DoesTheMath

Rheem RTGH-S10I vs RINNAI (RX180I) REU-NB2934FF-US: which uses less energy?

At the high draw pattern the RINNAI (RX180I) REU-NB2934FF-US uses 173 therms a year and the Rheem RTGH-S10I 176 — 4 therms (2%) less, about 36 therms over ten years. Both are filed as a gas tankless unit, so the comparison holds. Enter your ZIP below to price the difference.

Side by side

Compared within one class only: models filed as a gas tankless unit at the high draw pattern.

Filed and computed figures
Rheem RTGH-S10I RINNAI (RX180I) REU-NB2934FF-US
UEF (Uniform Energy Factor) 0.96 0.98 (better)
Energy use per year (therms) 176 173 (better)
Max flow (GPM) 5.2 5.3
Installed cost range $2,405–$3,460 $2,499–$3,570
ENERGY STAR certified yes yes
On the market since 2022-08-01 2024-03-01
DOE CCD · 2026-09-19

What it costs to run

Rheem RTGH-S10I: 176 therms a year RINNAI (RX180I) REU-NB2934FF-US: 173 therms a year

Rheem RTGH-S10I — 176 therms a year RINNAI (RX180I) REU-NB2934FF-US — 173 therms a year

RINNAI (RX180I) REU-NB2934FF-US is 3 therms a year lower than Rheem RTGH-S10I.

DOE CCD · 2026-09-19

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Reliability

Not published: Reliability, failure rates and each model’s actual warranty term.

The registries these figures come from certify energy performance under a federal test; none of them records how long a unit lasts or how often it fails, so this page does not rank the two on reliability.

What is here instead:

  • The filed efficiency, capacity and tank size of both models, side by side below.
  • Both are ENERGY STAR certified, which requires a manufacturer warranty of at least six years; the actual term for each model is not published.
  • Time on the market, as filed with ENERGY STAR: the Rheem RTGH-S10I since 2022-08-01, the RINNAI (RX180I) REU-NB2934FF-US since 2024-03-01. A sale date, not a lifespan.

Which should you pick?

  • Pick the RINNAI (RX180I) REU-NB2934FF-US if running cost matters most: it uses 4 therms less a year on the same test.
  • Pick the Rheem RTGH-S10I if it is the one you can get installed sooner or for less — on the filed figures it has no advantage over the RINNAI (RX180I) REU-NB2934FF-US.
  • Neither if you need more than 5.3 GPM at the same time — neither unit lists more. Check how to size a tankless unit by GPM.

Each model on its own

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