Instantaneous water heaters are compact water heating units that heat the water as it passes through the vessel. There’s no storage tank, so hot water is made only on demand. While there are many different makes and manufacturers, all have a common disadvantage. They are only adequate if you require a small volume of hot water. Some models have a larger volume capability, but also have a very high energy requirement. Before committing to installation of one of these, be sure to consult with your professional plumber to ensure it’s right for you.