Best High Efficiency Tankless Water Heaters for Instant Hot Water
Answering the search for high efficiency tankless water heaters, this guide compares five top options based on output, energy use, and indoor installation. Each choice is suited to different home sizes and hot-water needs. If you want compact, fast-heating performance for multiple outlets, consider models with higher GPM. For homes prioritizing energy savings, look for units with advanced controls or condensing designs. This overview helps you pick the right system for showers, laundry, and simultaneous hot-water use.
| Product | Max GPM | BTU | Best For |
|---|---|---|---|
| FOGATTI Instagas Comfort 170S | 7.5 | 170,000 | Multitasking households with 3–5 outlets |
| Rheem RTG-84DVLN-3 | 8.4 | 180,000 | 2–3 bathrooms, active with laundry |
| NEW Rinnai RX199iN | 11.1 | 199,000 | Homes needing high flow and versatility |
| MIZUDO 7.9 GPM | 7.9 | 180,000 | Indoor, app-controlled whole-house comfort |
| ThermoMate CZ422LP | 5 | 113,000 | Two showers simultaneously in small-to-medium homes |
These options cover a range from compact, high-speed gas models to versatile, WiFi-enabled units. Consider your climate, number of fixtures, and whether you prefer indoor-only or flexible indoor/outdoor installation when choosing.
FOGATTI Instagas Comfort 170S: Compact Power
The FOGATTI Instagas Comfort 170S delivers 170,000 BTU and up to 7.5 GPM, packed into a compact space. It uses a four-stage combustion system for consistent heating and precision water temperature control within ±1°F. Best for multitasking households needing reliable hot water across 3–5 outlets, this model excels when space is limited. A potential limitation is its higher BTU demand, which may require proper venting and gas supply checks.
Rheem RTG-84DVLN-3: Indoor Efficiency
Rheem’s RTG-84DVLN-3 provides up to 8.4 GPM and 180,000 BTU for dependable hot water in homes with two to three bathrooms. Designed for indoor installation, its durable cabinet supports wall mounting by a qualified installer. This unit is a solid choice for steady performance across moderate demand periods. Use this if you want a balance of high flow and straightforward installation, but beware of potential space needs behind panels and near venting paths.
NEW Rinnai RX199iN: Condensing Versatility
The Rinnai RX199iN is a condensing, smart-sense model offering up to 11.1 GPM and 199,000 BTU. It supports indoor or outdoor installation with Versa Vent, and it can run on propane or natural gas. Its compact design handles multiple fixtures efficiently, and its advanced features include a Quick Flush and a high energy efficiency factor (UEF .98). This is best for larger homes or scenarios needing higher simultaneous hot-water capacity; outdoor installs require an RX-OVC vent cap.
MIZUDO 7.9 GPM Indoor Tankless
The Mizudo 7.9 GPM model features a sealed combustion system that preserves indoor air quality and CSA certification for safety. It delivers up to 180,000 BTU and supports 3–5 usage points with minimal temperature fluctuation. The unit includes protections like freeze prevention and auto-relight. Choose this if you want app-controlled convenience and strong safety features, but note that WiFi control requires additional setup or accessories.
ThermoMate CZ422LP: Efficient for Two Showers
The ThermoMate CZ422LP delivers up to 5 GPM and 113,000 BTU, designed to supply two showers simultaneously with minimal water-temperature fluctuation. It emphasizes safety with protections like boil-dry, freeze, and flame-out, plus energy efficiency at 98% thermal performance. It’s best for smaller homes or when you need reliable, compact heat for two users. A potential limitation is the lower GPM compared with larger families or homes with significant concurrent demand.
Buying Guide
What size should you choose?
Estimate your peak hot-water needs by listing daily activities that run simultaneously (showers, laundry, dishwasher). A higher GPM unit handles more outlets at once. If you regularly run multiple fixtures, aim for 8–11 GPM ranges. For smaller homes or limited space, 5–7.9 GPM models can suffice.
What about energy efficiency?
Condensing units reclaim more exhaust heat, improving efficiency and reducing operating costs. Look for high UEF values and models with advanced controls for temperature stability. Indoor units often require venting, while some models offer outdoor-friendly or dual-install options.
Installation considerations
Indoor installation generally requires proper ventilation and clearance for venting. Check gas supply capacity and venting requirements. Some models include features like freeze protection and auto-relight—useful for colder climates. Verify whether professional installation is needed in your area.
Smart features and controls
WiFi or app-enabled controls can simplify temperature management and maintenance. Consider compatibility with existing home automation and whether remote monitoring offers tangible benefits for your household scenario.
Durability and safety
CSA or other safety certifications are important. Look for sealed combustion, dry-fire safeguards, and anti-freeze protections to reduce the risk of damage and keep water temperatures steady during winter.
Choosing the best option for you
– For the highest hot-water capacity: NEW Rinnai RX199iN offers up to 11.1 GPM and strong flexibility for indoor/outdoor use.
– For space-saving with solid performance: FOGATTI Instagas Comfort 170S provides 7.5 GPM in a compact form while maintaining ±1°F temperature accuracy.
– For balanced price and flow: Rheem RTG-84DVLN-3 gives 8.4 GPM with indoor installation and robust build.
– For app-controlled convenience: MIZUDO 7.9 GPM with CSA certification and WiFi/app options.
Short FAQ
- What is a high efficiency tankless water heater? A gas or electric on-demand heater that provides hot water with minimal standby energy waste, using advanced controls and heat exchange to improve efficiency.
- Do these units require venting? Most gas-powered models require venting; indoor installations must meet venting and clearance requirements.
- Can I install one myself? Some models allow DIY installation, but gas models typically require a licensed professional for safe operation.
- What is GPM and why does it matter? GPM (gallons per minute) measures hot-water flow. Higher GPM means more simultaneous use without temperature drops.
- Are condensing models worth it? Yes, condensing models are more energy-efficient and can reduce long-term operating costs, especially in colder climates.
- Which model is best for a large family? The NEW Rinnai RX199iN offers the highest flow with up to 11.1 GPM and flexible installation.