What made Sam realize that he needed ventilation when he used his furnace?

Sam fired up his furnace, closed the door, and was glad to be warm. Then he noticed that Frightful was siting on the bedpost with her head under her wings. She was topping. When he saw that she was sick, he opened the tent door to get her some water. The cold air revived her. That made him realize that there needed to be ventilation in the tree or they would be get sick and perhaps die.