Rheem Tankless Water Heater Size Calculator
Find the perfect Rheem tankless water heater model for your home based on flow rate, temperature rise, and usage patterns.
List of the Best Rheem Tankless Water Heater:
Rheem Tankless Water Heater Size Calculator: Your Complete Guide
Our Rheem tankless water heater size calculator makes it easy to select the right unit based on your home’s needs. Whether you're choosing a gas or electric model, proper sizing is critical to ensure reliable hot water during peak usage and maximize energy efficiency. This guide explains how sizing works, what factors to consider, and which Rheem models are best suited for your setup.
Why Choose Rheem Tankless Water Heaters?
Rheem is one of the most trusted names in residential and commercial water heating. Their tankless water heaters are known for durability, performance, and innovative features like Wi-Fi control, recirculation technology, and energy-saving modes. With options for both gas and electric systems, Rheem offers solutions for small apartments to large family homes.
How the Rheem Tankless Water Heater Size Calculator Works
This sizing calculator helps you estimate your total hot water demand in gallons per minute (GPM) and determine the required temperature rise. Based on that, it recommends the right Rheem model for your home. The calculator considers:
- Number of bathrooms
- Number of hot water appliances used simultaneously
- Local groundwater (incoming) temperature
- Type of heater: gas or electric
What is Flow Rate (GPM)?
Flow rate is the amount of water your household will use at once, measured in gallons per minute. For example, if you’re running a shower and dishwasher simultaneously, your system may need to supply 4.0 to 5.0 GPM. Each fixture or appliance has its own average GPM usage:
| Fixture/Appliance | Approximate GPM |
|---|---|
| Shower | 2.0–2.5 |
| Bathtub | 4.0 |
| Bathroom Sink | 0.5–1.0 |
| Kitchen Sink | 1.0–1.5 |
| Dishwasher | 1.5 |
| Washing Machine | 2.0–2.5 |
What is Temperature Rise?
Temperature rise is the difference between the temperature of incoming cold water and your desired hot water temperature (usually 120°F). If your incoming water is 50°F and you want 120°F, your unit must provide a 70°F rise.
Average Incoming Water Temperatures by Region
- Northern U.S.: 40–50°F
- Midwest: 45–55°F
- Southern U.S.: 55–70°F
Rheem Gas Tankless Water Heater Sizing
Rheem’s gas tankless models deliver high flow rates and fast recovery, making them ideal for whole-house systems. Their Performance Platinum and Professional Prestige lines offer models rated from 6.8 GPM up to 11+ GPM depending on climate and installation.
Popular Gas Models
- Rheem Performance Platinum 6.8 GPM: Suitable for small homes and warmer climates.
- Rheem RTGH-95DVLN: High-efficiency condensing unit for up to 3 bathrooms.
- Rheem Commercial Series: Large-capacity models for homes with 4+ bathrooms.
Rheem Electric Tankless Water Heater Sizing
Electric Rheem tankless units are great for point-of-use or small to medium homes. They are energy efficient and easy to install but require adequate amperage and voltage.
Popular Electric Models
- Rheem RTEX-13: Compact unit ideal for a single bathroom or sink.
- Rheem RTEX-18: Good for one shower and a sink in warmer climates.
- Rheem RTEX-27 or RTEX-36: Whole-home options with multiple bathroom support.
FAQs: Rheem Tankless Water Heater Size Calculator
How do I know which Rheem unit is right for me?
Use this Rheem tankless water heater size calculator to enter your bathroom count, appliance usage, and incoming temperature. It will recommend a model based on flow and temperature rise needs.
Can I use a Rheem electric unit for my whole home?
Yes, but only if your home has the required electrical capacity (typically 150A or more). The RTEX-36 is the most powerful residential electric unit from Rheem.
Do Rheem units work in cold climates?
Yes. Rheem gas models are designed to work in incoming temperatures as low as 37°F. You may need a higher GPM-rated model to compensate for the extra temperature rise.
What size do I need for a family of 4?
Families of 4 typically need 6–9 GPM depending on usage. For gas, the RTGH-95DVLN is a solid choice. For electric, consider RTEX-27 or RTEX-36 if power is available.
What’s better: gas or electric Rheem tankless units?
Gas is better for high flow and whole-house use. Electric is great for small homes or remote applications. Use this calculator to find the best fit based on your needs.
Installation Tips
- Gas units require proper venting (direct or power vent models).
- Electric units need double-pole breakers and correct wire gauge.
- All units should be installed by licensed professionals for warranty protection.
Conclusion: Choose the Right Rheem Tankless Model with Confidence
With this Rheem tankless water heater size calculator, you can make informed decisions quickly and easily. Whether you're upgrading an older model or installing for new construction, the right sizing ensures your Rheem unit performs efficiently, delivers continuous hot water, and avoids overload. Bookmark this page and refer back anytime you or your clients need accurate water heater sizing recommendations.