Here are some of the reasons a home might feel stuffy or humid:
The humid feeling in your home could be because of changes not related to your HVAC system, including a plumbing leak or water infiltration from outside due to home damage like a leaky roof or rotting windows. In these cases, solving the water infiltration will solve the problem, then the HVAC system can catch up. The system may need cleaning to remove mildew and other contaminants caused by the damp conditions, however you must understand that your home’s HVAC system isn’t a powerful enough dehumidifier for all situations and can’t always keep up with especially high levels of humidity in the air. You may need to consider adding a separate dehumidifier, air filtration system or other components to your system if the problem persists.