Enclosing utility room (furnace / water heater) basement
You really do need the fresh air make up when venting to another room. When in training the reason for the 1 vent up high and 1 low was to circulate the the air in the room. So if you have the total square inches needed for the open combustion systems it would be safe but just into another room that may not have access to the same area that this room has may be un-safe