Blueair Blue Pure 511i Max vs KNKA APH4000

The Blueair Blue Pure 511i Max is the better air purifier overall, rated 3.9 to 3.4. The Blueair Blue Pure 511i Max leads on filter cost, efficiency and auto mode. The KNKA APH4000 wins on value and warranty. The KNKA APH4000 costs $13 less ($99.99 vs $112.99).

Prices were last checked October 22, 2026 and are subject to change; the price and availability shown by the retailer at the time of purchase apply.

Which should you buy?

• Choose the Blueair Blue Pure 511i Max for filter cost ($59.98 a year in filters vs $143.96 a year in filters), efficiency (7.9 cfm per watt vs 4.3 cfm per watt) and auto mode (particle sensor and Wi-Fi app vs air quality (type not stated) sensor, auto mode).
• Choose the KNKA APH4000 for value ($0.44 per cfm of smoke CADR vs $0.74 per cfm of smoke CADR) and warranty (2-year warranty vs 1-year warranty).

Scores side by side

Each factor is scored 0-100 against the same fixed reference points for every medium rooms air purifier; a clear win is highlighted. How we rank explains each one.

Specifications

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