The right size is determined through a professional load calculation that takes into account your home's square footage, insulation quality, window efficiency, and local climate conditions, rather than just the overall house size.

You should consider replacement if you're experiencing frequent repairs, your system has reached 15-20 years of age, or if repair costs exceed 30% of the price of a new system.

Typically every 1-3 months, though homes with pets or family members with allergies should replace filters monthly.