Tankless max flow at a 67 °F rise: reading the GPM numbers

DOE rates a tankless water heater at the flow it sustains while heating 58 °F water to 125 °F, a 67 °F rise. Certified gas tankless units top out at 6.0 gal/min by that measure, and the figure tracks gas input almost in a straight line: the 13 sets at 199,000 Btu/h rate 5.4–6.0 gal/min.

Why the rise matters

Brochures often quote flow at a smaller temperature rise, such as 35 °F, which gives a much larger number. The certified figure uses the test’s 67 °F rise. This site only shows the certified one, always with its rise, and never mixes it with a brochure value.

Flow by gas input

Each step down in input lowers the flow ceiling. The reference line in the last column is simple arithmetic: a gallon of water weighs about 8.34 pounds, and one Btu raises one pound by 1 °F, so a burner of a given input can heat at most input ÷ (8.34 × 60 × 67) gallons per minute through a 67 °F rise if every Btu reaches the water.

The listed flows sit close to that line. Across the 40 tankless sets, listed flow is between 91% and 105% of the reference value, with a median of 98%. Of these, 8 sets sit slightly above the line; the test allows the inlet and outlet temperatures to vary a little (58 ± 2 °F in, 125 ± 5 °F out), so a small overshoot is possible.

The highest flow on the list, 6.0 gal/min, belongs to A. O. Smith ATM-199M-N 1 and Vesta VTP-199.

Rows we could not use

Twenty-three listed rows claimed a flow that their own gas input could not supply even at full efficiency, such as 6.0 gal/min at 160,000 Btu/h. We cannot tell whether the flow or the input is wrong, so both values were removed and those models do not appear. Most were propane versions of 180,000 and 199,000 Btu/h units, so propane-only tankless models are under-represented here.

Fuel listing

Counted by model number among the tankless units, 151 are listed for natural gas only, 103 for propane only and 113 for either. Separate natural-gas and propane versions usually carry their own model numbers, often differing by one letter.

The one medium-draw unit

Rinnai (RX130I) REU-NB2024FF-US is the exception on the list: with an input of 130,000 Btu/h, the unit rates 3.8 gal/min, just under the 4.0 gal/min line, so the unit was tested at the medium pattern. That unit’s UEF of 0.97 comes from a lighter test day than the other tankless units and is not ranked against them.

Gas use hardly moves with input

A bigger burner raises the flow, but not the rated gas use. Median high-draw gas use is 176 therms/yr at 199,000 Btu/h; 176 therms/yr at 170,000–189,000 Btu/h; 178 therms/yr at 160,000–169,000 Btu/h; 176 therms/yr at under 160,000 Btu/h. The test day delivers the same water whatever the input, so the energy needed is nearly the same; input only changes how fast it can be delivered.

The low end: minimum flow

The listing also gives a minimum flow, the smallest draw at which a unit fires. Of the 367 tankless model numbers, 210 list 1.0 gal/min, 70 list 0.4 and 49 list 0.5; the lowest listed value is 0.1 gal/min. Like max flow, it is a listed figure and not part of the UEF, and it is not compared across types on this site.

Venting

Every tankless unit on the list with a venting entry vents mechanically: the listings name power vent, direct vent or power direct vent, and 105 model numbers list more than one option. Venting is an installation matter, which this site does not cover; it is listed here only because it is part of each certified record.

Efficiency is tightly packed

Flow varies a lot between tankless sets; efficiency hardly does. At the high draw pattern every tankless set rates a UEF between 0.95 and 0.98, and rated gas use runs from 173 to 178 therms a year. See UEF explained for why the gas scale is so narrow.

The gas tankless page lists every set by input class, and the Heater finder filters by minimum flow at the 67 °F rise.