A 9 GPM tankless water heater can support substantial hot-water demand, but the flow number alone does not tell you how well a unit will perform in your home. Incoming water temperature, desired output temperature, fuel type, and how many fixtures run at once all matter. The picks below include gas models rated at or near 9 GPM and an electric option advertised at up to 8 GPM, so compare the specifications carefully before choosing.
Table of Contents
Quick Comparison
| Product | Fuel and installation | Key specifications | Best for |
|---|---|---|---|
| Rheem RTGH 9.0 GPM Natural Gas | Natural gas; indoor, direct vent | 180,000 BTU; 0.93 UEF; 9.0 GPM at 35°F rise | Large homes with natural-gas service |
| Rinnai CU160iN | Natural gas; indoor | Commercial model; 9 GPM; Circ-Logic recirculation | Commercial hot-water demand |
| ThermoMate 36kW Electric | Electric; 240V | Advertised up to 8.0 GPM; estimated 7.03 GPM at 35°F rise | Homes prepared for a high electrical load |
| Rheem RTGH 9.0 GPM Liquid Propane | Liquid propane; indoor, direct vent | 180,000 BTU; 0.93 UEF; 9.0 GPM at 35°F rise | Large homes using propane |
| Rinnai RXP160iN | Natural gas or propane; indoor or outdoor with required vent cap | Up to 9 GPM; 160,000 BTU; 0.97 UEF; built-in recirculation | Homes seeking flexible installation and recirculation |
Our Picks: Best 9 8 Gpm Tankless Water Heater reviews
1. Rheem RTGH 9.0 GPM Natural Gas: High-Capacity Indoor Pick
This indoor Rheem is a natural-gas condensing tankless water heater designed for whole-home use. Its listed output is 9.0 GPM at a 35°F temperature rise, and its 180,000 BTU rating gives it a strong capacity for a large household. The stated flow rate is tied to that specific temperature rise, so your actual output may differ depending on the temperature of your incoming water.
The unit has a 0.93 Uniform Energy Factor, direct-vent installation, freeze protection, and a digital control with temperature adjustment to within ±1°F. Rheem also lists CSA certification and ultra-low NOx compliance. The heat exchanger warranty is 15 years, with a five-year parts warranty.
This is a practical choice if your home already has natural-gas service and you need an indoor unit for several bathrooms. Its main trade-off is that it is not a fuel-flexible option: it is the natural-gas version, and professional installation is recommended. Confirm venting, gas supply, and local requirements with a qualified installer before buying.
Pros:
- Rated at 9.0 GPM at a 35°F rise
- Condensing design with a stated 0.93 UEF
- Digital temperature control with ±1°F precision
- Includes freeze protection and a 15-year heat exchanger warranty
Cons:
- Requires natural-gas service and indoor direct-vent installation
- Professional installation is recommended
Verdict: Choose this Rheem if you want a high-capacity indoor natural-gas model and its 35°F-rise rating fits your household’s needs.
2. Rinnai CU160iN: Commercial Natural-Gas Model with Recirculation Support
The Rinnai CU160iN is a commercial natural-gas condensing model intended for indoor installation. It is listed at 9 GPM and includes Circ-Logic, which supports recirculation patterns based on typical usage. Multiple venting options are also noted, giving an installer some flexibility when planning the setup.
This model may suit a business or another setting with steady hot-water demand. Its commercial classification and warranty terms are important to consider: the listed commercial warranty is eight years for the heat exchanger, five years for parts, and one year for labor. Those terms differ from the residential warranty on the Rinnai RXP160iN.
It is less straightforward as a default choice for a typical home because it is specifically presented as a commercial water heater. The product information also recommends professional installation and does not recommend installing it without a technician. If you are considering it for residential use, check suitability and warranty coverage with Rinnai or a qualified installer first.
Pros:
- Listed 9 GPM hot-water flow rate
- Condensing design with Circ-Logic recirculation support
- Multiple venting options are listed
- Commercial warranty includes heat exchanger, parts, and labor coverage
Cons:
- Commercial model, so confirm it is appropriate for your intended use
- Requires natural gas and professional installation
Verdict: Consider the CU160iN for a commercial application that needs natural gas and recirculation support, but verify its suitability before choosing it for a home.
3. ThermoMate 36kW Electric: High-Output Option for Electrically Ready Homes
The ThermoMate is a 36kW, 240-volt electric tankless water heater. The title advertises a maximum flow of 8.0 GPM, but its description gives an estimated 7.03 GPM at a 35°F rise. That distinction matters: compare the stated flow at your expected temperature rise rather than treating the maximum figure as a guaranteed whole-home output.
Its self-modulating system adjusts power to match hot-water demand, and the unit has a digital display with temperature adjustments from 80°F to 140°F in one-degree steps. The listed installation requirements are substantial: it draws up to 150 amps, calls for four dedicated double-pole 40A breakers and four 8 AWG copper wires, and strongly recommends a minimum 200-amp main service. A qualified electrician is essential to assess whether your home can support it.
Compared with the gas models here, this option avoids a gas connection but places a large demand on your electrical system. It is ETL certified and lists dry-fire, overheat, and anti-freezing protections. The electrical requirements are the key practical drawback, so check your panel and installation costs before deciding.
Pros:
- Electric operation for homes without a gas connection
- Self-modulating heating and one-degree temperature adjustments
- ETL certification and several listed safety protections
- Compact wall-mounted design
Cons:
- Estimated flow is 7.03 GPM at a 35°F rise, below the advertised 8.0 GPM maximum
- Requires a high-capacity electrical setup and professional installation
Verdict: This ThermoMate may fit a home with verified electrical capacity, but its demanding power requirements and flow-rate distinction deserve careful attention.
4. Rheem RTGH 9.0 GPM Liquid Propane: Whole-Home Capacity Without Natural Gas
This Rheem model shares the main specifications of the natural-gas version but is configured for liquid propane. It is rated at 9.0 GPM at a 35°F rise, uses a 180,000 BTU condensing system, and has a stated 0.93 UEF. The indoor unit uses direct venting and is designed for whole-home hot-water supply.
Its feature set includes a digital LED temperature control with ±1°F precision, built-in freeze protection, and a multi-point safety system. Rheem lists CSA certification, ultra-low NOx compliance, a 15-year heat exchanger warranty, and five years of parts coverage. These are useful specifications, but they do not remove the need to verify that the installation is appropriate for your home.
The main choice between this and the natural-gas Rheem is the fuel supply. This version is for homes using propane, while the similar natural-gas model is for homes with natural-gas service. Do not assume the two versions are interchangeable; choose the model that matches your fuel system and have a professional confirm the setup and venting requirements.
Pros:
- Rated at 9.0 GPM at a 35°F rise
- Condensing design with a stated 0.93 UEF
- Digital temperature control and built-in freeze protection
- Long heat exchanger warranty listed
Cons:
- Designed for liquid propane, not natural gas
- Indoor direct-vent installation and professional setup are needed
Verdict: Pick this Rheem over the natural-gas version only if propane is the right fuel for your home and the stated capacity fits your demand.
5. Rinnai RXP160iN: Flexible Fuel and Built-In Recirculation
The Rinnai RXP160iN is a residential condensing tankless water heater rated to provide up to 9 GPM. Its Smart Sense gas control system supports either natural gas or propane, and the unit can be installed indoors or outdoors. Outdoor installation requires the RX-OVC vent cap, so include that requirement in your planning.
A standout feature is the built-in pump with Smart-Circ, which learns hot-water usage patterns and schedules recirculation accordingly. The model has a stated 0.97 UEF, a Quick Flush feature, and a compact design. Wi-Fi control requires a separate Controlr module, so it is not included as a built-in feature.
Its flexibility makes it a strong alternative to the single-fuel Rheem models, especially if you value recirculation or need more installation options. The residential warranty is listed as 15 years on the heat exchanger, five years on parts, and five years on labor when registered within 90 days of installation; otherwise, the labor warranty defaults to one year. Professional installation is recommended.
Pros:
- Works with natural gas or propane
- Indoor or outdoor installation is possible with the required outdoor vent cap
- Built-in pump and Smart-Circ recirculation features
- Stated 0.97 UEF and residential warranty coverage
Cons:
- Outdoor installation requires a separate RX-OVC vent cap
- Wi-Fi control requires a separate module
Verdict: The RXP160iN is a compelling choice if you want a residential model with fuel flexibility and built-in recirculation, provided you account for its additional installation parts and registration terms.
How to Choose a 9 GPM Tankless Water Heater
Match the fuel to your home
Start by checking whether your home uses natural gas, propane, or electricity. The Rheem models in this list are fuel-specific, while the Rinnai RXP160iN supports natural gas or propane. The ThermoMate is electric, but its high electrical demand makes a panel assessment especially important. Do not select a unit based only on its advertised flow rate if the fuel or utility setup is a poor fit.
Understand flow rate and temperature rise
GPM means gallons per minute, but a tankless heater’s output depends on how much it must raise the incoming water temperature. The Rheem models list 9.0 GPM at a 35°F rise, and the ThermoMate lists an estimated 7.03 GPM at that same rise despite advertising up to 8.0 GPM. For other models, the supplied product information does not specify a temperature rise for the 9 GPM figure. Ask the manufacturer or installer for output at your local incoming-water temperature.
Estimate simultaneous hot-water use
Think about which fixtures and appliances may run at the same time, rather than counting the total number in the home. Several showers running together can create a different demand than one shower and a sink. Your installer can help estimate peak use and compare it with the heater’s output at the temperature rise your home needs.
Check venting and installation location
These models have different installation requirements. The Rheem units are indoor models with direct venting, while the Rinnai RXP160iN can be installed indoors or outdoors when the specified outdoor vent cap is used. Venting, clearances, and local code requirements should be confirmed before purchase. The Rinnai CU160iN also calls for professional installation and is listed as a commercial model.
Compare efficiency and recirculation features
UEF provides a way to compare stated energy performance, but efficiency is only one part of your decision. The Rheem models list a 0.93 UEF, and the Rinnai RXP160iN lists 0.97. The Rinnai models include recirculation-related features, which may be useful depending on your plumbing layout and hot-water habits. Check what accessories or additional controls are required; Wi-Fi control for the RXP160iN, for example, requires a separate module.
Review warranties and registration conditions
Warranty coverage varies by model and by whether the unit is classified for residential or commercial use. The RXP160iN’s listed labor warranty depends on registration within 90 days of installation. The CU160iN has commercial warranty terms that differ from the residential models. Read the full warranty details and confirm who can install the unit without affecting coverage.
Plan for professional installation
Gas, venting, electrical, and plumbing work can affect safety and performance. The product information recommends professional installation for the gas models, and the ThermoMate’s listed electrical requirements call for a qualified electrician. Before ordering, ask an installer to verify your utility capacity, fuel type, venting path, electrical service, and local code requirements.
Frequently Asked Questions
Is a 9 GPM tankless water heater enough for a whole house?
It may be, but the answer depends on your incoming-water temperature, desired temperature rise, and how many fixtures operate at once. A 9 GPM figure does not guarantee that every home can run several showers and appliances simultaneously. Compare the heater’s output at your expected temperature rise with your household’s peak demand.
Why does the ThermoMate list both 8.0 GPM and 7.03 GPM?
The product title advertises a maximum of 8.0 GPM, while its description estimates 7.03 GPM at a 35°F rise. Those figures describe different conditions or ratings, so use the specified temperature-rise figure for a more useful comparison and ask the manufacturer to confirm performance for your installation.
What is the difference between the two Rheem models?
The 9.0 GPM natural-gas Rheem is designed for natural gas, while the similar 9.0 GPM model is designed for liquid propane. Both are indoor direct-vent units with a listed 180,000 BTU rating, 0.93 UEF, and 9.0 GPM output at a 35°F rise. Choose the version that matches your fuel supply.
Can the Rinnai RXP160iN be installed outdoors?
Yes. Its product information lists indoor or outdoor installation, but outdoor installation requires the RX-OVC vent cap. Confirm the setup with a qualified installer before purchase.
Does the Rinnai RXP160iN include Wi-Fi control?
No. The product information says Wi-Fi control requires a separate Controlr module. The unit does include a built-in pump and Smart-Circ recirculation features.
Should I install a tankless water heater myself?
These products have gas, venting, plumbing, or high-load electrical requirements. Their product information recommends professional installation, and the ThermoMate specifically calls for a qualified electrician. A professional can also check local codes and confirm that your home’s utilities can support the unit.
Final Recommendation
For a residential natural-gas installation, the Rheem RTGH offers a clear 9.0 GPM rating at a 35°F rise. The propane Rheem is the corresponding option for homes using liquid propane. If you want built-in recirculation and support for either gas type, the Rinnai RXP160iN offers more flexibility, though its outdoor vent cap and optional Wi-Fi module should be factored into your plans. The ThermoMate is worth considering only after confirming that your electrical service can handle its substantial requirements. Whichever model you choose, have a qualified installer verify capacity, fuel, venting, and code fit before ordering.
