UpStart Battery 4-Pack Replacement Honeywell HCM-535-20 Humidifier Filter - Compatible Honeywell HAC-504, HAC-504AW Air Filter Replacement Honeywell HCM-535-20 Humidifier Filter - Quantity, 4 Replaces Honeywell HAC-504, HAC-504AW Air Filter Traps Water Impurities & Dissolved Solids Before They Become Airborne For best results, change air filter at least once per season or more often depending on air quality and use. Essential for Allergy Sufferers. On Sale for a Limited Time!