Rheem vs A. O. Smith: Tankless Water Heaters Compared by Specs
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When comparing rheem vs a o smith tankless water heater, you face two distinct approaches. Both brands deliver endless hot water, but their technologies differ significantly. Rheem offers a compact electric unit, while A. O. Smith provides a high-capacity natural gas model. This guide breaks down the key specs to help you choose the right system for your home.
Key differences
| Aspect | Rheem 18kW 240V Electric Tankless Water Heater | A. O. Smith 10 GPM Outdoor Condensing Tankless Heater |
|---|---|---|
| Fuel Type | Electric | Natural Gas |
| Flow Rate | 4.4 GPM | 10.0 GPM |
| Installation Location | Indoor | Outdoor |
| Venting Type | Not listed | Condensing |
| Wattage / BTU | 18 kW | 199,000 BTU/hr |
Which one should you choose?
Choose Rheem 18kW 240V Electric Tankless Water Heater if
- You need a compact unit for a small space or point-of-use application.
- You prefer electric power and do not have a gas line available.
- You want precise temperature control with an external digital display.
Choose A. O. Smith 10 GPM Outdoor Condensing Tankless Heater if
- You require high flow rates for a large home with multiple bathrooms.
- You have an outdoor space suitable for a condensing gas unit.
- You want energy efficiency with a high Uniform Energy Factor rating.
Verdict: Choose the Rheem unit if you need a compact electric solution for smaller demands. Select the A. O. Smith model if you need high capacity for a large household with gas access. Your choice depends entirely on your fuel type and hot water volume needs.
Rheem vs A. O. Smith Specs
Fuel type
You must choose between two different energy sources. The Rheem RTEX-18 runs on 240V electricity. This makes it suitable for homes with ample electrical capacity but no gas line. The A. O. Smith ATO-540H-N uses natural gas. It delivers 199,000 BTU/hr of heating power. Gas models often provide higher flow rates for larger households. Electric units are simpler to wire but depend on grid availability. Your existing utility infrastructure dictates which fuel type is viable for your installation site.
Flow rate
Flow rate determines how many fixtures can run simultaneously. The Rheem unit provides up to 4.4 gallons per minute. This is sufficient for a small bathroom or point-of-use sink. The A. O. Smith model delivers up to 10.0 gallons per minute. This high capacity supports a three-bath home in northern climates or a four-bath home in southern regions. If you need hot water for multiple showers at once, the gas unit handles the demand much better than the electric option.
Dimensions
Physical size affects where you can mount the unit. The Rheem heater is compact, measuring 3.5 inches by 14.5 inches by 18.5 inches. It weighs only 14.8 pounds. This small footprint allows installation in tight closets or utility spaces. The A. O. Smith unit is significantly larger at 23.6 inches by 17.8 inches by 17.8 inches. It weighs 59 pounds. Its size requires more wall space and structural support. You need ample room for the larger gas model.
Installation location
Location constraints vary by technology. The Rheem electric heater is designed for indoor use. It features simple bottom water connections for easy replacement. The A. O. Smith model is rated for outdoor installation. It uses a direct vent design that allows for flexible venting up to 100 feet using various pipe types. This outdoor rating means you can place it outside your home, saving interior space. You cannot install the A. O. Smith unit indoors.
Temperature control
Precision matters for comfort. The Rheem unit includes an external digital thermostatic control with an LED display. It offers plus or minus 1 degree Fahrenheit accuracy. This allows you to set exact temperatures for specific needs. The A. O. Smith data does not specify digital temperature accuracy in the provided specs. It includes a wall mount temperature remote controller for convenience. However, the Rheem offers verified precision in its control system, which helps maintain stable output.
Which tankless heater fits your home?
Small household with electric service
The Rheem RTEX-18 delivers 4.4 GPM at 18 kW. Its compact 3.5-inch height fits tight spaces. The ±1°F digital control ensures precise temperature stability. You get endless hot water for a single bathroom or small kitchen without needing gas lines or complex venting.
Large family needing high flow
The A. O. Smith ATO-540H-N provides up to 10.0 GPM. This high flow rate supports multiple simultaneous showers in larger homes. The 199,000 BTU output ensures consistent performance. You benefit from commercial-grade copper heat exchangers for superior heat transfer and durability in high-demand scenarios.
Outdoor installation with gas access
This unit is designed for outdoor use with direct vent capabilities. It supports venting up to 100 feet using standard pipes. The condensing technology offers high efficiency. You save indoor space and enjoy stable water temperature with advanced safety controls, including overheat cut-off and air-fuel ratio sensors.
The products compared
Rheem 18kW 240V Electric Tankless Water Heater
Best for: Compact whole-house electric hot water
- Fuel Type
- Electric
- Flow Rate
- 4.4 GPM
- Wattage
- 18 kW
- Voltage
- 240 V
- Temperature Control
- ±1°F
- Dimensions
- 3.5 x 14.5 x 18.5 inches
View on Amazon: Rheem 18kW 240V Electric Tankless Water Heater Affiliate link
The Rheem RTEX-18 is a 18 kW electric unit designed for whole-house use. It delivers up to 4.4 GPM at 240V. The compact gray body measures 14.5 by 18.5 by 3.5 inches and weighs 14.8 pounds. You get external digital control with ±1°F accuracy. This model suits homes needing reliable, space-saving hot water without complex venting.
Pros
- 18 kW output for whole-house supply
- High flow rate of 4.4 GPM
- ±1°F temperature precision
- Compact 3.5-inch depth
Cons
- Requires 240V electrical circuit
- No remote control included
- Electric unit needs adequate panel capacity
Good fit if
- You have a 240V electrical panel
- You need a compact whole-house unit
- You want precise temperature control
Skip if
- You only have 120V outlets
- You need gas fuel type
- You want remote operation
Verdict: Rheem RTEX-18 Electric Tankless Heater offers strong 4.4 GPM flow in a slim profile. It fits homes with 240V power seeking whole-house coverage.
A. O. Smith 10 GPM Outdoor Condensing Tankless Heater
Best for: Large homes needing high outdoor flow
- Fuel Type
- Natural Gas
- Flow Rate
- 10.0 GPM
- BTU
- 199,000 BTU/hr
- Installation Location
- Outdoor
- Dimensions
- 23.6 x 17.8 x 17.8 inches
- Venting Type
- Condensing
View on Amazon: A. O. Smith 10 GPM Outdoor Condensing Tankless Heater Affiliate link
This A. O. Smith unit delivers up to 10.0 GPM of hot water using 199,000 BTU of natural gas. Its condensing design achieves a .94 UEF, making it highly efficient for outdoor installation. The commercial-grade copper heat exchanger offers superior heat transfer. It includes a wall-mount remote controller and supports direct venting up to 100 feet. This model suits homes with three to four bathrooms requiring high flow rates.
Pros
- High 10.0 GPM flow rate for multiple simultaneous uses
- Efficient condensing technology with .94 UEF rating
- Commercial-grade copper heat exchanger for better heat transfer
- Includes wall mount temperature remote controller
- Flexible direct venting up to 100 feet
Cons
- Outdoor-only installation limits placement options
- Heavier build at 59 pounds requires sturdy mounting
- No indoor variant listed for this specific high-capacity model
Good fit if
- You need hot water for three to four bathrooms
- You have an outdoor space for direct venting
- You want high efficiency with a .94 UEF
Skip if
- You require indoor installation only
- You need propane fuel instead of natural gas
- You have limited wall space for a 59-pound unit
Verdict: A. O. Smith 10 GPM Outdoor Natural Gas Heater is ideal for large households needing high flow rates outdoors. Its copper heat exchanger and efficient condensing design make it a robust choice for northern or southern climates with multiple bathrooms.
Specifications side by side
| Spec | Rheem 18kW 240V Electric Tankless Water Heater | A. O. Smith 10 GPM Outdoor Condensing Tankless Heater |
|---|---|---|
| Fuel Type | Electric | Natural Gas |
| Flow Rate | 4.4 GPM | 10.0 GPM |
| BTU | not stated | 199,000 BTU/hr |
| Wattage | 18 kW | not stated |
| Installation Location | not stated | Outdoor |
| Voltage | 240 V | not stated |
| Temperature Control | ±1°F | not stated |
| Dimensions | 3.5 x 14.5 x 18.5 inches | 23.6 x 17.8 x 17.8 inches |
| Use | Whole House | not stated |
| Venting Type | not stated | Condensing |
| Weight | 14.8 pounds | 59 pounds |
| Size | Small | not stated |
| Color | Gray | White |
| Model | RTEX-18 | ATO-540H-N 100 |
Frequently asked questions
Which Rheem tankless water heater is the best choice for a small home?
The Rheem RTEX-18 is the top pick for compact whole-house electric hot water. It delivers 4.4 GPM flow using 18 kW of power at 240V. Its slim 3.5-inch height fits tight spaces easily. This unit is ideal for homes with strong electrical capacity but no gas line.
What is the best outdoor tankless heater for large families?
The A. O. Smith ATO-540H-N 100 is designed for large homes needing high outdoor flow. It produces 10 GPM using natural gas. The condensing design and copper heat exchanger make it robust for multiple bathrooms. It is suitable for both northern and southern climates.
How do Rheem and A. O. Smith differ in fuel type?
Rheem offers an electric model that runs on 240V power. A. O. Smith provides a natural gas unit that uses 199,000 BTU. You must choose between electricity for homes with ample electrical capacity or natural gas for those with a gas line available.
Which brand offers a more compact tankless water heater?
Rheem offers a significantly more compact unit. The RTEX-18 measures only 3.5 inches in height and weighs 14.8 pounds. In comparison, the A. O. Smith model is larger, standing 23.6 inches tall and weighing 59 pounds. The Rheem design fits better in tight indoor spaces.
Is the A. O. Smith heater suitable for outdoor installation?
Yes, the A. O. Smith ATO-540H-N 100 is built for outdoor use. It features a condensing venting type and a durable white exterior. This model is ideal for households needing high flow rates in outdoor settings, providing robust performance for large families.
What is the flow rate difference between these two models?
The A. O. Smith model provides a much higher flow rate. It delivers 10 GPM, making it suitable for large households. The Rheem unit offers 4.4 GPM, which is sufficient for smaller homes or single-bathroom setups. The difference depends on your household size and usage.
Does the Rheem heater require gas venting?
No, the Rheem RTEX-18 does not require gas venting. It is an electric unit that runs on 240V power. This eliminates the need for gas lines or complex venting systems. It is a simpler installation option for homes with adequate electrical service.