Old Liquid Heaters, also known as ETNAs, are a type of heating system that uses liquid fuel to generate heat. They are an older form of heating technology, and are still used in some parts of the world today. ETNAs are typically used in areas where electricity is not available or is unreliable. They are also used in areas where natural gas is not available or is too expensive. ETNAs are efficient and cost-effective, making them a popular choice for many households.