Natural draft water heaters are a type of water heating system that relies on the natural buoyancy of hot air to create a draft and remove combustion gases from the heating process. These water heaters typically have a vertical vent or chimney that allows warm air and gases to rise and escape through it due to the temperature differential and pressure changes. As the hot gases rise, they create a negative pressure in the combustion chamber, pulling in fresh air for combustion. This design is relatively simple and cost-effective, as it doesn't require a fan or external power source to expel combustion products. However, its efficiency and hot water output may be influenced by external factors like weather conditions and altitude.