I see a lot of questions online about filament drying, and wanted to clear up some common misconceptions. Most filament dryers work by heating the air, not by removing moisture. As the air heats up, its capacity to hold moisture increases, which causes the relative humidity (RH) to drop (even though the amount of moisture in the air remains constant). This creates a humidity gradient which encourages moisture to migrate out of the filament and into the surrounding air, effectively drying the filament out. You don’t need a constant flow of fresh air into the dryer. The air inside the dryer has the same moisture content as the room air (except for what’s added by the filament). Some ventilation is useful to allow the moisture to escape, but a few small holes or a cracked lid are usually enough to maintain low RH inside the dryer. I made the chart below to show the minimum RH values you can expect from your dryer when starting with room temp of 25 C and various indoor humidity levels. If your dryer bottoms out around 15–25% RH, that’s totally normal and the driest condition that heat alone will achieve for your filament.  TL;DR: Filament dryers work by heating the air to lower the RH, which helps moisture escape from the filament. They don’t need a constant flow of fresh air. RH values around 15-25% are about as low as you can expect to see when using heat alone to dry filament.