Rinnai Tankless Water Heater Sizing Calculator

Not sure which Rinnai tankless water heater is right for your home? This calculator helps you choose the best model by estimating your household’s peak hot water demand and the temperature rise required. Whether you’re installing a new unit or replacing an old one, this tool considers how many fixtures you use at once, your incoming water temperature, and your desired output temperature.

How to Use the Rinnai Tankless Water Heater Sizing Calculator?

Step 1: Enter the Number of Devices Running at the Same Time

Begin by estimating how many hot water fixtures or appliances you expect to use simultaneously. This includes showers, faucets, dishwashers, and washing machines. For example, if two people are likely to shower while the kitchen sink runs, you should enter “3”. This step helps calculate your total hot water flow rate in gallons per minute (GPM).

Step 2: Input the Incoming Water Temperature

Next, type in the temperature of the cold water entering your home. This varies depending on where you live and the season. In colder climates, it could be as low as 40°F, while warmer areas might be closer to 60°F. If you’re unsure, using an estimate like 50°F provides a reliable baseline.

Step 3: Set the Desired Hot Water Temperature

Most homes prefer a hot water temperature around 120°F. You can adjust this based on your comfort preferences. The calculator uses the difference between this and your incoming water temperature (called the temperature rise) to determine how much energy is needed to heat the water to your desired level.

Step 4: Click the “Calculate Recommendation” Button

After filling in all inputs, press the Calculate Recommendation button. The calculator will estimate:

• Your total GPM (based on number of devices)
• Required temperature rise (°F)
• Total BTU/hr output required to meet demand
• A recommended Rinnai tankless model (such as RU160iN or RU199iN) that matches your needs

Use this information to select a Rinnai water heater that delivers reliable performance without being oversized or inefficient.

Frequently Asked Questions (FAQ)

Why does the number of devices matter?

Each hot water device draws a certain amount of water per minute. Showers typically use 1.5–2.5 GPM, faucets 1–1.5 GPM, and dishwashers or washing machines around 2 GPM. The more devices you run at once, the more hot water your heater must supply instantly.

What is temperature rise and why is it important?

Temperature rise is the difference between your incoming cold water temperature and your desired hot water output. A larger temperature rise means your heater must work harder to heat water quickly. Rinnai models can provide higher GPM at lower temperature rises, and less GPM at higher rises.

How is the BTU/hr requirement calculated?

The calculator uses a simplified formula to estimate your home’s energy demand for water heating:
BTU/hr = GPM × Temperature Rise × 500
This gives a rough estimate of the burner output required. Rinnai’s gas tankless models range up to 199,000 BTU/hr, making them suitable for even large households.

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

If the unit is undersized, it may not be able to provide enough hot water when multiple fixtures are used at the same time. This could lead to lower flow rates or inconsistent water temperature, especially during winter when the incoming water is colder.

Can I choose a larger model than recommended?

Yes, oversizing slightly is okay — and sometimes preferred — especially if your water demand might increase in the future. However, larger models may cost more and require larger gas supply lines and venting systems, so balance your needs and installation conditions.

Is this calculator for gas or electric Rinnai models?

This tool is specifically designed for gas-powered Rinnai tankless water heaters, such as the RU Series (e.g., RU160iN, RU199iN). Electric models are sized differently and are typically used for point-of-use or smaller applications.

Can I use this tool for commercial sizing?

No, this calculator is intended for residential use. Commercial environments — such as restaurants, gyms, or hotels — require more detailed sizing based on peak hour demand, recovery time, and fixture type. Rinnai offers commercial calculators and multi-unit solutions for that purpose.

What if I have a recirculation loop or large home?

If your home has long plumbing runs, recirculation, or multiple bathrooms, it’s wise to consider a higher-capacity unit or even multiple units in parallel. This calculator gives a general recommendation, but unique home layouts may require further analysis by a plumber or installer.

Check out our reviews of the best tankless water heaters below:

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• Best Natural Gas Tankless Water Heater
• Best Non-Condensing Tankless Water Heater
• Best RV Tankless Water Heate