Furnace Air Filters: How Often to Change and Why It Matters

Codes that a dirty filter can trigger

Code	Manual lists blocked filters or restricted airflow as a cause
Goodman EE3	Primary limit circuit open
Carrier 33 / 13	Limit circuit fault / lockout
Lennox red slow / green on	Primary or secondary limit open
Trane E04	Open limit

How often

• DOE: clean or replace once a month or as recommended.
• ENERGY STAR: inspect, clean or change monthly; a dirty filter can increase energy costs and damage equipment.
• Check more often if you have pets, construction dust or run the system constantly.

MERV ratings

MERV (Minimum Efficiency Reporting Value) rates how well a filter captures particles; EPA explains that higher MERV filters capture more small particles. Follow your furnace manual for allowed filter sizes and types.

Changing the filter

1. Turn the thermostat off or the furnace switch off.
2. Note the size and airflow arrow on the old filter.
3. Insert the new filter with the arrow pointing toward the furnace.

Safety first: If you smell gas, don’t touch light switches or use a phone inside. Leave the building and call your gas utility from outside. Turn off power to the furnace before opening any panel, and never bypass or jumper a safety switch. Gas, venting, wiring and control-board work is for a licensed HVAC technician.