If you’re tired of running out of hot water or paying to keep a bulky storage tank heated all day, a gas tankless water heater could be the perfect solution. And if you want to keep your system inside for protection from the weather, the best choice is a gas tankless water heater designed for indoor installation.
Indoor units are compact, energy-efficient, and built to deliver a steady supply of hot water on demand. In this guide, we’ll cover the top gas tankless water heaters for indoor use in 2025, explain what makes them stand out, and share tips on choosing the right one for your home. Plus, at the end, we’ll talk about a crucial home maintenance product that can save you thousands in plumbing repair costs.
Why Choose an Indoor Gas Tankless Water Heater?
Indoor gas tankless units are installed inside your home, often in utility closets, basements, or garages. They’re ideal for climates with harsh winters and have several benefits:
- Year-round protection: No exposure to freezing temperatures or heavy weather.
- Higher efficiency: Protected from temperature extremes, so they run more consistently.
- Longer lifespan: Reduced exposure to the elements means less wear and tear.
- Quiet operation: Newer models are quiet enough to run without disturbing daily life.
The only downside is they require proper venting to safely exhaust combustion gases, but with professional installation, this is straightforward.
Best Indoor Gas Tankless Water Heater Overall: Rinnai RU199iN Super High Efficiency Plus
The Rinnai RU199iN remains the top choice for indoor gas tankless heaters in 2025. It’s powerful, reliable, and extremely efficient, making it ideal for large households or anyone who never wants to think twice about hot water availability.
- Flow rate up to 11 gallons per minute (GPM) – easily handles multiple showers and appliances.
- ENERGY STAR® certified with condensing technology to save energy.
- Compact, wall-mounted design saves floor space.
- Excellent build quality and long-term warranty.
This model’s ability to deliver consistent performance even in cold weather makes it a standout.
Other Top Indoor Gas Tankless Water Heater Picks
Noritz NRC66DV-NG
The Noritz NRC66DV is a high-efficiency condensing unit designed for smaller to medium-sized homes. It’s compact, reliable, and known for low NOx emissions, making it environmentally friendly.
Navien NPE-240S2
Navien’s NPE-240S2 offers a stainless steel heat exchanger, excellent efficiency, and a reputation for reliability. It’s also compatible with a recirculation system for faster hot water delivery.
Rheem Prestige Series Indoor 9.5 GPM
The Rheem Prestige Series combines strong performance with advanced features like Wi-Fi connectivity, so you can monitor and adjust settings from your phone.
Bosch Greentherm 9000 Series
German-engineered for precision and efficiency, the Bosch Greentherm 9000 series offers high performance and a sleek, compact design that fits well in tight spaces.
Key Features to Look For
When shopping for an indoor gas tankless water heater, consider:
- Flow rate (GPM): Match it to your household’s peak demand.
- Venting type: Direct vent systems are common for indoor units.
- Energy efficiency: Look for ENERGY STAR® certification.
- Build quality: Stainless steel heat exchangers offer better durability.
- Warranty: A good warranty reflects the manufacturer’s confidence in the product.
Pro Tip: Have your gas line checked before installation—many high-capacity tankless units need more fuel flow than older systems.
Installation Considerations
Professional installation is essential for indoor gas tankless units. Proper venting, gas line sizing, and water connections ensure safe and efficient operation. The installer will also position the unit for easy access during maintenance.
Maintenance Tips
With regular upkeep, an indoor gas tankless water heater can last 20 years or more. Recommended maintenance includes:
- Annual flushing to remove mineral buildup from the heat exchanger.
- Cleaning or replacing air intake filters as needed.
- Checking venting and gas connections for any issues.
The Forgotten Side of Your Plumbing System
Installing the best indoor gas tankless water heater solves your “hot water in” problem. But what about your “water out” problem? If you have a septic system, it’s vital to keep it in good shape. Increased water flow from a new heater can put extra strain on an already stressed septic system.
Ignoring septic maintenance can lead to slow drains, foul odors, and costly backups. Many homeowners don’t think about it until there’s a problem—and by then, repairs can be expensive.
SEPTIFIX – The Easiest Septic Care Solution
SEPTIFIX is the #1 septic tank treatment in the US, trusted by homeowners to keep their systems clean and odor-free. These easy-to-use tablets dissolve in your toilet, releasing oxygen and beneficial bacteria to break down waste, grease, and sludge.
Benefits include:
- Removes odors in just a few days.
- Prevents costly backups and pump-outs.
- Safe for all septic systems and environmentally friendly.
- Just one tablet a month keeps your system in top shape.
Pairing your indoor tankless heater with regular SEPTIFIX treatment means your entire plumbing system stays efficient and problem-free.
Final Thoughts
In 2025, the Rinnai RU199iN is the best indoor gas tankless water heater for most households, thanks to its unmatched blend of performance, efficiency, and reliability. Still, excellent alternatives like the Noritz NRC66DV-NG, Navien NPE-240S2, Rheem Prestige Series, and Bosch Greentherm 9000 Series ensure there’s a perfect fit for every budget and home size.
And remember—hot water is only half the equation. Keeping your septic system healthy with SEPTIFIX ensures you’ll enjoy years of trouble-free comfort.
Order SEPTIFIX Today – Protect Your Septic Tank
Endless hot water without septic problems—that’s the real luxury.