HVAC filter
The air filter that protects your heating/cooling system and indoor air.
Your furnace or air handler pulls air through a filter that catches dust before it reaches the equipment. A clogged filter chokes airflow, spikes energy bills, and can freeze coils or burn out the blower. Check it monthly and replace it every 1–3 months depending on type, pets, and allergies. It's the single highest-leverage maintenance habit there is.