Central HVAC systems shall have air filters installed in which location?

• A. Upstream from any exchanger coil
• B. Downstream from the exchanger coil
• C. Inside the exchanger housing only
• D. At the exterior of the unit

Answer: (A)

Placing the air filter in the return air path before the heat exchanger coil ensures that all air entering the system passes through the filter. This protects the coil from dust and debris, keeps heat transfer efficient, and reduces energy use and maintenance. If the filter is placed after the coil, dirty air would reach the coil without being filtered, causing faster fouling, lower cooling or heating performance, and more frequent cleaning. Filters tucked inside the exchanger housing or located on the exterior of the unit don’t reliably protect the coil or the indoor air, and they’re harder to service or prone to exposure to outdoor elements.