Friday, July 15, 2011

Sizing Tankless Water Heaters

Tankless water heaters only produce hot water when you need it, and the supply will never run out – provided that you do not exceed the unit’s GPM capacity. There is a limit to the amount of hot water every tankless unit can produce in one minute. This gallons per minute (GPM) rating will be quite small for units designed to supply only a sink or two, but larger for whole-house heaters. The GPM rating fluctuates based on the “temperature rise” the unit needs to achieve. If the incoming water temperature is 50°F and the shower water must be 110°F, you have a 60°F temperature rise. The specification sheets for tankless heaters display a chart with the maximum flow rates across the spectrum of temperature rises.

At lower temperature rises, tankless water heaters can make more hot water. This means that it is easier to get more hot water during the summer or in warmer climates. Even in the coldest areas of the continental United States, incoming water temperatures do not fall much below 40°F. In warmer southern climates, average ground water temperatures hover close to 70°F. Keep this in mind as you browse the selection of tankless water heaters on