How to Size, Buy, and Replace Geothermal Furnace Filters Easily? A ground-source heat pump might draw energy from the earth, but the air you breathe still passes through a filter. Geothermal filters do the same job as ordinary furnace pads, as they capture dust before it reaches the coil. However,... How to Size, Buy, and Replace Geothermal Furnace Filters Easily? A ground-source heat pump might draw energy from the earth, but the air you breathe still passes through a filter. Geothermal filters do the same job as ordinary furnace pads, as they capture dust before it reaches the coil. However,...