Section 4

Fixture flow rates for tankless water heater sizing

This page lists the federal maximum flow rates for showerheads and faucets, the EPA WaterSense maximums, and the fixture flows that Stiebel Eltron, EcoSmart, Rinnai and Eccotemp use in their sizing sheets. Required flow is the sum of the fixtures that run hot at the same time.

The flow for a fixture can come from the federal maximum, a WaterSense label or the rating stamped on the fixture. Each manufacturer uses its own default.

4.3 Why the manufacturers' flows differ

A shower at 105 F mixes hot and cold water. When the heater's outlet is hotter than 105 F, only part of the showerhead flow passes through the heater. Stiebel Eltron and EcoSmart size as if the whole showerhead flow passes through the heater at 105 F. That is the case when the heater is set to 105 F. Rinnai's table uses a 120 F set point with a 105 F shower mix and a 2 GPM head^[7]. The sizer follows the first approach, with the outlet temperature set to the temperature at the fixture.

In a small ADU the showerhead is the largest single load. A 1.5 GPM head in place of a 2.5 GPM head cuts the shower load by 40 percent. That can lower the electric unit by one or two sizes in Table 2.1.

The sizer presets are shower 2.0 GPM, bathroom sink 1.0 GPM, kitchen sink 1.5 GPM, dishwasher 2.0 GPM and washer 3.0 GPM. They match the EcoSmart sheet. Each preset can be changed in the sizer to match the rating printed on the fixture.