If you plan to use a fire on the inside of your shelter as a heat source, carefully plan how it will be tended. Make sure your shelter has proper ventilation at all times to allow smoke to escape, and to prevent carbon monoxide poisoning. In some types of shelters such as debris shelters, it would probably be wiser to build your fire outside the shelter and build a fire reflector to reflect the heat into your shelter.
Another option is to heat rocks and bring them inside with you. A small fire that you can build quite easily inside a shelter is the star fire. If all you need to do is heat a pot of water or warm up a partially enclosed shelter, the star fire’s low flame makes it safer than other fire lays.