Best Tankless Water Heaters with 6.6 GPM: 7 Spec-Based Picks
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Finding the best tankless water heaters with 6 6 gpm requires checking specific output ratings. You need a unit that maintains flow for multiple fixtures. The Thermomate 27kW Electric Tankless Water Heater leads because its 27 kW wattage supports this high volume in a compact design. We compared 7 models based strictly on published specifications. These picks focus on flow rate, fuel type, and physical dimensions. This guide helps you decide which heater fits your home’s electrical or gas setup.
Our top pick: Thermomate 27kW 6.6 GPM Electric Tankless Water Heater.
Quick picks
The best fit for you
- You have an all-electric home with a 150-amp panel Thermomate 27kW 6.6 GPM Electric Tankless Water Heater The 27 kW electric unit delivers 6.6 GPM without gas lines.
- You need outdoor propane heating with LED diagnostics Rinnai 6.1 GPM Outdoor Propane Water Heater This outdoor propane model provides 6.6 GPM with clear LED status.
- You want indoor natural gas with integrated recirculation Rinnai Smart-Circ 6.1 GPM Natural Gas Heater It supplies 6.6 GPM and includes built-in recirculation for convenience.
Our 7 picks
Thermomate 27kW 6.6 GPM Electric Tankless Water Heater
Best for: all-electric homes running multiple fixtures simultaneously
- Flow Rate
- 6.6 GPM (what this guide is about)
- Fuel Type
- Electric
- Wattage
- 27 kW
- Installation Location
- Indoor
- Voltage
- 240 V
- Temperature Control
- 80°F to 140°F
View on Amazon: Thermomate 27kW 6.6 GPM Electric Tankless Water Heater
The Thermomate 27kW Electric Tankless Water Heater delivers up to 6.6 GPM of continuous hot water using 240-volt electric power. It features an adjustable temperature range from 80°F to 140°F and operates at 99% thermal efficiency. Measuring 17.09 by 13.15 by 3.15 inches, this wall-mounted unit suits you if you want an all-electric system capable of running multiple fixtures without gas venting.
Pros
- Delivers up to 6.6 GPM flow rate with 99% thermal efficiency
- Compact 17.09 x 13.15 x 3.15-inch wall-mounted profile
- Cast aluminum heating element isolates water from electrical components
- Self-modulating control regulates water temperature between 80°F and 140°F
Cons
- Requires a 150-amp panel, three 40-amp double-pole breakers, and three 8 AWG wire runs
- Achievable flow rate and fixture count depend on incoming water temperature
Good fit if
- You have an electrical panel with at least 150 amps of capacity
- You prefer an all-electric water heating setup without gas venting
- You want to supply two showers and a sink simultaneously
Skip if
- You cannot accommodate three 40-amp double-pole breakers in your breaker box
- You prefer a natural gas or propane water heater
Verdict: Thermomate 27kW Electric Tankless Water Heater delivers 6.6 GPM electric performance in a compact footprint for homes with the required 150-amp panel capacity. It provides reliable temperature control from 80°F to 140°F without requiring gas lines.
Rinnai 6.1 GPM Outdoor Propane Water Heater
Best for: outdoor propane multi-fixture hot water setups
- Flow Rate
- 6.6 GPM (what this guide is about)
- Fuel Type
- Propane
- BTU
- 160k BTU
- Installation Location
- Outdoor
- Dimensions
- 24.6 x 14.1 x 10.8 in
- Venting Type
- Non-Condensing
This Rinnai unit is an outdoor non-condensing tankless water heater running on liquid propane. It delivers up to a 6.6 GPM flow rate to serve multiple fixtures simultaneously. Measuring 24.6 by 14.1 by 10.8 inches and weighing 49.2 pounds, it mounts outside to save interior space. It suits you if you need a propane-fueled outdoor system with an onboard LED diagnostic display.
Pros
- Up to 15-year heat exchanger and 5-year parts warranty coverage
- Outdoor mounting eliminates the need for indoor venting runs
- LED screen displays service reminders and diagnostic error codes
- Compact frame measuring 24.6 by 14.1 by 10.8 inches
Cons
- WiFi control requires buying a separate Controlr module
- Outdoor installation design cannot be installed indoors
- Labor warranty drops to 1 year if not registered within 90 days
Good fit if
- You have a dedicated exterior mounting location with a propane supply.
- You want onboard LED diagnostics for service reminders and error codes.
- You plan to register within 90 days to retain 5-year labor coverage.
Skip if
- You need an indoor installation or run on natural gas.
- You want factory-integrated WiFi without purchasing extra accessories.
Verdict: Rinnai 6.6 GPM Outdoor Propane Water Heater provides dedicated exterior propane heating backed by clear LED diagnostics and up to a 15-year heat exchanger warranty. It delivers steady multi-fixture flow without taking up indoor living space.
Rinnai Smart-Circ 6.1 GPM Natural Gas Heater
Best for: indoor natural gas setups needing integrated recirculation
- Flow Rate
- 6.6 GPM (what this guide is about)
- Fuel Type
- Natural Gas
- Installation Location
- Indoor
- Dimensions
- 27.3 x 14.1 x 10.8 inches
- Weight
- 51 pounds
View on Amazon: Rinnai Smart-Circ 6.1 GPM Natural Gas Heater
This indoor natural gas unit delivers hot water across up to five fixtures using non-condensing technology. It includes a built-in recirculation pump with Smart-Circ scheduling that tracks your daily water usage. Measuring 27.3 by 14.1 by 10.8 inches and weighing 51 pounds, it mounts inside your home to conserve space. It suits you if you run natural gas and need dedicated internal recirculation without adding an external pump.
Pros
- Built-in recirculation pump with Smart-Circ usage tracking
- Compact 27.3 by 14.1 by 10.8-inch dimensions weighing 51 pounds
- Powers up to five fixtures at once
- Up to 15-year heat exchanger and 5-year parts warranty coverage
Cons
- WiFi control requires buying a separate Controlr module
- Labor warranty defaults to one year if unregistered within 90 days
Good fit if
- You have an indoor natural gas line ready for connection.
- You want an integrated recirculation pump to eliminate external plumbing add-ons.
- You need to supply hot water to as many as five fixtures concurrently.
Skip if
- You plan to mount your heater outdoors or run on propane.
- You require out-of-the-box WiFi connectivity without extra hardware.
Verdict: Rinnai 6.6 GPM Indoor Natural Gas Water Heater provides integrated recirculation and multi-fixture supply for indoor natural gas setups. It eliminates the need for an external pump while maintaining a compact wall-mounted footprint.
A. O. Smith ProLine XE Outdoor Natural Gas Heater
Best for: outdoor natural gas setups needing 6.6 GPM
- Flow Rate
- 6.6 GPM (what this guide is about)
- Fuel Type
- Natural Gas
- BTU
- 120,000 BTU/hr
- Installation Location
- Outdoor
- Dimensions
- 22.9 x 13.9 x 13.9 inches
- Weight
- 50 pounds
View on Amazon: A. O. Smith ProLine XE Outdoor Natural Gas Heater
The A. O. Smith ProLine XE Outdoor Natural Gas Heater provides continuous hot water up to 6.6 GPM for your home. Running on natural gas at 120,000 BTU per hour, this condensing outdoor unit mounts directly to an exterior wall to save indoor space. It includes a commercial-grade copper heat exchanger alongside integrated diagnostics. It suits you if you need outdoor natural gas water heating at a steady 6.6 GPM flow rate.
Pros
- Delivers up to 6.6 GPM continuous hot water flow rate
- Commercial-grade copper heat exchanger transfers heat quickly
- Wall-mount exterior profile measuring 22.9 by 13.9 by 13.9 inches
- Supports 3-inch PVC venting to 70 feet or 4-inch PVC to 100 feet
Cons
- Configured strictly for outdoor wall mounting
- Requires an existing natural gas supply line
Good fit if
- You need an exterior natural gas water heater delivering up to 6.6 GPM.
- You want an outdoor wall-mounted design that preserves interior storage.
- You need flexible PVC venting capability reaching up to 100 feet.
Skip if
- You require an indoor water heater installation.
- You use liquid propane or electricity rather than natural gas.
Verdict: A. O. Smith ProLine XE Outdoor Natural Gas Heater delivers 6.6 GPM flow from an outdoor condensing wall-mount design powered by 120,000 BTU natural gas. It is a dependable exterior choice if you want quick heat transfer and built-in diagnostics.
Noritz Outdoor Natural Gas Water Heater 6.6 GPM
Best for: outdoor natural gas water heating setups
- Flow Rate
- 6.6 GPM (what this guide is about)
- Fuel Type
- Natural Gas
- BTU
- 140,000 Btuh
- Installation Location
- Outdoor
- Dimensions
- 26.6 x 17.6 x 10.7 inches
- Weight
- 36 pounds
View on Amazon: Noritz Outdoor Natural Gas Water Heater 6.6 GPM
This Noritz outdoor tankless water heater runs on natural gas and delivers up to 6.6 GPM. It produces up to 140,000 BTUh of heat output with a 0.81 Uniform Energy Factor. The unit weighs 36 pounds and mounts directly outside, saving interior utility space. It suits your home if you have natural gas hookups and need an exterior-mounted system without indoor venting requirements.
Pros
- Delivers up to 6.6 GPM maximum flow rate
- 140,000 BTUh natural gas heating capacity
- Exterior installation eliminates indoor venting
- 0.81 UEF rating with Low NOx operation
Cons
- Outdoor installation only
- Requires a natural gas line connection
Good fit if
- You want an exterior-mounted tankless unit that frees up indoor space.
- You have a natural gas supply line available for heating.
Skip if
- You need an indoor tankless water heater.
- You use liquid propane or electricity instead of natural gas.
Verdict: Noritz 6.6 GPM Outdoor Natural Gas Heater provides 140,000 BTUh of exterior heating power at a 0.81 UEF rating. It is a solid choice when you need outdoor placement and run on natural gas.
Noritz 6.6 GPM Outdoor Liquid Propane Tankless Heater
Best for: outdoor liquid propane installations needing 6.6 GPM
- Flow Rate
- 6.6 GPM (what this guide is about)
- Fuel Type
- Liquid Propane
- BTU
- 140,000 Btuh
- Installation Location
- Outdoor
- Dimensions
- 26.6 x 17.6 x 10.7 inches
- Weight
- 36 pounds
View on Amazon: Noritz 6.6 GPM Outdoor Liquid Propane Tankless Heater
This unit is an exterior-mount tankless water heater engineered for liquid propane supply lines. It delivers up to 6.6 GPM using a 140,000 BTU burner and features low NOx operation. At 36 pounds with an outdoor configuration, it eliminates the need for indoor venting systems. This model suits you if you run liquid propane and want an exterior installation to save interior utility space.
Pros
- Delivers up to 6.6 GPM flow rate
- 140,000 BTU maximum heating power
- Low NOx burner design
- Outdoor installation eliminates indoor venting runs
- Weighs 36 pounds for straightforward mounting
Cons
- Outdoor installation only
- Requires liquid propane supply
Good fit if
- You have an exterior wall location for installation.
- You use liquid propane as your fuel source.
- You need a flow rate up to 6.6 GPM.
Skip if
- You need an indoor-mounted water heater.
- You use natural gas or an electric utility line.
Verdict: Noritz 6.6 GPM Outdoor Propane Tankless Water Heater provides high heating capacity and 6.6 GPM for setups with external mounting space and liquid propane lines. It frees up indoor room while meeting standard domestic hot water demands.
Noritz 6.6 GPM Indoor Natural Gas Tankless Heater
Best for: indoor natural gas installations using PVC venting
- Flow Rate
- 6.6 GPM (what this guide is about)
- Fuel Type
- Natural Gas
- Installation Location
- Indoor
- Venting Type
- Condensing
- Dimensions
- 6.7 × 13.8 × 23.6 inches
- Weight
- 38 pounds
View on Amazon: Noritz 6.6 GPM Indoor Natural Gas Tankless Heater
The Noritz 6.6 GPM Indoor Condensing Natural Gas Heater delivers on-demand hot water inside your home using condensing technology. It provides up to 6.6 GPM of flow and vents through standard Schedule 40 PVC pipe and fittings. At 38 pounds, this wall-mounted natural gas unit includes a remote thermostat and a six-foot power cord. It suits you if you need indoor natural gas heating with standard PVC venting requirements.
Pros
- Condensing design vents with Schedule 40 PVC pipe and fittings
- Delivers up to 6.6 GPM flow rate
- Includes remote thermostat and 6-foot power cord
- Low NOx emissions
Cons
- Indoor installation only
- Natural gas only with no propane compatibility listed
Good fit if
- You have an indoor installation space with a natural gas supply.
- You want a 6.6 GPM condensing unit that uses Schedule 40 PVC venting.
Skip if
- You need an outdoor-rated tankless water heater.
- You heat water using liquid propane or electricity.
Verdict: Noritz 6.6 GPM Indoor Condensing Natural Gas Heater pairs a 6.6 GPM delivery rate with condensing efficiency and standard PVC venting. It is an effective indoor solution if your home runs on natural gas.
Specifications side by side
| Spec | Thermomate 27kW 6.6 GPM Electric Tankless Water Heater | Rinnai 6.1 GPM Outdoor Propane Water Heater | Rinnai Smart-Circ 6.1 GPM Natural Gas Heater | A. O. Smith ProLine XE Outdoor Natural Gas Heater | Noritz Outdoor Natural Gas Water Heater 6.6 GPM | Noritz 6.6 GPM Outdoor Liquid Propane Tankless Heater | Noritz 6.6 GPM Indoor Natural Gas Tankless Heater |
|---|---|---|---|---|---|---|---|
| Fuel Type | Electric | Propane | Natural Gas | Natural Gas | Natural Gas | Liquid Propane | Natural Gas |
| Flow Rate | Up to 6.6 GPM | 6.1 GPM | 6.6 GPM | 6.6 GPM | 6.6 Gpm | 6.6 Gpm | 6.6 GPM |
| BTU | not specified | 160k BTU | not specified | 120,000 BTU/hr | 140,000 Btuh | 140,000 Btuh | not specified |
| Wattage | 27 kW | not specified | not specified | not specified | not specified | not specified | not specified |
| Installation Location | Indoor | Outdoor | Indoor | Outdoor | Outdoor | Outdoor | Indoor |
| Voltage | 240 V | not specified | not specified | not specified | not specified | not specified | not specified |
| Temperature Control | 80°F to 140°F | not specified | not specified | not specified | not specified | not specified | not specified |
| Certification | ETL Listed | not specified | not specified | not specified | not specified | not specified | not specified |
| Dimensions | 17.09 x 13.15 x 3.15 inches | 24.6 x 14.1 x 10.8 in | 27.3 x 14.1 x 10.8 inches | 22.9 x 13.9 x 13.9 inches | 26.6 x 17.6 x 10.7 inches | 26.6 x 17.6 x 10.7 inches | not specified |
| Venting Type | not specified | Non-Condensing | not specified | not specified | not specified | not specified | Condensing |
| Weight | 19 pounds | 49.2 pounds | 51 pounds | 50 pounds | 36 pounds | 36 pounds | 38 pounds |
| Color | Gray | Silver | Silver | White | No Finish | No Finish | White |
| Model | EH270 | REU-VE2125WD-US-P | REU-VEP2125FFD-US-N | ATO-140H 100 | NR662-OD-NG | NR662-OD-LP | NRC661-DV-NG |
| Size | not specified | 6.6 GPM | 6.6 GPM | not specified | not specified | not specified | 13.8 inches (W) x 23.6 inches (H) x 10 inches (D) |
Is 6.6 GPM Worth It for Your Home?
A 6.6 GPM flow rate supports simultaneous use of multiple fixtures. You can run a shower, kitchen sink, and laundry machine without noticeable pressure drops. This volume suits larger households or homes with extensive plumbing layouts. Smaller units may struggle to maintain temperature under heavy load. Verify your electrical panel or gas line capacity matches the heater’s requirements before choosing this output level.
Worth it if
- You run two or more hot water fixtures at once
- Your household has three or more bathrooms
- You need consistent temperature during high-demand periods
Skip it if
- You only use one faucet or shower at a time
- Your electrical panel lacks 150-amp capacity for electric models
- You have limited outdoor space for propane or gas units
To maintain incoming flow capacity, you can evaluate the best 12 GPM whole house water treatments for your system.
How we picked these 6.6 GPM models
We identified all products on 7 listings that explicitly state a 6.6 GPM flow rate in their titles or feature lists. We reviewed only the published specifications to confirm this capacity. We did not handle any units or test them ourselves. All details come directly from the manufacturer data provided on each Amazon page.
The list is ordered by availability first. We then ranked items by their overall rating weighted by the number of ratings. Finally, we considered how many guide criteria each product's specs met. We included only one variant per product family. This method ensures you see the most relevant options for high-flow needs.
Key Specs for Choosing 6.6 GPM Heaters
-
Check electrical panel capacity
Electric units require significant power. The Thermomate 27kW model delivers 6.6 GPM but needs a 150-amp panel. You must verify your home’s electrical service before buying. Gas models do not have this constraint. They use natural gas or propane instead. If your panel is smaller, an electric unit will trip breakers. Always match the wattage to your available amperage. This ensures the heater runs without shutting down your home power.
-
Select the correct fuel source
Your existing utility lines dictate the heater type. Gas models like the Rinnai REU-VE2125WD-US-P use propane. Others, such as the A. O. Smith ProLine XE, run on natural gas. Electric options like the Thermomate need no gas lines. You cannot switch fuels easily. Check if you have gas lines near the installation spot. If you only have electricity, stick to electric models. Mismatching fuel types leads to costly retrofitting or installation failure.
-
Decide on installation location
Indoor and outdoor units differ in design. Outdoor models, like the Noritz NR662-OD-NG, handle weather exposure. They free up indoor space. Indoor units, such as the Rinnai REU-VEP2125FFD-US-N, require protected environments. Some indoor models, like the Noritz NRC661-DV-NG, use condensing tech. This affects venting needs. Choose outdoor placement if you have exterior wall space. Choose indoor placement for climate control and noise reduction.
-
Match venting requirements
Venting type changes installation complexity. Condensing models, like the Noritz NRC661-DV-NG, use PVC venting. This is easier and cheaper to install. Non-condensing units, like the Rinnai REU-VE2125WD-US-P, need metal venting. Metal vents are heavier and cost more. Check your local codes for venting materials. If you already have PVC vents, a condensing unit fits best. Non-condensing units require specific clearance and metal pipe routing for safe exhaust.
-
Verify flow rate consistency
All listed models claim 6.6 GPM. This supports multiple fixtures. You can run showers and sinks simultaneously. However, input temperature affects actual output. Cold inlet water reduces effective flow. Electric units like the Thermomate 27kW handle this with high wattage. Gas units use BTU ratings. The A. O. Smith ProLine XE uses 120,000 BTU. Ensure your inlet water temperature aligns with the unit’s capacity for full flow.
Frequently asked questions
Can a 6.6 GPM tankless heater run two showers at once?
Yes, a 6.6 GPM unit can support multiple fixtures simultaneously. This flow rate allows you to run a shower, kitchen sink, and laundry machine without noticeable pressure drops. The Thermomate 27kW model provides this capacity in a compact footprint for all-electric homes. Other gas models also deliver steady multi-fixture flow for larger households.
What electrical panel capacity is needed for a 6.6 GPM electric heater?
You need a 150-amp electrical panel for the Thermomate 27kW Electric Tankless Water Heater. Its 27 kW wattage supports the high volume required for 6.6 GPM output. Most other picks in this range run on natural gas or propane, so they do not have this specific electrical constraint. Verify your home’s service before buying an electric unit.
Which 6.6 GPM heater is best for outdoor propane setups?
The Rinnai Outdoor Propane Water Heater is the best premium choice for exterior use. It delivers 6.6 GPM with non-condensing venting and clear LED diagnostics. The Noritz Outdoor Propane Tankless Heater is another solid option that frees up indoor room while meeting standard domestic hot water demands using liquid propane lines.
Do I need a separate pump for recirculation with these units?
No, the Rinnai Smart-Circ 6.1 GPM Natural Gas Heater includes integrated recirculation. It eliminates the need for an external pump while maintaining a compact wall-mounted footprint. This indoor natural gas model provides multi-fixture supply and is ideal for setups that need instant hot water without additional hardware.
What is the difference between condensing and non-condensing venting?
Condensing models, like the Noritz NRC661-DV-NG, use standard PVC venting for higher efficiency. Non-condensing units, such as the Rinnai Outdoor Propane model, typically require metal venting. The A. O. Smith ProLine XE is an outdoor condensing option that offers quick heat transfer. Your installation location and existing venting infrastructure will determine which type suits your home.
How large are the physical dimensions of these 6.6 GPM heaters?
Dimensions vary by model and fuel type. The Thermomate electric unit is compact, measuring 13.2 inches wide and 17.1 inches high. Gas models are generally taller, such as the Rinnai Smart-Circ at 27.3 inches high. The Noritz indoor condensing model measures 13.8 inches wide and 23.6 inches high. Check your available wall space before installation.
Can I use a 6.6 GPM heater in an indoor closet?
You can use indoor models like the Rinnai Smart-Circ or Noritz NRC661-DV-NG in enclosed spaces. The Rinnai model is designed for indoor natural gas setups. The Noritz condensing unit uses PVC venting, which is suitable for indoor installation. Ensure you follow manufacturer clearance requirements. Outdoor models must remain outside for proper ventilation and safety.