I have an outside-air line hooked into the cold air returns of my furnaces. I also live in an area where we can burn leaves. Is there a way to add a filter (HEPA filter?) to this fresh air line to eliminate the furnace from filling the house with smoke? One of the furnaces is in the attic, so a manual damper would be a no-go, plus I would like to keep the positive pressure to keep the smoke from seeping in. Any ideas?