Tankless water heaters have matured to the point where they genuinely work for most whole-house applications, and these two — Rinnai's RU199iN and Navien's NPE-A2 240A — are the models that plumbers and HVAC contractors recommend most often when a homeowner wants the best available. Both are condensing units, both have built-in recirculation pumps, and both are natural gas. The differences between them are real and worth understanding before you commit to a $1,500+ installation.
Rinnai RU199iN
The Rinnai RU199iN wins on maximum flow rate and long-term reliability data; the Navien NPE-A2 wins on hot-water delivery speed thanks to its built-in buffer tank.
Specs Comparison
| Spec | Rinnai RU199iN | Navien NPE-A2 240A |
|---|---|---|
| Max Flow Rate | 11.1 GPM | 11.2 GPM |
| UEF Rating | 0.95 | 0.99 |
| Buffer Tank | None | 0.8 gallon built-in |
| Recirculation Pump | Built-in | Built-in |
| Min Activation Flow | 0.26 GPM | 0.5 GPM |
| Reliability Track Record | Industry leading | Very good (improving) |
| MSRP | ~$1,399 | ~$1,299 |
Flow Rate and Whole-House Capacity
The Rinnai RU199iN is rated at 11.1 GPM (gallons per minute) at a 35°F temperature rise. At a more typical 55°F rise (65°F inlet to 120°F output), you're looking at roughly 8–9 GPM depending on inlet water temperature. That's enough for two simultaneous showers plus a dishwasher in most climates.
The Navien NPE-A2 240A is rated at 11.2 GPM — effectively identical to the Rinnai at maximum throughput. For most households, neither machine will hit its flow ceiling. The real-world performance differences show up in low-demand scenarios and hot-water delivery speed rather than at peak load.
Both units modulate burner output across a wide range rather than cycling on and off — this is what gives condensing tankless units their efficiency advantage over older non-condensing models. At partial load, both achieve efficiency ratings above 95% UEF.
Navien's Buffer Tank: The Hot-Water Delivery Advantage
The Navien NPE-A2 includes a small internal buffer tank (0.8 gallons) that stores pre-heated water and eliminates the 'cold water sandwich' effect common in tankless units — the brief burst of cold water that arrives between hot draws. Owners on r/Plumbing who switched from Rinnai to Navien frequently cite this as the feature they didn't know they needed.
Rinnai's RU199iN doesn't have an internal buffer. It does include a recirculation pump that can maintain hot water in supply lines, which addresses delivery delay on long runs, but it doesn't eliminate the cold water sandwich effect on its own without a separate buffer tank installation.
For households with multiple bathrooms, kids who turn taps off mid-shower, or anyone who's been burned (figuratively) by cold water surprises, Navien's built-in buffer tank is a meaningful quality-of-life improvement.
Efficiency and Long-Term Operating Cost
Both units are condensing tankless water heaters with UEF (Uniform Energy Factor) ratings above 0.95 — among the most efficient gas water heaters available. The Rinnai RU199iN achieves a UEF of 0.95. The Navien NPE-A2 240A achieves a UEF of 0.99.
That 0.04 UEF difference translates to roughly $15–$25/year in gas savings at typical residential consumption. Over 10 years, that's meaningful but not transformative. Both qualify for the federal 30% energy efficiency tax credit under current IRA provisions, which substantially reduces upfront cost.
Condensing units also require a condensate drain — a PVC pipe that routes acidic water away from the unit. Both machines produce condensate that must be properly handled. This is a minor installation consideration but worth confirming with your plumber before purchase.
Reliability and Service
Rinnai has been manufacturing tankless water heaters longer than any other brand in the U.S. market, and their long-term reliability data is strong. Yale Appliance and independent plumber surveys consistently place Rinnai among the top two tankless brands for service call frequency.
Navien has improved its reliability significantly since early generations that had heat exchanger and venting issues. The current NPE-A2 series has a much better track record, but Rinnai still leads on long-term service data. Both brands maintain parts availability and service networks.
MSRP: Rinnai RU199iN lists around $1,399. Navien NPE-A2 240A lists at $1,299. Installed costs typically run $3,000–$5,000 depending on venting complexity and local labor rates. Both qualify for federal tax credits that meaningfully reduce total cost.
Rinnai RU199iN Strengths
- Longest reliability track record of any tankless brand in the U.S. per Yale Appliance and plumber service data
- 11.1 GPM maximum flow rate handles simultaneous whole-house demands in most climates
- Built-in recirculation pump with multiple scheduling modes for instant hot water at fixtures
Navien NPE-A2 240A Strengths
- Built-in 0.8-gallon buffer tank eliminates cold water sandwich effect — no chilly mid-shower surprises
- UEF 0.99 vs Rinnai's 0.95 — slightly more efficient, saving $15–$25/year in gas costs
- Lower MSRP ($1,299 vs $1,399) with competitive installer availability
Rinnai RU199iN Weaknesses
- No internal buffer tank — cold water sandwich effect possible without separate buffer installation
- UEF 0.95 vs Navien's 0.99 — marginally less efficient over the life of the unit
- Rinnai's recirculation scheduling via the app requires initial setup that some owners find finicky
Navien NPE-A2 240A Weaknesses
- Early NPE generations had heat exchanger reliability issues — current NPE-A2 is improved but Rinnai's track record is longer
- Condensate neutralizer is sold separately rather than included — adds ~$50 to installation
- Navien's error codes are harder to interpret for DIY diagnosis than Rinnai's straightforward code system
Best For
- a: Households prioritizing maximum long-term reliability and plumbers who know and prefer Rinnai's service ecosystem
- b: Households who've experienced cold water sandwich frustration and want hot water immediately without separate buffer tank installation
FAQ
What size gas line do these units require?
Both units require a 3/4-inch gas line minimum for full BTU delivery. If you're upgrading from a traditional tank water heater, confirm your existing gas line can support the higher BTU draw of a condensing tankless. Many older homes have 1/2-inch lines that need upsizing — budget for this in your plumber estimate.
Can a tankless water heater run out of hot water?
No — that's the defining advantage. As long as the unit has gas supply and cold water inlet, it heats on demand without limit. The caveat: if demand exceeds the unit's GPM rating simultaneously (running three showers at once in cold-climate inlet conditions), output temperature will drop. Size correctly for your household.
Do both units need to be vented through the roof?
No — both the Rinnai RU199iN and Navien NPE-A2 support direct vent (two pipes through an exterior wall) or concentric venting through the roof. Wall venting is typically easier and cheaper to install. Confirm venting requirements with your plumber before choosing a unit location.