Q: The dealer we talked to told us if we go with a vent-free system it is recommended that we leave a window open in the room? Is this true?

A: Your dealer is partially correct.  All gas burning appliances (vented and vent-free) need fresh air for proper combustion.  If your house is of very tight construction, then you will need to provide air into the room to replace that which is being consumed in the combustion process (the burning of the gas).  Vent-free gas logs and heaters are considered to be "supplemental" heaters, not your primary source of heat.  The longer the vent-free gas log or heater is operated, the more the need for fresh air.

On the other side of the issue is to consider whats left over after the gas is burned.  The vent-free standard limits the amount of gas that can be burned, and Chilbuster burners are very efficient, so the amount of combustion products is quite low compared to other gas appliances, but still, there are combustion products (water vapor, Carbon Dioxide and small amounts of Carbon Monoxide are the largest components).  You can either dilute them or vent them.  Fresh air will dilute them, and cracking your damper will vent-them (but will also send some of the heat out of the house and up the chimney).

Your nose and eyes can be you guide as to whether you need to open a window and/or crack the damper.  Since vent-free gas logs draw the air it needs for combustion from the room, any impurities in the air (like dust, cigarette smoke, pet hair, incense, scented candles, off-gassing from new carpet, paint or varnish, etc.) will be recycled through the flame and be sent back into the room as odors, which are most noticeable in the absence of fresh air.  Even without any of these influences, there is a natural odor of combustion, to which some people are more sensitive than others.  Water vapor condenses on cold surfaces (like windows and walls) when not diluted or vented (just like in your bathroom after taking a shower).  In all such cases, "dilution is the solution", and/or slightly opening the damper to vent the combustion products.

Slightly opening a window may provide enough fresh air.  If your home is very active, with people opening and closing the doors to the outside often, or if your home is older and drafty, then you may naturally have enough fresh air input.

In summary, vent-free gas log sets can be a very good source of heat for a room, but like all heaters, you need fresh air.  Slightly opening a window and/or slightingly opening the damper can be the solution to the need for fresh air or to reduce any odors from having a tight house or impurities in the air.