Just wondering if anyone uses a special furnace filter for allergies? My gf gets pretty stuffed up sleeping, but its only when the furnace is on, we moved in last winter, shes been fine all summer, but the furnace was on again lately and its just started up again. She has a couple cats and is fine and the landlord upstairs smokes.. but it would seem as though its dust in the furnace or something triggering it, since the air exchanger in the summer never caused anything....