I have a Thermo Pride WC-27 Wood/Coal burning furnace in my basement and I am having a perplexing issue. In my den which is right above the fireplace, I have a vent that is beside my recliner. When the stove is not in use, I can feel cold drafts of air coming through the vent that correlate with the wind blowing outside. I know that air comes back down my chimney and flue but can't figure out how it is coming in to the vents. I thought that all the heat comes from convection and then then blower circulates the heat through the duct work throughout the house and back to the cold air return. Please help me figure out where this cold air is coming in from. Thanks!!