They are called “On Demand Boilers” and they begin heating the water at the moment you turn on the faucet. The rest of the day, no energy is being consumed keeping a tank of water up to temperature. It makes sense. Winterizing a house is easier too. You don’t have to empty the water tank. Just the pipes. This model of Rennai burns either propane or natural gas. This is their light duty model. They have models for a number of use loads. Rinnai also produces gas fired whole-house heating units for circulating water or air. This is not the only reputable manufacturer of these components. But its a nice website for learning your options.