Humidity & Ventilation Load Calculator

ANA›Life Services Authority›National Calculator Authority›Humidity and Ventilation Load Calculator — Account for WV's variable humidity levels in HVAC system sizing

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Humidity & Ventilation Load Calculator

Account for West Virginia's variable humidity levels in HVAC system sizing. Calculates latent and sensible ventilation loads per ASHRAE 62.1 and Manual J principles.

### Space & Occupancy

Conditioned Floor Area (sq ft)

Number of Occupants

Average Ceiling Height (ft)

Building Type

Residential Office / Commercial Retail School / Classroom Restaurant

### WV Climate Conditions

WV Region

Charleston (Central) Huntington (SW) Morgantown (N Central) Elkins (Mountain) Bluefield (Southern)

Design Season

Summer (Cooling / Dehumidification) Winter (Heating / Humidification)

Outdoor Dry-Bulb Temp (°F)

Outdoor Relative Humidity (%)

### Indoor Design Conditions

Indoor Dry-Bulb Temp (°F)

Indoor Relative Humidity (%)

### Ventilation Parameters

Air Changes per Hour (ACH)

Ventilation Method

Natural Infiltration Mechanical Ventilation (ERV/HRV) Exhaust-Only

ERV/HRV Sensible Effectiveness (%) (if mechanical)

ERV Latent Effectiveness (%) (if ERV)

Calculate Ventilation & Humidity Loads

### Results

Ventilation Airflow Required — CFM

Outdoor Humidity Ratio — gr/lb

Indoor Humidity Ratio — gr/lb

Humidity Ratio Difference — gr/lb

Latent Ventilation Load — BTU/hr

Sensible Ventilation Load — BTU/hr

Total Ventilation Load — BTU/hr

Total Load (Tons) — Tons

Moisture Removal Rate — pints/day

Outdoor Enthalpy — BTU/lb

Indoor Enthalpy — BTU/lb

Sensible Heat Ratio —

ERV/HRV Energy Recovery Savings

Sensible Saved— Latent Saved— Net Load After Recovery—

// Pre-fill outdoor conditions based on WV region + season document.getElementById('hum-wv-region').addEventListener('change', humPrefill); document.getElementById('hum-season').addEventListener('change', humPrefill);

// WV design conditions: [summer_db, summer_rh, winter_db, winter_rh] const WV_REGIONS = { charleston: { name:"Charleston", summer:[91,72], winter:[14,60] }, huntington: { name:"Huntington", summer:[92,70], winter:[16,58] }, morgantown: { name:"Morgantown", summer:[89,68], winter:[10,62] }, elkins: { name:"Elkins", summer:[85,74], winter:[5, 65] }, bluefield: { name:"Bluefield", summer:[87,71], winter:[12,60] } };

// ASHRAE 62.1 ventilation rates [cfm/person, cfm/sqft] const VENT_RATES = { residential: [7.5, 0.01], office: [5.0, 0.06], retail: [7.5, 0.12], school: [10.0,0.12], restaurant: [7.5, 0.18] };

function humPrefill() { const region = document.getElementById('hum-wv-region').value; const season = document.getElementById('hum-season').value; const data = WV_REGIONS[region]; if (!data) return; const [db, rh] = season === 'summer' ? data.summer : data.winter; document.getElementById('hum-outdoor-db').value = db; document.getElementById('hum-outdoor-rh').value = rh; // Flip indoor setpoint for winter document.getElementById('hum-indoor-db').value = season === 'summer' ? 75 : 70; document.getElementById('hum-indoor-rh').value = season === 'summer' ? 50 : 35; }

// Saturation pressure of water vapor (psia) using Antoine-like equation // Valid -40°F to 200°F function satPressure(tF) { const tC = (tF - 32) / 1.8; // Magnus formula in kPa, convert to psia const pKpa = 0.6108 * Math.exp((17.27 * tC) / (tC + 237.3)); return pKpa * 0.145038; // kPa to psia }

// Humidity ratio W (lb water / lb dry air) from dry-bulb (°F) and RH (%) // W = 0.62198 * (phi * Pws) / (P - phi * Pws) // P = 14.696 psia (sea level; WV avg ~14.0 psia at ~1000 ft elevation) function humidityRatio(tF, rhPct, elevFt) { const P = 14.696 * Math.pow(1 - 6.8754e-6 * elevFt, 5.2559); // altitude correction const phi = rhPct / 100; const Pws = satPressure(tF); const Pw = phi * Pws; return 0.62198 * Pw / (P - Pw); // lb/lb }

// Enthalpy of moist air BTU/lb dry air // h = 0.240T + W(1061 + 0.444*T) function enthalpy(tF, W) { return 0.240 * tF + W * (1061 + 0.444 * tF); }

function clearErrors() { ['hum-floor-area','hum-occupants','hum-ceiling-height','hum-outdoor-db', 'hum-outdoor-rh','hum-indoor-db','hum-indoor-rh','hum-ach', 'hum-erv-efficiency','hum-erv-latent'].forEach(id => { const el = document.getElementById(id + '-err'); if (el) el.textContent = ''; }); }

function showError(id, msg) { const el = document.getElementById(id + '-err'); if (el) el.textContent = msg; }

function humCalc() { clearErrors(); let valid = true;

const floorArea = parseFloat(document.getElementById('hum-floor-area').value); const occupants = parseInt(document.getElementById('hum-occupants').value); const ceilHeight = parseFloat(document.getElementById('hum-ceiling-height').value); const buildingType = document.getElementById('hum-building-type').value; const region = document.getElementById('hum-wv-region').value; const season = document.getElementById('hum-season').value; const outDB = parseFloat(document.getElementById('hum-outdoor-db').value); const outRH = parseFloat(document.getElementById('hum-outdoor-rh').value); const inDB = parseFloat(document.getElementById('hum-indoor-db').value); const inRH = parseFloat(document.getElementById('hum-indoor-rh').value); const ach = parseFloat(document.getElementById('hum-ach').value); const ventMethod = document.getElementById('hum-vent-method').value; const ervSens = parseFloat(document.getElementById('hum-erv-efficiency').value) / 100; const ervLat = parseFloat(document.getElementById('hum-erv-latent').value) / 100;

if (isNaN(floorArea) || floorArea 100000) { showError('hum-floor-area', 'Enter 100–100,000 sq ft.'); valid = false; } if (isNaN(occupants) || occupants 500) { showError('hum-occupants', 'Enter 1–500 occupants.'); valid = false; } if (isNaN(ceilHeight) || ceilHeight 30) { showError('hum-ceiling-height', 'Enter 7–30 ft.'); valid = false; } if (isNaN(outDB) || outDB 110) { showError('hum-outdoor-db', 'Enter -20 to 110 °F.'); valid = false; } if (isNaN(outRH) || outRH 100) { showError('hum-outdoor-rh', 'Enter 1–100%.'); valid = false; } if (isNaN(inDB) || inDB 85) { showError('hum-indoor-db', 'Enter 60–85 °F.'); valid = false; } if (isNaN(inRH) || inRH 65) { showError('hum-indoor-rh', 'Enter 20–65%.'); valid = false; } if (isNaN(ach) || ach 20) { showError('hum-ach', 'Enter 0.1–20 ACH.'); valid = false; } if (isNaN(ervSens) || ervSens 0.95) { showError('hum-erv-efficiency', 'Enter 0–95%.'); valid = false; } if (isNaN(ervLat) || ervLat 0.85) { showError('hum-erv-latent', 'Enter 0–85%.'); valid = false; } if (!valid) return;

// WV average elevation by region (ft) for altitude correction const ELEV = { charleston:600, huntington:565, morgantown:975, elkins:1994, bluefield:2611 }; const elevFt = ELEV[region] || 1000;

// ── ASHRAE 62.1 Ventilation Rate Procedure ────────────────────────────── const [cfmPerson, cfmSqft] = VENT_RATES[buildingType]; const ventCFM_62 = occupants * cfmPerson + floorArea * cfmSqft;

// ── Infiltration-based airflow ─────────────────────────────────────────── const volumeCuFt = floorArea * ceilHeight; const infiltCFM = (ach * volumeCuFt) / 60;

// Use the larger of ASHRAE 62.1 and infiltration const totalCFM = Math.max(ventCFM_62, infiltCFM);

// ── Psychrometric calculations ─────────────────────────────────────────── const Wout = humidityRatio(outDB, outRH, elevFt); // lb/lb const Win = humidityRatio(inDB, inRH, elevFt); // lb/lb const Hout = enthalpy(outDB, Wout); // BTU/lb const Hin = enthalpy(inDB, Win); // BTU/lb

// Effective outdoor conditions after ERV/HRV recovery let effOutDB = outDB; let effWout = Wout; let ervSensSaved = 0, ervLatSaved = 0;

if (ventMethod === 'mechanical') { // Sensible recovery: T_supply = T_out + eff_s(T_in - T_out) effOutDB = outDB + ervSens * (inDB - outDB); // Latent recovery: W_supply = W_out + eff_l(W_in - W_out) effWout = Wout + ervLat * (Win - Wout); }

const effHout = enthalpy(effOutDB, effWout);

// ── Air density at altitude ────────────────────────────────────────────── // rho = 0.075 * (P/14.696) * (530 / (T+460)) lb/ft³ (approx) const P_site = 14.696 * Math.pow(1 - 6.8754e-6 * elevFt, 5.2559); const rhoOut = 0.075 * (P_site / 14.696) * (530 / (outDB + 460));

// Mass flow rate: lb/hr = CFM * 60 * rho const massFlow = totalCFM * 60 * rhoOut; // lb dry air / hr

// ── Sensible load: Q_s = 1.1 * CFM * ΔT (BTU/hr) ────────────────────── // Standard: 1.1 = 60 min/hr * 0.075 lb/ft³ * 0.240 BTU/lb·°F const deltaT_eff = effOutDB - inDB; const sensibleLoad = 1.1 * totalCFM * deltaT_eff; // BTU/hr (+ = cooling, - = heating)

// ── Latent load: Q_l = 0.68 * CFM * ΔW_grains ────────────────────────── // 0.68 = 60 * 0.075 * 1076 / 7000 (1076 BTU/lb latent heat, 7000 gr/lb) const deltaW_grains = (effWout - Win) * 7000; // grains/lb const latentLoad = 0.68 * totalCFM * deltaW_grains; // BTU/hr

// ── Total load ─────────────────────────────────────────────────────────── const totalLoad = sensibleLoad + latentLoad; // BTU/hr const totalTons = totalLoad / 12000;

// ── Moisture removal ───────────────────────────────────────────────────── // lbs water/hr = massFlow * |ΔW| const moistureLbHr = massFlow * Math.abs(effWout - Win); const moisturePintsDay = moistureLbHr * 24 / 0.1043; // 1 pint water ≈ 1.043 lb

// ── ERV savings ────────────────────────────────────────────────────────── if (ventMethod === 'mechanical') { ervSensSaved = 1.1 * totalCFM * Math.abs(outDB - effOutDB); ervLatSaved = 0.68 * totalCFM * Math.abs((Wout - effWout) * 7000); }

// ── SHR ────────────────────────────────────────────────────────────────── const absSens = Math.abs(sensibleLoad); const absTotal = Math.abs(totalLoad); const shr = absTotal > 0 ? absSens / absTotal : 1;

// ── Display ────────────────────────────────────────────────────────────── const fmt = (v, d=0) => isFinite(v) ? v.toFixed(d) : '—'; const fmtS = (v, d=0, unit='') => { const s = isFinite(v) ? Math.abs(v).toFixed(d) : '—'; const dir = v >= 0 ? (season==='summer'?'(cooling)':'(heating)') : (season==='summer'?'(heating)':'(cooling)'); return s + (unit?' '+unit:'') + ' ' + dir; };

document.getElementById('hum-cfm').textContent = fmt(totalCFM,1) + ' CFM'; document.getElementById('hum-out-w').textContent = fmt(Wout7000,1) + ' gr/lb'; document.getElementById('hum-in-w').textContent = fmt(Win7000,1) + ' gr/lb'; document.getElementById('hum-delta-w').textContent = fmt(Math.abs(deltaW_grains),1) + ' gr/lb'; document.getElementById('hum-latent').textContent = fmtS(latentLoad,0,'BTU/hr'); document.getElementById('hum-sensible').textContent = fmtS(sensibleLoad,0,'BTU/hr'); document.getElementById('hum-total').textContent = fmtS(totalLoad,0,'BTU/hr'); document.getElementById('hum-tons').textContent = fmt(Math.abs(totalTons),2) + ' Tons'; document.getElementById('hum-moisture').textContent = fmt(moisturePintsDay,1) + ' pints/day'; document.getElementById('hum-out-h').textContent = fmt(Hout,2) + ' BTU/lb'; document.getElementById('hum-in-h').textContent = fmt(Hin,2) + ' BTU/lb'; document.getElementById('hum-shr').textContent = fmt(shr,3);

const ervBlock = document.getElementById('hum-erv-savings-block'); if (ventMethod === 'mechanical') { ervBlock.style.display = 'block'; document.getElementById('hum-erv-sens-saved').textContent = fmt(ervSensSaved,0) + ' BTU/hr'; document.getElementById('hum-erv-lat-saved').textContent = fmt(ervLatSaved,0) + ' BTU/hr'; document.getElementById('hum-erv-net').textContent = fmt(Math.abs(totalLoad),0) + ' BTU/hr'; } else { ervBlock.style.display = 'none'; }

// ── Recommendations ────────────────────────────────────────────────────── const regionName = WV_REGIONS[region].name; let recs = [];

if (season === 'summer') { if (outRH > 70) recs.push(⚠️ ${regionName} summer RH of ${outRH}% is high — size dehumidification capacity generously. Latent loads often exceed sensible in WV summers.); if (shr 50) recs.push(💧 Moisture removal of ${fmt(moisturePintsDay,0)} pints/day is significant — verify condensate drain capacity.); if (totalTons > 0) recs.push(❄️ Size cooling equipment for at least ${fmt(Math.abs(totalTons),2)} tons of ventilation load (add envelope loads separately).); } else { if (inRH '' + r + '').join('') + '';

document.getElementById('hum-result').style.display = 'block'; }

#### Formulas Used

Humidity Ratio (W): W = 0.62198 × (φ × Pws) / (Psite − φ × Pws)  [lb water / lb dry air] where Pws = saturation pressure (Magnus formula), φ = relative humidity fraction, Psite = barometric pressure corrected for WV elevation.

Enthalpy of Moist Air: h = 0.240·T + W·(1061 + 0.444·T)  [BTU/lb dry air]

Sensible Ventilation Load: Qs = 1.1 × CFM × ΔT  [BTU/hr] (1.1 = 60 min/hr × 0.075 lb/ft³ × 0.240 BTU/lb·°F)

Latent Ventilation Load: Ql = 0.68 × CFM × ΔWgrains  [BTU/hr] (0.68 = 60 × 0.075 × 1076 / 7000; 1076 BTU/lb = latent heat of vaporization)

ASHRAE 62.1 Ventilation Rate: CFM = (Occupants × Rp) + (Floor Area × Ra)

ERV/HRV Recovery: Tsupply = Tout + εs·(Tin − Tout) Wsupply = Wout + εl·(Win − Wout)

Altitude Correction (WV elevations 565–4,863 ft): Psite = 14.696 × (1 − 6.8754×10⁻⁶ × elevation)5.2559 psia

#### Assumptions & References

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