Why are there two air filter slots in my HVAC system?

A lot of HVAC air handlers do not have covers over the filter slot and this is costing homeowners

The FilterLock seals furnace and air conditioning air filter slots to reduce utility bills and improve indoor air quality The Filter Slot Cover eliminates

furnace filter slot FR20253-2SLOT BELVENT FILTER RACK 20 X 25 W 2 SLOT 2 FILTERS MFC2025 COVER FOR 3X20 FILTER RACK MAGNET MFC2025 COVER MFC2025

slot gacor 2024 terpercaya slot arrangement ; and said filter element being releasably secured furnace : b ) means within said furnace for holding molten glass ; c