Efficient tankless gas water heaters heat water when it passes through the heater, eliminating the need to wait for a tank of water to heat up before use. Because they do not have a storage tank, they use less energy and take up less space than standard gas water heaters. They are powered by natural or liquid propane gas and require venting.
When choosing a tankless water heater, select a unit with a maximum flow rate that exceeds the total flow rate for the fixtures that will be supplied by the unit and a maximum water temperature that exceeds the water temperature required by the fixtures. On average, kitchen sinks and dishwashers use 1.5 gpm of water at 110° F, washing machines use 2 gpm of water at 120° F, showers use between 2.5 to 3 gpm of water at 104° F, and bathtubs use 4 gpm of water at 102° F.