We have had ventless for years. Like you, we have a flu and on the recommendation of the plumber who hooked up the fireplace, we leave it open slightly. The advantage of more heat is nice at the cost of looking slightly less realistic than vented logs. I have never encountered any issues with moisture. YMMV.
Previous: 1993 Prostar 205
Red 1998 Closed Bow Ski Boat, PCM GT40, 310 hp, Standard Gear Reduction, Acme 4 blade, Perfect Pass SG/Zbox.
Tyler Ski Club
Be kind. Have fun.