Alen Air Purifier Replacement Filters | Filterbuy
November 6, 2026
Every Alen air purifier takes a filter made for that model, so the right replacement depends on which BreatheSmart model you own and how much odor control your home needs. This guide matches each model to its filter and walks through replacing the one in a BreatheSmart 45i. Because Filterbuy makes its own HEPA, carbon, and pleated filters, we read purifier specs the way a filter maker does, with fit and depth first and media second.
TL;DR
Each Alen air purifier uses its own filter family, and the BreatheSmart 45i shares its B4 filter with the FLEX. Most families come in three tiers: Pure (HEPA only), Fresh (HEPA plus activated carbon), and Odor (HEPA plus a carbon and catalyst blend). Alen recommends a new 45i filter every 9 to 12 months at 24/7 use, and sooner with carbon tiers, smoke, or pets.
Key Takeaways
The Alen BreatheSmart 45i filter is the B4, shared with the FLEX and sold in Pure, Fresh, Odor, and VOC/Smoke versions.
Essential, Advanced, and Ultimate are Alen's newer names for its Pure, Fresh, and Odor filters.
By Filterbuy's math, a 45i carbon filter costs roughly twice as much per year as Pure at Alen's list prices.
When the air quality index (AQI) tops 100, Filterbuy's rule is an EPA-recommended MERV 13 HVAC filter plus HEPA air purifiers in each room.
Which Replacement Filter Does My Alen Air Purifier Use?
Alen gives each model its own filter family, such as B4 for the BreatheSmart 45i and FLEX or B7 for the 75i. A filter from one family will not fit another model, so check your unit's label or order history for the model name, then find it below.
Alen Air Purifier Filter Finder
Sizes (L x W x H) come from Alen's published filter dimension chart.
Filterbuy built this table from Alen's dimension chart, product pages, manuals, and guarantee terms, checked November 2026. Where Alen's pages disagree, we used the more conservative figure.
What Filter Does The Alen BreatheSmart 45i Use?
The Alen BreatheSmart 45i filter is the B4, the same filter the FLEX uses, at about 11 x 3 x 15 inches. It comes as Essential (Pure), Advanced (Fresh), Ultimate (Odor), or a VOC/Smoke version (part FL40-H), and Alen rates the Pure filter for 9 to 12 months at 24/7 use.
Alen rates the 45i at a clean air delivery rate (CADR) of 255 cubic feet per minute on its top fan speed. That rating assumes a fresh filter, and Alen's 45i pages list maximum coverage as both 1,600 and 1,900 square feet. Filterbuy sizes by the more conservative AHAM two-thirds rule below, and our guide shows how to size a purifier to its room.
Pure, Fresh, Or Odor: Which Alen Filter Tier Fits Your Home?
Pure is HEPA only and handles dust, pollen, and pet dander. Fresh adds activated carbon for cooking odors, smoke, and volatile organic compounds (VOCs), while Odor adds a carbon and catalyst blend for pets and lingering smells. All three tiers include HEPA media.
Alen 45i Filter Tiers: Fit, Life, And Yearly Cost
Life and list prices ($69, $89, $99) are from Alen's 45i pages, November 2026. Costs exclude Alen's 10% subscription discount.
Filterbuy's take: start with Pure unless odors are the real problem. Carbon tiers cost roughly twice as much per year (see the table), so buy gas control only when you need it. In wildfire season, pair the VOC/Smoke version with these steps for protecting your home from wildfire smoke. A sour smell usually means the carbon is spent, as our guide on why an air purifier smells explains. Our carbon vs. HEPA comparison shows what each layer captures.
How Often Should You Replace An Alen Air Purifier Filter?
At 24/7 use, Alen recommends a new filter every 6 to 9 months for the BreatheSmart 25i and 35i, 9 to 12 months for the 45i and FLEX, and 12 to 15 months for the 75i. Carbon tiers, smoke, pets, and heavy cooking all shorten those windows.
Alen's filter light tracks run time, particle levels, and fan speed, but Filterbuy trusts the calendar more, and so does Alen, which suggests replacing a filter once its lifespan passes even with a green light. Date the frame at install.
Weaker airflow and a stale smell are other clues. Watch for these signs your air purifier filter is clogged, or read our breakdown of how often to replace pre-filter, HEPA, and carbon filters.
How To Change An Alen 45i Or FLEX Filter And Reset The Light
To change a BreatheSmart 45i or FLEX filter, remove the magnetic front cover, pull the old filter out by its strings, slide the new one in with the mesh prefilter facing out, and hold FILTER for five seconds to reset the light.
Unplug the purifier and stand it upright.
Pull the front cover off at the intake vents to release its magnets.
Pull the used filter out by its pull strings and bag it.
Slide the new filter in with the pull strings and black mesh prefilter facing out.
Hook the cover's bottom tabs, then press the top until it snaps.
Power the unit on and hold FILTER for five seconds until the light turns green.
If the light will not reset, try these stuck filter light fixes, and use Filterbuy's air purifier filter direction guide to double-check orientation.
Can You Wash Or Reuse An Alen HEPA Filter?
No. Alen's manuals say its filters are not designed to be washed, cleaned, or reused. Water damages HEPA media and saturates carbon. Only the black mesh prefilter can be cleaned, and vacuuming it every month or two helps each filter reach its rated life. Filterbuy's guide explains how long HEPA air purifier filters last.
Are Compatible Filters A Good Choice For An Alen Air Purifier?
A compatible Alen filter can work well if it matches the original's exact dimensions, uses true HEPA media, and has a real carbon layer for odor tiers. The trade-off is warranty coverage, because Alen's Forever Guarantee for years two and three requires proof of Alen filter purchases.
Fit matters most. When Filterbuy makes carbon pre-filters for Honeywell purifiers, we cut each one to that model's exact dimensions instead of selling a roll. Air always takes the easiest path, and any gap around a filter becomes that path. Before buying a compatible filter, make sure:
The listing names your exact Alen model and filter family.
Length, width, and depth match your current filter, with no gaps once installed.
The efficiency claim names a particle size and percentage.
Odor tiers disclose the carbon layer, ideally by weight.
The Federal Trade Commission's consumer guide to how product warranties work explains the federal rules on required parts.
How An Alen Purifier Works Alongside Your HVAC Filter
An Alen air purifier cleans one room, while your HVAC filter cleans the air moving through the whole house every time the system runs. The U.S. Environmental Protection Agency (EPA) Guide to Air Cleaners in the Home advises replacing filters according to the manufacturer's recommendations and avoiding air cleaners that intentionally produce ozone.
When the air quality index (AQI) rises above 100, Filterbuy recommends two steps. If you have an HVAC system, switch to an EPA-recommended MERV 13 filter, and run HEPA air purifiers in each room, your Alen included. Homes without ductwork skip the first step and lean on room purifiers. Filterbuy makes MERV 13 air filters in hundreds of sizes, and our guide to the best MERV filter for wildfire smoke explains why, but check your HVAC system's manual before upgrading. A MERV 13 filter is not a HEPA filter, which is why the room purifiers matter, and this guide covers how long to run an air purifier in your bedroom.
An effective air purifier starts with a quality replacement filter. At Filterbuy, we believe cleaner air comes from more than just high-efficiency filter media. It takes a precise fit, dependable construction, and consistent filtration performance. Our goal is simple: to provide reliable replacement filters that help your air purifier perform at its best, so every breath at home feels a little fresher.
Supporting Statistics
HEPA Filtration Works In Real Homes
A randomized, single-blinded trial tested HEPA purifiers in 29 homes.
PM2.5 fell 78.8% in the room with the purifier.
PM2.5 fell 57.9% in a second room.
Source: Real-World Effectiveness of Portable Air Cleaners in Reducing Home Particulate Matter Concentrations
Most Of Your Air Is Indoor Air
Americans spend an average of 87% of their time in enclosed buildings.
At 24/7 use, Alen's 9–12 month 45i interval equals about 6,500–8,800 run hours.
Size By The Two-Thirds Rule
AHAM advises a CADR of at least two-thirds of the room's square footage.
Alen lists the 45i at 255 CFM.
That covers rooms up to about 380 square feet, well below Alen's coverage claims.
Takeaway: for larger open spaces, consider the 75i or a second unit.
Source: Air Filtration Standards
Frequently Asked Questions
Will A 45i Filter Fit The Alen BreatheSmart 75i?
No. The BreatheSmart 75i takes the larger B7 filter (15 x 3 x 20 inches), so a 45i filter would leave gaps.
How Do I Know When To Change My Alen Air Purifier Filter?
Replace the filter as soon as the light turns red, Alen's interval for your model passes, or airflow weakens, whichever happens first. Filterbuy's air purifier filter replacement schedule covers other brands.
How Much Do Alen 45i Filters Cost Per Year?
At Alen's November 2026 list prices and 24/7 use, a 45i Pure filter costs about $69 to $92 a year, Fresh about $134 to $178, and Odor about $149 to $198.
Where Can I Buy Alen Replacement Filters?
Genuine Alen filters are sold on Alen's website, where a filter subscription saves 10%, and through major retailers. Match your filter family first.
Does Using A Non-Alen Filter Affect The Alen Warranty?
It can. Alen's Forever Guarantee covers years two and three only with proof of Alen filter purchases within the last 12 months, or 15 months for the 75i.
Filterbuy's Growing Air Purifier Filter Line
Filterbuy does not carry Alen-compatible filters yet, but our air purifier filter line keeps growing, with factory-direct pricing behind every filter. Browse our air purifier replacement filters, read our guide to HEPA air purifier replacement filters, and pair your Alen with a whole-home HVAC filter, all with fast and free 2-day delivery to most US states.
Glossary
A family-owned air filter manufacturer that sells direct at filterbuy.com .
A U.S. maker of portable HEPA air purifiers, including the BreatheSmart line.
High-efficiency particulate air filter, dense pleated media that captures very fine particles.
A porous carbon layer that adsorbs odors and gases HEPA media misses.
Clean air delivery rate, the volume of filtered air a purifier delivers, rated for smoke, pollen, and dust.
Volatile organic compounds, gases released by household products such as paints and cleaners.
The Association of Home Appliance Manufacturers, which runs CADR verification for air cleaners.
Air quality index, the EPA's outdoor air scale, where readings above 100 are unhealthy for sensitive groups.
Minimum Efficiency Reporting Value, the HVAC filter efficiency scale.
Disclaimer: Filterbuy is not affiliated with, authorized by, or endorsed by Alen.