Draw patterns, the hidden label behind every UEF

A draw pattern is the daily hot-water schedule the DOE test uses to measure a heater’s UEF. The test assigns it from the heater’s first-hour rating, or from its maximum flow for tankless units: very small, low, medium or high. The high pattern covers 814 of the 1,138 certified model numbers. Two UEFs are only comparable when their patterns match.

The four bins

The bins come from the federal test method (10 CFR Part 430, Appendix E, Tables I and II). Each pattern prescribes a different number and size of draws over the simulated day, so a heater on the high pattern delivers more water during the test than one on the low pattern.

We checked every model on the list against these bins. All 1,138 kept model numbers sit in the pattern their first-hour rating or flow predicts.

Which heaters land where

The gas tanks all test at the high pattern: their first-hour ratings start at 86 gal. The tankless units do too, with one exception: Rinnai (RX130I) REU-NB2024FF-US, rated 3.8 gal/min, falls in the medium bin. The heat pumps split between high and medium, with a handful at low. Solar systems carry a pattern as well, but they are rated by solar UEF, so the pattern matters less for reading them.

What happens at the edge

The boundary at 75 gallons is sharp. Rinnai REHP50 rates 73 gal in the first hour and is tested at the medium pattern; Carnot Labs N51 rates 75 gal and is tested at the high pattern. Their rated use is 844 kWh/yr and 1,404 kWh/yr. Part of that gap is the heaters themselves, whose UEFs are 3.75 and 3.53; part is the test day, because the high pattern asks for more hot water.

This is also why a list sorted by annual energy alone misleads. Of the 20 heat-pump sets with the lowest rated kWh, all 20 test at the medium or low pattern, because their test day is lighter.

Tankless units and the flow bins

For tankless heaters the bins use maximum flow instead of first-hour rating. Every gas tankless set on the list rates between 3.8 gal/min and 6.0 gal/min, so all of them land in the top two bins, and all but one in the high bin. A tankless UEF therefore almost always comes from the same test day as a high-draw tank, which makes the gas tankless and gas storage UEFs on this list comparable with each other, though not with a heat-pump UEF.

The low and very small patterns

The two lower bins are thin on this list. The low pattern holds 32 model numbers, most of them solar systems and small heat pumps; the very small pattern holds 1, a single solar system. Certified water heaters are, in practice, rated at the medium and high patterns.

Patterns follow size

Because the pattern comes from the first-hour rating, and the first-hour rating grows with the tank, the pattern tracks rated volume closely. Of the 36 heat-pump sets under 50 gallons, 31 test at the medium pattern; of the 35 sets at 70 gallons and up, 33 test at the high pattern. The 50 to 69 gallon group is mostly high (24 of 30).

The step in rated energy

The pattern shows up directly in annual energy. The median heat-pump set uses 1,269 kWh/yr at the high pattern, 895 kWh/yr at the medium pattern and 724 kWh/yr at the low pattern. Part of that is bigger tanks and part is the test day itself. Among the 101 heat-pump sets, 19 have a first-hour rating between 70 and 79 gallons, on either side of the 75-gallon line between the medium and high patterns.

How the site handles patterns

• Every set table on a type or maker page is grouped by draw pattern, and each UEF rank names its pattern.
• The Heater finder shows the pattern on every result and refuses to subtract UEFs or annual energy across patterns.
• Explainers that compare groups, such as the heat-pump subtype comparison, keep each pattern in its own column.

The pattern is a test classification. It describes the water schedule used in the lab, not the size of household a heater suits, and this site does not translate it into one.