I have a Empire CIBV-20-30 natural gas fireplace I installed in my home. It has 5.5 ft of vertical stack and 19 inches of horizontal stack going through the wall. I used the specified 4" bvent.
When it gets below 35 degrees outside the fire place will not vent. I have tried a horizontal cap, turning the pipe down 45 degrees, 90 degrees, and adding 2 ft of vertical outside. Nothing seems to work. It burns until the temp limit switch shuts it down and no venting happens. When its 40 degrees out or above it vents beautifully.