary HVAC pints conversion differs by about 4.6% from a direct water-mass calculation.

Match capacity to operating conditions

The required removal rate is a capacity at the conditions you entered. Manufacturer ratings at 80°F / 60% RH (AHAM) or 73°F / 60% RH (whole-house DOE) do not establish capacity at 75°F / 50% RH. Portable DOE ratings use 65°F / 60% RH and should not be equated with whole-house ratings. Check the specific model’s performance data, duct resistance, operating limits, and ventilation configuration before selecting equipment.

Manufacturer comparison

A verified AprilAire example for an existing 2,000 sq. ft. Orlando home, slab foundation, three bedrooms, 75°F and 50% RH returned 61.95 PPD. Its E130 series reference was 84 PPD. This tool’s Central Florida home example uses 90 CFM of ventilation, 26.7 CFM of residual infiltration, a 76°F outdoor dew point and 18.2 PPD from occupants: 146.8 PPD before the 20-hour operating allowance, or 176.2 PPD of required capacity. These are different load assumptions. Identical floor area alone does not make an equivalent sizing case.

Aprilaire does not expose all climate and infiltration assumptions. Benchmark testing found behavior consistent with a lower area-based airflow term; that is an empirical observation, not a current ventilation requirement. If you already have a complete manufacturer or Manual J moisture load, use the known-load path and enter it once.

Account for the enclosure

Use attic and crawl-space estimates for controlled enclosures with a defined moisture load. Ground vapor, roof-deck drying, outdoor openings, and liquid-water problems need their own assessment. Square footage cannot capture those effects by itself.

This planning tool expands on manufacturer sizing approaches. It does not reproduce their proprietary selection methods, guarantee an equivalent model recommendation, replace a complete load calculation, or establish code compliance.

• NREL — Procedures for Calculating Residential Dehumidification Loads (2016): Orlando climate data in Table 5 and discussion of the 2% design condition. This tool uses a simplified daily moisture budget, not the report’s complete method.
• DOE / NREL — Impact of Mechanical Ventilation (2014): historical gross ventilation equation, section 1.4.2.7.
• Tim De Stasio / HVAC School — How to Properly Size a Dehumidifier: design moisture load and capacity at actual conditions.
• AprilAire — Dehumidifier Selection Assistant: manufacturer selection based on the space and building conditions.
• Santa Fe — Dehumidifier Sizing Calculator: manufacturer sizing reference; follow the selected model’s published performance data.
• Santa Fe — Ultra Series Performance Chart: published capacity at specified entering conditions and static pressure.
• Energy Vanguard — The Confusing World of Dehumidifier Capacity: why rating conditions and installed capacity differ.
• Energy Vanguard — Humidity in a Spray Foam Attic: enclosure moisture and air exchange.