Rheem Tankless Water Heater Size Calculator

Looking to install a Rheem tankless water heater but unsure which model fits your needs? This calculator helps you quickly estimate the right Rheem tankless water heater for your home based on how many hot water fixtures you’ll use at once, your local incoming water temperature, and your desired output temperature. Whether you’re upgrading your system or building new, this calculator is a fast and reliable way to get started.

Number of hot water devices running at once:

Incoming water temperature (°F):

Desired hot water temperature (°F):

How to Use the Rheem Tankless Water Heater Size Calculator?

Step 1: Enter the Number of Devices in Use

Start by estimating how many hot water appliances or fixtures may be used at the same time in your home. This includes items like showers, kitchen or bathroom faucets, washing machines, and dishwashers. Each one contributes to the total water flow demand. If you expect two people might be showering while the dishwasher runs, enter “3”.

Step 2: Input the Incoming Water Temperature

Enter the temperature of the water coming into your home. In cold climates, this could be as low as 40°F, while in warmer areas, it might reach 60–70°F. If you’re unsure, a general estimate like 50°F is a good starting point. The calculator uses this to determine how much your system needs to heat the water.

Step 3: Set the Desired Hot Water Temperature

Choose the temperature you want your hot water to reach. Most homes aim for 120°F, which is hot enough for comfort and hygiene without being scalding. The difference between this value and your incoming water temperature is called the temperature rise — a key factor in heater sizing.

Step 4: Click the “Calculate Recommendation” Button

After filling in the fields, press the Calculate Recommendation button. The calculator will process your data and show:

The estimated kilowatts (kW) required to meet your demand
A suggested Rheem model that can handle the flow rate and temperature rise

This helps you choose a properly sized heater from Rheem’s RTEX series without needing technical specs or trial and error.

Frequently Asked Questions (FAQ)

What is GPM and why does it matter?

GPM stands for gallons per minute, which measures how much hot water you use at once. Each shower or faucet typically uses 1.5 to 2.5 GPM. The calculator multiplies your number of devices by an average flow rate to estimate your total demand.

What if my fixtures are low-flow or high-efficiency?

If your home uses low-flow fixtures, you might be able to enter fewer devices than actual. For example, if your shower uses only 1.2 GPM, it may count as half a typical fixture. Adjusting device count to reflect real water usage makes the estimate more accurate.

What happens if I choose a model that’s too small?

An undersized tankless water heater won’t be able to meet your peak demand. This means you could run out of hot water when multiple devices are in use, or experience fluctuations in temperature. It’s best to follow the calculator’s recommendations or choose a slightly larger unit for flexibility.

Can I oversize the heater just in case?

Yes, you can choose a larger model than the one recommended. Oversizing offers extra capacity and performance margin, especially if your usage may grow. However, larger units draw more electrical power, so be sure your home’s electrical system can support it.

Are these results specific to Rheem products?

Yes. The calculator is tailored for Rheem’s electric tankless water heaters, especially the RTEX series. It estimates heater power based on Rheem’s rated performance at specific temperature rises. While other brands may perform similarly, sizing may vary.

What if my incoming temperature changes by season?

If you live in a region with cold winters and warmer summers, it’s best to size your heater based on the coldest incoming water temperature you expect during the year. This ensures you’ll always have enough hot water, even in winter.

Is this tool suitable for gas-powered Rheem models?

No. This calculator is designed specifically for electric Rheem tankless water heaters. Gas models use different performance and sizing considerations, including gas flow rate (BTU/hr) and venting requirements. For gas units, refer to Rheem’s gas sizing charts or consult a technician.

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