Rheem High Efficiency Non-Condensing Outdoor Tankless Natura

The Verdict

If your water heater lives outside your house, you already know the pain of a tank rusting out in five to seven years. The Rheem High Efficiency Non-Condensing Outdoor Tankless Natura (model RTEX-18, for the natural gas version) attacks that problem from a different angle — it simply doesn’t store water at all, so there’s no tank to corrode. With 229 Amazon ratings averaging 4.6 stars, the consensus is clear: this unit delivers endless hot water without eating your garage or basement floor space. But it’s not the right choice for every household, and I’m going to tell you exactly why.

What You Actually Get

The box contains the unit itself — roughly 28.5 inches tall, 16 inches wide, and 9.5 inches deep — plus a wall-mounting bracket, a condensation drain fitting (for the condensate that does form even in non-condensing designs), and the installation manual. Rheem rates it for outdoor installation only, which is the point: no venting hardware, no PVC pipes running through your roofline, no indoor clearance requirements. The included bracket is heavy-gauge steel; mounting takes about 15 minutes with a drill and a level.

One thing buyers consistently mention in the reviews: the unit ships well-protected. Multiple customers note the packaging survived freight shipping without dents or damage, which matters when you’re installing something that holds gas and water under pressure.

Performance and Build Quality

This unit delivers up to 6.5 gallons per minute at a 35°F temperature rise — enough for a tankless to handle a shower and a dishwasher running simultaneously. At a 45°F rise (typical for northern winters), you’re looking at closer to 5 GPM, which still covers most two-bathroom homes. The gas burner modulates down to about 15,000 BTU/hr when you’re just washing hands, so you’re not paying full-fire rates for small draws.

The heat exchanger is copper with a protective coating. Several reviewers mention they’ve installed these in coastal areas where salt air eats standard units — the coating on this one holds up better than the cheaper Rheem RTG models. One buyer wrote: “Three years on the Texas coast and no corrosion issues. My old tank was rusted through in four years.”

The digital display shows set temperature (95°F to 120°F in one-degree increments) and error codes when something’s wrong. It’s simple, but it works. What you don’t get is Wi-Fi connectivity or app control — for that, you’d need to step up to Rheem’s condensing models.

What Buyers Consistently Praise

Across the 229 ratings, the recurring themes are installation simplicity and reliability. Because there’s no venting, a competent DIYer with basic gas plumbing experience can install this in an afternoon — though I’d still recommend a licensed plumber for the gas line connection if you’re not comfortable with it.

Buyers also praise the size. Multiple reviews say something like: “I reclaimed my entire utility closet. This thing hangs on the outside wall and I don’t even notice it.” The compact footprint is genuinely one of the strongest selling points.

Where It Falls Short

Let’s be direct: this is a non-condensing unit, which means it’s less efficient than Rheem’s condensing models. The 0.82 Energy Factor is decent — better than any tank water heater — but condensing units push into the 0.95+ range. Over a 10-year lifespan, you might spend $200–$400 more in gas with this unit versus a condensing model. The tradeoff is that condensing units are bulkier and more expensive upfront.

The freeze protection is also a real limitation. The unit has built-in freeze protection down to about 32°F, but in climates that see sustained sub-freezing temps, you’ll need to add the optional electric heat tape kit (sold separately) or risk internal damage. That’s an extra cost and an extra installation step that isn’t obvious from the product listing.

One more quirk: at very low flow rates (under 0.4 GPM), the burner may not ignite, so you’ll get cold water if you’re trying to fill a tiny sink stream. This is common to most tankless units, but worth knowing before you buy.

Who Should Buy This

Buy it if: You have exterior wall space, your climate doesn’t see extended hard freezes (or you’re willing to add heat tape), and you want to eliminate a tank that’s going to fail in 6–8 years. It’s also ideal for vacation homes or rental properties where indoor space is at a premium.

Skip it if: You live in a severe cold climate and don’t want the extra freeze-protection hassle, you want smart-home integration, or you’re planning to stay in your home long enough to recoup the efficiency difference with a condensing unit.

FAQ

Q: Can I install this myself? A: The water and mounting are DIY-friendly. The gas line connection requires a licensed plumber in most jurisdictions — and you’ll need a permit in many areas. Budget $200–$400 for professional gas hookup if you don’t do it yourself.

Q: How much does it cost to run versus my old tank heater? A: Expect 20–30% gas savings compared to a standard 40-gallon tank. The exact number depends on your usage patterns and local gas rates, but most buyers report noticeable utility bill reductions within the first billing cycle.

Q: What maintenance does it need? A: Annual descaling with vinegar or citric acid solution is the main task — a 30-minute job with the included service valves. Outdoor units should also have the air intake screen checked seasonally for debris.

The Verdict

Rated 4.6/5. The Rheem High Efficiency Non-Condensing Outdoor Tankless Natura is a solid, reliable choice for the right application. It’s not the most efficient option on the market, and the freeze-protection caveat is real. But for the price point, the ease of installation, and the space savings, it’s hard to beat. Buy it if you want endless hot water without sacrificing indoor space. Skip it if you need maximum efficiency in a cold climate — spend the extra for a condensing unit instead.

Where to Buy

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Last updated: 2026-08-06. Ratings and prices current as of review date. Verify on Amazon before purchasing.