Installed Heat Commander with 26 feet of vertical double wall. There is a one inch diameter opening right under the door that lets in air to the front of firebox. When the stove is first lit, air flows into the stove but after a while it reverses and flows out! Called manufacturer and said I need a barometric dampener. Does not make sense since the dampener would only reduce draft. The stove burns well when hot, so there is negative pressure in the box to be able to draft. Thoughts?