Dehumidifier Capacity Sizing Calculator

ANA›Life Services Authority›National Calculator Authority›Dehumidifier Capacity Sizing Calculator

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Dehumidifier Capacity Sizing Calculator

Estimate the dehumidifier capacity (in pints per day) needed for your space based on room dimensions, current humidity level, and environmental conditions.

Room Length (ft)

Room Width (ft)

Ceiling Height (ft)

Current Relative Humidity (%)

Target Relative Humidity (%)

Room Temperature (°F)

Moisture Condition

Moderately Damp – slight musty odor, damp feeling Very Damp – musty odor, damp spots on walls/floors Wet – seepage, wet walls/floors Extremely Wet – standing water, flooding

Number of Windows / Doors

Number of Occupants

Calculate Capacity

function dehCalc() { const errEl = document.getElementById('deh-error'); const resEl = document.getElementById('deh-result'); errEl.style.display = 'none'; resEl.style.display = 'none';

// --- Gather inputs --- const length = parseFloat(document.getElementById('deh-length').value); const width = parseFloat(document.getElementById('deh-width').value); const height = parseFloat(document.getElementById('deh-height').value); const rhCurrent = parseFloat(document.getElementById('deh-humidity').value); const rhTarget = parseFloat(document.getElementById('deh-target').value); const tempF = parseFloat(document.getElementById('deh-temp').value); const condFactor = parseFloat(document.getElementById('deh-condition').value); const windows = parseFloat(document.getElementById('deh-windows').value); const occupants = parseFloat(document.getElementById('deh-occupants').value);

// --- Validation --- const errors = []; if (isNaN(length) || length 30) errors.push("Ceiling height must be between 4 and 30 ft."); if (isNaN(rhCurrent) || rhCurrent 100) errors.push("Current humidity must be between 30% and 100%."); if (isNaN(rhTarget) || rhTarget 80) errors.push("Target humidity must be between 30% and 80%."); if (!isNaN(rhCurrent) && !isNaN(rhTarget) && rhTarget >= rhCurrent) errors.push("Target humidity must be lower than current humidity."); if (isNaN(tempF) || tempF 120) errors.push("Temperature must be between 32°F and 120°F."); if (isNaN(windows) || windows 0) { errEl.innerHTML = errors.join(''); errEl.style.display = 'block'; return; }

// --------------------------------------------------------------- // FORMULA // --------------------------------------------------------------- // Step 1: Room volume (cubic feet) const volumeCuFt = length * width * height;

// Step 2: Saturation vapor pressure at temperature T (°F) // Magnus formula (Tetens): es(T_C) = 6.1078 * exp(17.27 * T_C / (T_C + 237.3)) [hPa] const tempC = (tempF - 32) * 5 / 9; const es_hPa = 6.1078 * Math.exp(17.27 * tempC / (tempC + 237.3));

// Step 3: Absolute humidity at current and target RH // AH (g/m³) = (RH/100) * es * 216.7 / (T_K) // where T_K = tempC + 273.15, 216.7 = Mw/R (g·K/J) const T_K = tempC + 273.15; const ahCurrent = (rhCurrent / 100) * es_hPa * 216.7 / T_K; // g/m³ const ahTarget = (rhTarget / 100) * es_hPa * 216.7 / T_K; // g/m³

// Step 4: Moisture to remove (g/m³) const deltaAH = ahCurrent - ahTarget; // g/m³

// Step 5: Convert room volume to m³ const volumeM3 = volumeCuFt * 0.0283168;

// Step 6: Total moisture to remove per day (grams) // Assume 12 air changes per day (ACH=0.5/hr typical residential) const ACH_day = 12; const moisturePerDay_g = deltaAH * volumeM3 * ACH_day;

// Step 7: Convert grams to pints (1 pint water ≈ 473.176 g) const GRAMS_PER_PINT = 473.176; let basePints = moisturePerDay_g / GRAMS_PER_PINT;

// Step 8: Apply condition severity multiplier basePints *= condFactor;

// Step 9: Add load adjustments // Each window/door adds ~0.5 pints/day (infiltration) // Each occupant adds ~0.5 pints/day (respiration/perspiration) const windowLoad = windows * 0.5; const occupantLoad = occupants * 0.5; const totalPints = basePints + windowLoad + occupantLoad;

// Step 10: Recommend standard capacity tier let recommended = ""; let tier = ""; if (totalPints Room Area ${(length * width).toFixed(1)} sq ft Room Volume ${volumeCuFt.toFixed(1)} cu ft (${volumeM3.toFixed(2)} m³) Saturation Vapor Pressure ${es_hPa.toFixed(2)} hPa at ${tempF}°F (${tempC.toFixed(1)}°C) Absolute Humidity (Current) ${ahCurrent.toFixed(2)} g/m³ at ${rhCurrent}% RH Absolute Humidity (Target) ${ahTarget.toFixed(2)} g/m³ at ${rhTarget}% RH Moisture Differential ${deltaAH.toFixed(2)} g/m³ Base Capacity (room only) ${basePints.toFixed(1)} pints/day Window/Door Load +${windowLoad.toFixed(1)} pints/day Occupant Load +${occupantLoad.toFixed(1)} pints/day Total Required Capacity ${totalPints.toFixed(1)} pints/day Recommended Unit Size ${recommended} ${tier} `; resEl.style.display = 'block'; }

#### Formulas Used

1. Saturation Vapor Pressure (Magnus / Tetens formula): es(T) = 6.1078 × exp(17.27 × TC / (TC + 237.3)) [hPa] where TC = (TF − 32) × 5/9

2. Absolute Humidity: AH = (RH / 100) × es × 216.7 / TK [g/m³] where TK = TC + 273.15, and 216.7 = Mw/R (g·K/J)

3. Moisture to Remove per Day: Moisture (g/day) = (AHcurrent − AHtarget) × Volume (m³) × ACHday ACHday = 12 (equivalent to 0.5 air changes per hour, typical residential)

4. Base Capacity (pints/day): Base = Moisture (g/day) / 473.176 × Condition Factor (1 US pint of water ≈ 473.176 g)

5. Total Capacity: Total = Base + (Windows × 0.5) + (Occupants × 0.5) [pints/day]

#### Assumptions & References

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