Rheem Tankless Water Heater Size Calculator
Calculate which Rheem tankless water heater best fits your needs based on your hot water usage and temperature requirements.
List of the Best Rheem Tankless Water Heater:
Why Use a Rheem Tankless Water Heater Size Calculator?
Choosing the correct tankless water heater size is essential to ensure that your household always has hot water when it’s needed. Rheem is one of the most trusted names in the industry, known for producing reliable and efficient electric tankless water heaters. Our Rheem tankless water heater size calculator is designed specifically to help homeowners, plumbers, and contractors find the perfect Rheem model for their specific water usage and temperature needs.
How Does the Rheem Tankless Water Heater Size Calculator Work?
This calculator uses two key inputs to determine the ideal model:
- Total Flow Rate (GPM): This measures the amount of hot water needed per minute across all fixtures in use.
- Temperature Rise (°F): This is the difference between your incoming groundwater temperature and your desired output temperature (typically 120°F).
Using this data, the calculator computes the required kilowatt (kW) output, then cross-references this with available Rheem models to give you a precise match.
Why Size Accuracy Matters for Rheem Units
Rheem offers a wide range of electric tankless models—from compact 13kW units to high-capacity 36kW systems. Installing a properly sized heater ensures:
- Reliable hot water delivery during peak usage times
- Maximum energy efficiency
- Longer system lifespan
- Reduced utility costs
Undersizing may result in lukewarm showers and user frustration, while oversizing increases electricity costs and installation complexity.
Rheem Electric Tankless Models at a Glance
| Model | Max GPM | Max Temp Rise (°F) | Power (kW) | Best For |
|---|---|---|---|---|
| RTEX-13 | 3.0 | 60 | 13 kW | Point-of-use or small apartment |
| RTEX-18 | 4.4 | 65 | 18 kW | Small homes or moderate demand |
| RTEX-24 | 5.9 | 70 | 24 kW | 2–3 fixtures simultaneously |
| RTEX-27 | 6.6 | 75 | 27 kW | Average-sized homes |
| RTEX-36 | 8.0 | 80 | 36 kW | High-demand homes |
Common Fixture Flow Rates
Use this list to estimate your total GPM usage:
- Showerhead: 1.5–2.5 GPM
- Bathroom faucet: 0.5–1.0 GPM
- Kitchen sink: 1.0–2.2 GPM
- Dishwasher: 1.5–2.0 GPM
- Washing machine: 2.0–2.5 GPM
If you run a shower and a sink at the same time, your required GPM could easily exceed 3.5–4.0. Use our calculator to ensure your Rheem unit can keep up.
Tips for Sizing and Efficiency
- Choose a lower desired output temperature (e.g., 110°F instead of 120°F) to reduce energy demand.
- Install low-flow fixtures to reduce GPM requirements.
- Insulate hot water pipes to preserve heat during transport.
- Use point-of-use heaters in areas far from your main water supply line.
FAQs About Rheem Tankless Water Heater Sizing
What if I live in a cold climate?
Colder incoming water means a higher temperature rise is required. For example, if your inlet temperature is 40°F and your desired output is 120°F, you’ll need an 80°F rise. Larger Rheem units like the RTEX-27 or RTEX-36 are ideal for colder regions.
Can I install two Rheem units together?
Yes. For large homes or multiple bathrooms, you can install two Rheem tankless water heaters in parallel to split the demand and maintain hot water supply efficiently.
What voltage is required for Rheem electric models?
Most high-capacity Rheem units require a 240V connection. Lower capacity models may run on 208V but perform best on 240V circuits.
Do Rheem electric models support outdoor installation?
Most Rheem electric units are designed for indoor use. For outdoor tankless options, Rheem’s gas models are better suited with proper weatherproofing.
Final Thoughts
Finding the right model is easier than ever with our Rheem tankless water heater size calculator. Whether you're replacing an old system or building a new home, sizing your water heater correctly is the first and most important step toward comfort, efficiency, and long-term performance. Rheem offers versatile, reliable options—and with our tool, you can pick the one that's just right for your home.