So, I scored a 3000 BTU CartCat heater cleaning up at my dad's house. Is it safe to use in an enclosed area like a pop up? Among other things, the instructions say: 5. This appliance consumes air. When used in any inside area, provide a fresh air opening of at least 6 square inches (39 square centimeters). Do not use other burning appliances inside. 6. Never operate the heater while sleeping. With that in mind, I was wondering about preheating the p'up before we go to bed. Will it deplete the O2 and leave CO behind if I warm the p'up with an air inlet and outlet on opposite ends of the p'up?