The answer to 'air filter blowing cold air' is that a clogged or dirty air filter can cause the HVAC system to blow cold air. When the air filter is blocked, it restricts airflow through the system, which can lead to the evaporator coil freezing up. This causes the cold air to blow out instead of warm air.
Some common causes of a clogged air filter include dust buildup, pet hair, and debris. Symptoms include the system blowing cold air, poor airflow, and the evaporator coil freezing. The solution is to replace the dirty air filter with a clean one. This is a simple DIY task, but if the problem persists, it's best to call an HVAC professional to inspect the system.