I have installed two DIRECT VENT gas fireplaces, one upstairs and one downstairs in a new home. The upstairs fireplace runs fine. The fireplace downstairs has had some issues. When I light the fireplace it runs for a few seconds and then the flames ghost out and the unit shuts off. The only way I can keep the unit running is by opening a door to the outside about two inches. Is this caused by negative pressure in the newly constructed house?