Air Purifier Sizing & Placement
An undersized purifier in a corner is expensive white noise. Correct sizing, placement, and filter maintenance determine whether you actually breathe cleaner air.
CADR formula
Assumes 8-ft ceilings, delivers ~4.8 ACH. CADR 200 covers 310 sq ft. CADR 300 covers 465 sq ft.
Oversize by 20-30%. A larger unit on medium speed is quieter and still effective versus a correctly-sized unit on high constantly.
Placement
Center when possible. Against a wall blocks intake/exhaust, reducing effectiveness 20-50%.
Minimum 3 ft from walls on the intake side if center is not practical.
Elevated (2-3 ft) improves circulation versus floor placement.
Near the pollution source. Kitchen for cooking fumes, bedroom for allergens.
Away from open windows/doors. Cross-drafts push dirty air past before cleaning.
Air changes per hour (ACH)
2 ACH: minimum for general improvement.
4-5 ACH: recommended for allergy sufferers.
6+ ACH: medical grade. Severe asthma or wildfire smoke events.
Fan speed strategy
High: during/after cooking, cleaning, or outdoor AQI spikes. Run 30-60 min to drop particulates, then switch to medium.
Low/medium: continuous maintenance. Oversized unit on low still delivers 3+ ACH at a whisper.
Filter replacement
HEPA: every 6-12 months. Pets/wildfire shorten to 4-6 months.
Carbon: every 3-6 months. Saturated carbon off-gasses captured VOCs.
Pre-filter: vacuum/wash monthly. Extends HEPA life significantly.