Rheem vs Thermomate: Tankless Water Heaters Compared by Specs
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This rheem vs thermomate tankless water heater comparison highlights two distinct electric solutions. Both units operate on electricity for reliable hot water delivery. The Rheem model is a true tankless system designed for whole-house use with high flow rates. The Thermomate unit is a compact mini-tank intended for point-of-use installation under sinks. You need to decide between continuous flow and stored hot water based on your specific household needs.
Key differences
| Aspect | Rheem 18kW 240V Electric Tankless Water Heater | Thermomate 6.5-Gallon Under-Sink Electric Mini-Tank |
|---|---|---|
| Type | Tankless | Mini-tank (6.5 Gallons) |
| Use | Whole House | Point of Use (Under Sink) |
| Wattage | 18 kW | 1.44 kW |
| Voltage | 240 V | 120 V |
| Flow Rate | 4.4 GPM | Not listed |
Which one should you choose?
Choose Rheem 18kW 240V Electric Tankless Water Heater if
- You need hot water for multiple fixtures simultaneously.
- You have a 240V electrical circuit available.
- You want an on-demand system with no storage tank.
Choose Thermomate 6.5-Gallon Under-Sink Electric Mini-Tank if
- You only need hot water for a single sink or faucet.
- You want to plug the unit into a standard 120V outlet.
- You prefer a compact unit that stores water for fast recovery.
Verdict: Choose the Rheem if you require high-volume hot water for whole-house applications. Select the Thermomate if you need a simple, low-power solution for a single under-sink location.
Rheem vs Thermomate Spec Comparison
Use Case
You need to decide where hot water matters most. The Rheem 18kW unit is designed for whole-house use. It delivers up to 4.4 gallons per minute, enough for multiple fixtures. The Thermomate 6.5-gallon unit is an under-sink mini-tank. It targets point-of-use locations like a bathroom or kitchen sink. It provides instant hot water for a single faucet. You cannot run a shower on the Thermomate. If you need hot water for the entire home, the Rheem is the only choice. If you just need a warm hand wash, the Thermomate fits that specific spot.
Flow Rate
Flow rate determines how much hot water you get at once. The Rheem unit offers a flow rate of up to 4.4 GPM. This supports simultaneous use, such as a shower and a sink. The Thermomate does not list a continuous flow rate. It is a storage tank that holds 6.5 gallons. Once that hot water is used, you must wait for recovery. The Rheem heats water on demand continuously. The Thermomate relies on its small tank capacity. For high demand, the Rheem provides a clear advantage in volume and consistency.
Voltage
Your home’s electrical setup dictates which unit you can install. The Rheem requires a 240V connection. This is standard for high-power electric tankless heaters but needs specific circuit wiring. The Thermomate runs on a standard 120V outlet. It plugs into a regular wall socket. This makes the Thermomate easier to install in locations without heavy electrical infrastructure. You do not need a dedicated breaker panel upgrade for the Thermomate. If you have a standard kitchen or bathroom outlet, the Thermomate is simpler. If you have a 240V circuit, the Rheem is ready.
Wattage
Wattage shows how much power the unit uses to heat water. The Rheem draws 18 kW of power. This high wattage allows it to heat water quickly for whole-house needs. The Thermomate uses only 1.44 kW. This lower power is sufficient for a small tank but cannot heat large volumes instantly. The Rheem’s higher wattage ensures consistent temperature at high flow rates. The Thermomate’s low wattage limits its output to a single faucet. You trade power for convenience with the Thermomate. You get high performance with the Rheem, but it demands more electrical capacity.
Dimensions
Space is often limited in kitchens and bathrooms. The Rheem is compact, measuring 14.5 inches wide, 3.5 inches high, and 18.5 inches long. It weighs 14.8 pounds. It mounts on a wall and takes up minimal vertical space. The Thermomate is larger and boxier. It measures 17.7 inches wide, 14.6 inches deep, and 21.2 inches high. It weighs 26.5 pounds. The Rheem is slimmer and lighter, making it easier to fit in tight utility closets. The Thermomate needs more floor or shelf space under a sink. If space is tight, the Rheem is more versatile.
Temperature Control
Precision matters for comfort and safety. The Rheem features an external digital thermostat with LED display. It offers ±1°F accuracy. You can set the exact temperature you want. The Thermomate has a thermostat with three level settings. It ranges from 50°F to 149°F. It also includes a freeze mode. The Rheem provides finer control over your water temperature. The Thermomate offers basic settings with safety protections. If you need precise temperature adjustments, the Rheem is superior. The Thermomate is adequate for general use but less precise.
Choose the Right Unit for Your Needs
Homeowner needing whole-house hot water
This Rheem unit handles whole-house demands with a 4.4 GPM flow rate. Its 18kW power and 240V connection ensure consistent heating. You get precise temperature control with ±1°F accuracy. The compact 18.5-inch length fits easily in tight spaces. It is built for continuous use without running out of hot water.
User wanting instant hot water at one sink
This Thermomate mini-tank provides instant hot water for point-of-use applications. The 6.5-gallon capacity ensures you do not wait for heating. It plugs into a standard 120V outlet, making installation simple. The adjustable thermostat ranges from 50°F to 149°F. This unit is ideal for under-sink mounting where space is limited.
DIYer seeking easy 120V installation
The Thermomate model connects directly to a 120V outlet, avoiding complex electrical work. Its 12-amp nominal current suits standard circuits. The included 2-foot power cord adds flexibility. You can mount it on the wall or floor. This setup is straightforward for renters or those avoiding new wiring runs for high-voltage appliances.
User prioritizing precise digital temperature control
The Rheem heater features an external digital thermostat with LED display. You can adjust settings with ±1°F accuracy. This precision prevents temperature fluctuations during use. The self-modulation technology adjusts power to match demand. This level of control is superior for households requiring consistent water temperatures across multiple fixtures.
The products compared
Rheem 18kW 240V Electric Tankless Water Heater
Best for: Whole-house electric hot water demand
- 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 tankless water heater designed for whole-house use. It delivers up to 4.4 GPM at 240V. The unit features external digital thermostatic control with LED display and ±1°F accuracy. Its compact dimensions of 3.5 x 14.5 x 18.5 inches allow for flexible installation. You get durable copper immersion heating elements that are field serviceable. This model suits homes needing reliable, on-demand hot water with precise temperature management.
Pros
- 18 kW power output for whole-house demand
- High flow rate of up to 4.4 GPM
- Precise temperature control with ±1°F accuracy
- Field serviceable copper heating elements
- Compact 3.5-inch depth for easy mounting
Cons
- Requires 240V electrical service
- No wireless app or remote control connectivity
- Heavier at 14.8 pounds than point-of-use models
Good fit if
- You need a high-flow unit for multiple bathrooms
- You want precise temperature stability for comfort
- You have space for a compact wall-mounted unit
Skip if
- You only have 120V electrical outlets available
- You need a small point-of-use heater under a sink
- You prefer wireless smart home integration
Verdict: Rheem RTEX-18 18kW Electric Tankless is a robust whole-house solution for homes with adequate electrical capacity. It delivers consistent hot water with precise control, making it ideal for households prioritizing performance and reliability over smart features.
Thermomate 6.5-Gallon Under-Sink Electric Mini-Tank
Best for: single sink point-of-use hot water
- Fuel Type
- Electric
- Wattage
- 1.44 kW
- Capacity
- 6.5 Gallons
- Voltage
- 120V
- Temperature Control
- 50°F to 149°F
- Certification
- UL Listed
View on Amazon: Thermomate 6.5-Gallon Under-Sink Electric Mini-Tank Affiliate link
This Thermomate unit is a 6.5-gallon electric mini tank designed for point-of-use installation under sinks. It operates on a standard 120V circuit with 1.44 kW of power. The glass-lined tank offers 99% thermal efficiency and includes a UL Listed safety rating. You get an adjustable thermostat ranging from 50°F to 149°F. Its compact dimensions allow for wall or floor mounting. This heater suits small households needing localized hot water without waiting.
Pros
- 6.5-gallon capacity provides instant hot water for point-of-use tasks
- 120V plug-in connection simplifies installation into standard outlets
- Adjustable thermostat offers three temperature settings from 50°F to 149°F
- UL Listed certification ensures safety compliance
- Glass-lined tank provides durable insulation and corrosion resistance
Cons
- 1.44 kW power output is low for high-demand applications
- 6.5-gallon capacity limits simultaneous use with other fixtures
- Only a 3-year warranty covers the tank
Good fit if
- You need hot water for a single bathroom sink
- You want to install the unit under a counter or sink
- You prefer a standard 120V electrical connection
Skip if
- You require hot water for multiple bathrooms at once
- You need a high-flow rate for showers or tubs
- You are looking for a whole-house heating solution
Verdict: Thermomate 6.5 Gal Under Sink Water Heater is a practical solution for small spaces. It fits buyers who need reliable, localized hot water for sinks without complex plumbing.
Specifications side by side
| Spec | Rheem 18kW 240V Electric Tankless Water Heater | Thermomate 6.5-Gallon Under-Sink Electric Mini-Tank |
|---|---|---|
| Fuel Type | Electric | Electric |
| Flow Rate | 4.4 GPM | not stated |
| Wattage | 18 kW | 1.44 kW |
| Capacity | not stated | 6.5 Gallons |
| Voltage | 240 V | 120V |
| Temperature Control | ±1°F | 50°F to 149°F |
| Certification | not stated | UL Listed |
| Dimensions | 3.5 x 14.5 x 18.5 inches | 17.7"H x 17.7"W x 14.6"D |
| Use | Whole House | Under Sink |
| Weight | 14.8 pounds | 26.5 pounds |
| Size | Small | 6.5 Gallon |
| Color | Gray | white,blue |
| Model | RTEX-18 | ES700 |
Frequently asked questions
Is Rheem a better brand than Thermomate for tankless water heaters?
Rheem is a major manufacturer of traditional tankless systems, while Thermomate focuses on compact under-sink units. The Rheem RTEX-18 is a true tankless heater designed for whole-house use. Thermomate offers a 6.5-gallon mini-tank for point-of-use applications. Choose Rheem for high flow rates and Thermomate for small space efficiency.
What is the difference between Rheem and Thermomate installation requirements?
The Rheem RTEX-18 requires a 240V electrical connection and adequate capacity for whole-house heating. It weighs 14.8 pounds and measures 18.5 inches in length. The Thermomate ES unit operates on 120V power. It is heavier at 26.5 pounds and designed for under-sink installation. Verify your home’s electrical panel supports the chosen voltage.
Does Rheem make smaller tankless heaters like Thermomate?
The Rheem RTEX-18 is a full-capacity 18kW unit for whole-house needs. It is not a mini-tank. Thermomate provides a 6.5-gallon storage tank for single sink use. Rheem does not list a mini-tank in this comparison. If you need localized hot water for one faucet, the Thermomate option fits that specific use case.
Which unit is better for a bathroom sink?
The Thermomate 6.5-gallon unit is ideal for a single sink. It provides localized hot water without complex plumbing. The Rheem RTEX-18 is overkill for one fixture. It is designed for whole-house demand with a 4.4 GPM flow rate. Use Thermomate for point-of-use and Rheem for multiple simultaneous fixtures.
How do Rheem and Thermomate dimensions compare for mounting?
The Rheem RTEX-18 is small, measuring 14.5 inches wide and 3.5 inches high. It fits easily on a wall. The Thermomate ES is taller at 21.2 inches high and 17.7 inches long. It is designed to fit under a sink. Measure your available space carefully, as the form factors differ significantly.
Can Thermomate replace a Rheem whole-house heater?
No. The Thermomate ES is a 6.5-gallon mini-tank for single sinks. It cannot supply hot water to multiple fixtures. The Rheem RTEX-18 delivers up to 4.4 gallons per minute for whole-house use. They serve different purposes. You need Rheem for whole-home coverage and Thermomate for specific, isolated locations.