overall
Rinnai RU199iN
9.5
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Tankless water heaters deliver endless hot water on demand while saving space and energy. Our top pick, the Rinnai RU199iN, offers exceptional flow rate and efficiency for whole-home use.
| # | Product | Best for | Flow Rate (GPM) | Energy Factor (UEF) | Venting Type | |
|---|---|---|---|---|---|---|
| 1 | Rinnai RU199iN | overall | 9.8 GPM | 0.96 UEF | Direct Vent | Check Price |
| 2 | Navien NPE-240S2 | efficiency | 8.0 GPM | 0.98 UEF | Direct Vent | Check Price |
| 3 | Rheem RTEX-36 | electric | 7.0 GPM | 0.99 UEF | No Venting | Check Price |
| 4 | Stiebel Eltron Tempra 36 Plus | advanced electric | 7.2 GPM | 0.99 UEF | No Venting | Check Price |
| 5 | Rinnai V65iP | outdoor | 6.5 GPM | 0.82 UEF | Outdoor | Check Price |
Why we picked it: The Rinnai RU199iN delivers an impressive 9.8 GPM flow rate, enough to run two showers simultaneously. Its 0.96 UEF rating makes it one of the most energy-efficient gas models available. The unit features CircLogic recirculation for instant hot water at every tap. With a robust stainless steel heat exchanger, it promises long-term durability.
Ideal for large households needing high simultaneous hot water demand.
Not for budget-conscious buyers or those with limited gas line capacity.
Key specs: 9.8 GPM - 0.96 UEF - Natural Gas - Direct Vent - 199,000 BTU - 30.5 x 18.5 x 10.5 inches
Why we picked it: The Navien NPE-240S2 boasts a 0.98 UEF, the highest efficiency among tankless water heaters. Its built-in recirculation pump reduces water waste and provides instant hot water. The unit operates quietly and has a compact footprint suitable for tight spaces. Advanced diagnostics via the Navien Smart Control app simplify maintenance.
Best for eco-conscious homeowners seeking maximum energy savings.
Not for large households requiring very high flow rates.
Key specs: 8.0 GPM - 0.98 UEF - Natural Gas - Direct Vent - 199,000 BTU - 28.5 x 18.5 x 10.5 inches
Why we picked it: The Rheem RTEX-36 is a top-performing electric tankless water heater with a 0.99 UEF. It provides up to 7.0 GPM, sufficient for two simultaneous showers in mild climates. The self-modulating technology adjusts power usage based on demand, saving energy. Its compact size and no-venting requirement simplify installation.
Perfect for homes without natural gas or where venting is impractical.
Not suitable for very cold climates or large families with high hot water demand.
Key specs: 7.0 GPM - 0.99 UEF - Electric - No Venting - 36 kW - 17.5 x 17.5 x 8.8 inches
Why we picked it: The Stiebel Eltron Tempra 36 Plus features advanced flow control technology that maintains consistent outlet temperature regardless of inlet temperature. It delivers up to 7.2 GPM with 0.99 UEF. The unit includes a large digital display for easy monitoring. Its premium build quality ensures long-lasting performance.
Ideal for homeowners wanting top-tier electric performance and temperature stability.
Not for those with limited electrical capacity or tight budgets.
Key specs: 7.2 GPM - 0.99 UEF - Electric - No Venting - 36 kW - 18 x 18 x 8.5 inches
Why we picked it: The Rinnai V65iP is designed for outdoor installation, saving indoor space. It provides 6.5 GPM, suitable for a typical household. The unit is built with weather-resistant materials and operates quietly. Its compact size and simple installation make it a convenient choice.
Best for homeowners who want to keep the water heater outside to free up interior space.
Not for cold climates where outdoor units may freeze without proper precautions.
Key specs: 6.5 GPM - 0.82 UEF - Natural Gas - Outdoor - 150,000 BTU - 24 x 18 x 10 inches
Size is determined by flow rate (GPM) needed for your home. A typical household requires 6-8 GPM for two simultaneous showers. Consider your climate and number of fixtures. Use a sizing calculator or consult a professional.
Yes, for many homes. They save energy (up to 34% vs. storage heaters), last longer (20+ years), and provide endless hot water. Higher upfront cost is offset by lower utility bills and rebates.
Electric models can be DIY if you have electrical experience, but gas models require professional installation due to venting and gas line requirements. Improper installation can be dangerous.
Yes, but flow rate may decrease because incoming water is colder. Gas models with higher BTU output are better for cold climates. Some electric models may struggle; consider a gas unit for northern areas.
Choose gas for higher flow rates and better performance in cold climates. Electric models are easier to install and more efficient but may have lower flow rates. Consider availability and cost of fuel in your area.
Calculate the peak hot water demand: add flow rates of fixtures likely used simultaneously (shower 2.5 GPM, faucet 1.5 GPM). Choose a unit with GPM rating that meets or exceeds your total. Oversizing is better than undersizing.
Look for high UEF (Uniform Energy Factor) ratings. Condensing gas models (0.95+ UEF) are most efficient. Electric models typically have UEF near 1.0. Higher efficiency saves money over time.
| Feature | Why it matters |
|---|---|
| Flow Rate (GPM) | Determines how much hot water the unit can deliver per minute. Higher GPM allows multiple fixtures to run simultaneously. Essential for meeting household demand. |
| Energy Efficiency (UEF) | Measures how efficiently the heater converts fuel to hot water. Higher UEF means lower operating costs and environmental impact. Critical for long-term savings. |
| Venting Type | Direct vent systems pull air from outside and exhaust outdoors, safe for indoor installation. Outdoor units need no venting but must be weather-resistant. Important for installation location. |
| Temperature Control | Digital controls or smart apps allow precise temperature adjustment and monitoring. Some models offer recirculation for instant hot water. Enhances convenience and user experience. |
Every product above was scored out of 10 on the same six-part rubric, then sorted into an S to C tier. We do not accept free units or payment for placement, and price or affiliate commission never factors into the score.
| Criterion | What we check | Weight |
|---|---|---|
| Core performance | The numbers that define the category: capacity, power, resolution, battery life, speed or output, taken from manufacturer specs and cross-checked against independent test data where it exists. | High |
| Build & reliability | Materials, warranty length, brand track record, and how often the model shows up in long-term failure or return complaints. | High |
| Real-world usability | Weight, dimensions, noise level, setup difficulty and day-to-day friction, drawn from owner reviews and published measurements. | Medium |
| Running cost | Ongoing costs beyond the purchase: subscriptions, consumables, energy use or maintenance, where they apply to the category. | Medium |
| Owner feedback | Patterns across aggregated verified owner reviews: recurring praise, recurring complaints, and whether the experience matches the marketing. | Medium |
| Value | What you get relative to the rest of the field at a similar price band, not an absolute price judgment. | Medium |
Sources: manufacturer spec sheets and manuals, retailer listing data, aggregated verified owner reviews, and published independent test results where available for the category.
Honesty note: We have not hands-on tested every product on this page. Where we have not personally used a product, its ranking is based on verified specs, aggregated owner feedback, availability and editorial comparison rather than a hands-on review. Hands-on impressions, when included in a product entry above, are clearly written from direct use.
We don't accept free units or payment for placement. Our rankings combine verified manufacturer specifications, real owner feedback and availability, compared on one transparent S to C rubric.