Teri, your H can do some calculations using the actual BTU of the actual fp… If our paper napkin math is correct, a typical fireplace (depending on its BTU) would be equivalent to evaporation of 3-5 pounds of water into the air per hour. In my neck of the woods, where folks freak out about mold, any extra moisture is undesirable. YMMV. We axed the idea of ventless and went with vented. Vented fp lose 20-25% heat compared to ventless, so for folks like us who sometimes have to use them for the whole house heat during windstorms and other power outages, the small tradeoff in efficiency was worth it. CO is easy to deal with by monitoring and proper maintenance. Extra moisture is theinevitable result of natural gas burn and can condense on the windows, ruin the wood, and lead to mold problems. It looks like our approximations were close:
http://www.ehow.com/info_12140163_amount-water-natural-gas-propane-ventless-heaters.html
Another thing to consider. Does the house have other gas appliances and therefor by code did it require a whole house vent? Ours is all gas (appliances where heating is involved except the ovens and D/W), and by codes it was required to have a whole house ventilation in place (it is run on a timer built into the furnace and is wired into the fan in the downstairs laundry room).
Anyway… Congrats!!! It sounds like an awesome place. One more piece of advice from these folks who have a 2-story high ceilings in the living room: a fan is really nice to have. Hot air rises to the top… It is a good idea to whirl it around. 