Envirosept 2424 Furnace Filter Disposable Pads, Box of 12 12 EnviroSept air filter collector pads for Envirosept Furnace Panel HVAC air purifier model 2424 Panel NOT included . Exact size is 22 inches by 21 inches. The electronic air cleaner panel is 23.5 x 23.5 x 1 inch. Unlock the air filter panel after you've unplugged it and removed it from the furnace filter slot. Replace both filter pads. Two replacement pads are needed for each filter change unless you remove the dirtier pad and replace it with the cleaner one.Replace filter pads monthly or more often if your home or business requires it. Note Measure the panel insert including the black plastic piece to get accurate dimensions of your Envirosept insert. Replacement furnace filter pads cover only the inside mesh part of the furnace air cleaner panel. Replacement furnace filters are made from a custom polywool blend of proprietary fibers without latex or adhesives. We suggest a label on your furnace to remind you of the size. To Replace Furnace Filters Unplug the unit. Remove furnace filter panel from the slot. Open the two retaining clips. Remove both filters. If desired, move the less dirty filter to where the dirtier filter was. Add second filter. Push the clips closed, slide panel into slot and plug in. This item ships from a separate warehouse and may require extra time for delivery.