Filter Replacement Interval Calculator
ANA›Life Services Authority›National Calculator Authority›Filter Replacement Interval Calculator
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Filter Replacement Interval Calculator
Estimate how often you should replace your HVAC filter based on filter type, home size, occupancy, pets, and local air quality conditions.
Filter Type (MERV Rating)
MERV 1–4 (Fiberglass / Basic Panel) – 1 month base MERV 5–8 (Pleated) – 3 month base MERV 9–12 (High-Efficiency Pleated) – 6 month base MERV 13–16 (Hospital-Grade / HEPA-style) – 12 month base
Home Size (sq ft)
Number of Occupants
Number of Pets (dogs/cats)
Allergy / Asthma Sufferers in Home?
No Yes
Outdoor Air Quality
Good (rural / low pollution) Moderate (suburban) Poor (urban / high traffic / wildfire-prone)
Average Daily HVAC Runtime (hours/day)
Calculate Replacement Interval
function filCalc() { // --- Read inputs --- var filterType = parseInt(document.getElementById('fil-filter-type').value); var homeSize = parseFloat(document.getElementById('fil-home-size').value); var occupants = parseFloat(document.getElementById('fil-occupants').value); var pets = parseFloat(document.getElementById('fil-pets').value); var allergy = parseInt(document.getElementById('fil-allergy').value); var airQuality = parseInt(document.getElementById('fil-air-quality').value); var runtime = parseFloat(document.getElementById('fil-runtime').value);
var resultDiv = document.getElementById('fil-result');
// --- Validation --- var errors = []; if (isNaN(homeSize) || homeSize 10000) errors.push("Home size must be between 200 and 10,000 sq ft."); if (isNaN(occupants) || occupants 20) errors.push("Occupants must be between 0 and 20."); if (isNaN(pets) || pets 10) errors.push("Pets must be between 0 and 10."); if (isNaN(runtime) || runtime 24) errors.push("Daily runtime must be between 1 and 24 hours.");
if (errors.length > 0) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }
// --- Base interval by filter type (days) --- // MERV 1-4: 30 days, MERV 5-8: 90 days, MERV 9-12: 180 days, MERV 13-16: 365 days var baseIntervalDays; var filterLabel; if (filterType === 1) { baseIntervalDays = 30; filterLabel = "MERV 1–4 (Fiberglass)"; } else if (filterType === 2) { baseIntervalDays = 90; filterLabel = "MERV 5–8 (Pleated)"; } else if (filterType === 3) { baseIntervalDays = 180; filterLabel = "MERV 9–12 (High-Efficiency Pleated)"; } else { baseIntervalDays = 365; filterLabel = "MERV 13–16 (Hospital-Grade)"; }
// --- Factor: Home Size --- // Larger homes circulate more air volume; smaller homes concentrate particles faster. // Reference size: 2,000 sq ft. Factor = sqrt(2000 / homeSize) clamped [0.5, 1.5] var sizeFactor = Math.sqrt(2000 / homeSize); sizeFactor = Math.min(Math.max(sizeFactor, 0.5), 1.5);
// --- Factor: Occupants --- // Each person beyond 1 adds ~5% load. Base = 1 occupant. // Factor = 1 / (1 + 0.05 * max(occupants - 1, 0)) var occupantFactor = 1 / (1 + 0.05 * Math.max(occupants - 1, 0));
// --- Factor: Pets --- // Each pet adds ~15% additional particulate load. // Factor = 1 / (1 + 0.15 * pets) var petFactor = 1 / (1 + 0.15 * pets);
// --- Factor: Allergy / Asthma --- // Recommended to replace 50% more frequently for sensitive occupants. // Factor = 0.5 if yes, 1.0 if no var allergyFactor = (allergy === 1) ? 0.5 : 1.0;
// --- Factor: Outdoor Air Quality --- // Good = 1.0, Moderate = 0.75, Poor = 0.5 var airFactor; var airLabel; if (airQuality === 1) { airFactor = 1.00; airLabel = "Good"; } else if (airQuality === 2) { airFactor = 0.75; airLabel = "Moderate"; } else { airFactor = 0.50; airLabel = "Poor"; }
// --- Factor: Runtime --- // Standard runtime assumed 8 hrs/day. More runtime = faster clogging. // Factor = 8 / runtime (clamped to reasonable range) var runtimeFactor = 8 / runtime; runtimeFactor = Math.min(Math.max(runtimeFactor, 0.33), 3.0);
// --- Adjusted Interval --- // AdjustedDays = BaseInterval × sizeFactor × occupantFactor × petFactor // × allergyFactor × airFactor × runtimeFactor var adjustedDays = baseIntervalDays * sizeFactor * occupantFactor * petFactor * allergyFactor * airFactor * runtimeFactor;
// Clamp to sensible bounds: minimum 7 days, maximum 365 days adjustedDays = Math.min(Math.max(adjustedDays, 7), 365);
// --- Convert to human-readable --- var weeks = adjustedDays / 7; var months = adjustedDays / 30.44;
var intervalText; if (adjustedDays ' + 'Filter Type' + filterLabel + '' + 'Base Interval' + baseIntervalDays + ' days' + 'Adjusted Replacement Interval' + intervalText + '' + 'Estimated Filters / Year' + filtersPerYear.toFixed(1) + '' + 'Size Factor' + sizeFactor.toFixed(3) + ' (home: ' + homeSize + ' sq ft)' + 'Occupant Factor' + occupantFactor.toFixed(3) + ' (' + occupants + ' occupants)' + 'Pet Factor' + petFactor.toFixed(3) + ' (' + pets + ' pets)' + 'Allergy Factor' + allergyFactor.toFixed(2) + (allergy ? ' (sensitive occupants)' : ' (none)') + '' + 'Air Quality Factor' + airFactor.toFixed(2) + ' (' + airLabel + ')' + 'Runtime Factor' + runtimeFactor.toFixed(3) + ' (' + runtime + ' hrs/day)' + '' + '' + urgency + '
'; }
#### Formula
Adjusted Interval (days) = Base Interval × Size Factor = √(2000 / HomeSizeSqFt) [clamped 0.5–1.5] × Occupant Factor = 1 / (1 + 0.05 × max(Occupants − 1, 0)) × Pet Factor = 1 / (1 + 0.15 × Pets) × Allergy Factor = 0.5 if yes, 1.0 if no × Air Quality Factor = 1.0 (Good) | 0.75 (Moderate) | 0.50 (Poor) × Runtime Factor = 8 / DailyRuntimeHours [clamped 0.33–3.0]
Base Intervals by MERV Rating: MERV 1–4: 30 days | MERV 5–8: 90 days | MERV 9–12: 180 days | MERV 13–16: 365 days
Filters per Year = 365 / Adjusted Interval
Final interval is clamped between 7 days (minimum) and 365 days (maximum).
#### Assumptions & References
- Reference home size is 2,000 sq ft; the square-root scaling reflects that air volume (and filter load) scales sub-linearly with floor area.
- Each additional occupant beyond one is assumed to increase particulate load by 5% (skin cells, activity, cooking).
- Each dog or cat is assumed to increase particulate load by 15% per pet (dander, fur, tracked debris).
- Allergy/asthma households are advised to replace filters 50% more frequently per ACAAI recommendations.
- Standard HVAC runtime is assumed to be 8 hours/day; runtime factor scales linearly with actual usage.
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