Tankless ratings
Tankless temperature rise, and why advertised GPM falls short
A tankless unit’s maximum GPM is measured at a 67°F rise, 58°F water heated to about 125°F. If your incoming water is colder or you want it hotter, your rise is bigger and the unit delivers less. A unit rated 5.0 gpm delivers about 4.2 gpm at an 80°F rise.
Updated
Convert a rated GPM to your rise
Flow at your 75°F rise
4.5gpm (estimate)
Your rise is larger than the rated 67°F, so the unit delivers less: 5.0 × 67 ÷ 75. At full fire a tankless unit's heat output is roughly fixed, so flow falls as the rise grows. Confirm on the manufacturer's curve.
Why flow falls as the rise grows
At full fire, a tankless heater puts a roughly fixed amount of heat into the water each hour. Heat = 500 × gpm × rise, so if the rise goes up, the flow it can heat comes down in proportion. Double the rise, half the flow.
flow at your rise = rated flow × rated rise ÷ your rise = 5.0 × 67 ÷ 80 = 4.19 gpm heat (constant) = 500 × 5.0 × 67 = 167,500 BTU/hr
This is the same rule the iPhone app’s model check uses. It only ever scales down: when your rise is smaller than the rated rise, the published flow is used as-is, because real units have a maximum flow limit and the extra isn’t guaranteed.
Rated flow at larger rises
| Rated at 67°F | 67°F rise | 70°F rise | 75°F rise | 80°F rise | 90°F rise |
|---|---|---|---|---|---|
| 4.0 gpm | 4.0 | 3.8 | 3.6 | 3.4 | 3.0 |
| 5.0 gpm | 5.0 | 4.8 | 4.5 | 4.2 | 3.7 |
| 6.0 gpm | 6.0 | 5.7 | 5.4 | 5.0 | 4.5 |
| 7.0 gpm | 7.0 | 6.7 | 6.3 | 5.9 | 5.2 |
| 8.0 gpm | 8.0 | 7.7 | 7.1 | 6.7 | 6.0 |
Finding your rise
- Incoming water: measure it at a cold tap in the coldest month, or use an estimate from the incoming water guide.
- Outlet temperature: 120°F is the common setting. Remember that a shower at 105°F still needs the heater to deliver its outlet temperature; the valve blends it down.
- Rise = outlet − incoming. 120°F with 40°F water is 80°F; with 60°F water, 60°F.
Reading a manufacturer’s flow curve
Spec sheets show flow against rise as a curve or a table. Find your rise along the bottom, go up to the curve, and read the flow. That flow must be at least your required flow from the tankless calculator. The calculator’s chart draws curves of constant heat output so you can see where your requirement sits.
Also check the unit’s minimum activation flow. A large unit sized for two showers may not fire for a trickle at one bathroom sink.
Temperature rise questions
What is temperature rise on a tankless water heater?
The number of degrees the unit has to heat the water: outlet temperature minus incoming water temperature. Heating 45°F water to 120°F is a 75°F rise.
What temperature rise is the GPM rating based on?
The federal maximum GPM rating for tankless water heaters is measured in the test in 10 CFR 430 Appendix E, with 58°F supply water and the outlet set to about 125°F, a nominal 67°F rise. ENERGY STAR's tankless criterion, for example, is a maximum of at least 2.8 gpm over a 67° rise.
How do I convert a tankless GPM rating to my rise?
If your rise is larger than the rated rise, multiply the rated flow by rated rise ÷ your rise. A unit rated 5.0 gpm at 67°F delivers about 4.2 gpm at an 80°F rise. If your rise is smaller, the unit may deliver more, but only up to its maximum flow; check the manufacturer's curve.
Why do some tankless units advertise 9 or 10 gpm?
Some product pages quote the flow at a small rise, such as 35°F, which makes the number large. Check which rise the figure belongs to and convert it to yours.
Sources
Water Heater Sizing Calculator for iPhone
Take the calculator to the job site
The app runs the same sizing engine as this site, offline. Basic tank and tankless sizing is free. Lifetime Pro, a one-time $4.99 purchase, adds tank vs. tankless comparison, custom fixtures, saved jobs with notes and PDF reports you can send to a customer.
