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March 18, 2023

Sizing Tankless Water Heaters

Making sure you have a properly sized water heater for your residential or commercial application is very important. If you end up with a water heater that’s too small, your home (for example) will not have enough hot water to meet the demand, which is an obvious inconvenience, and overloading the unit may cause premature wear and tear.

Oversizing a tankless water heater may not cause any issues, but if the heater is too large it will never operate up to full capacity, and the system may cost more than necessary. Our Navien water heater sizing tool, NaviSizer, is especially effective in helping people pair the right water heater with the right application. 

How Tankless Water Heater Sizing Works

There are two primary factors that need to be considered when determining the right size water heater for your application. The first is the maximum flow rate required by the hot water system, which is measured in gallons per minute (GPM). Maximum flow is determined by the number of fixtures that will be used at the same time, and the flow rate of each of those fixtures. The system water pipe sizing, especially the incoming service supply, can also have an effect on flow volume. 
Temperature rise is the other critical factor. Temperature rise is simply the difference between the desired set temperature of the water heater, and the inlet water temperature from the building supply. For example, the normal residential set temperature is 120 degrees Fahrenheit, and if the incoming water temperature is 55 degrees the resulting temperature rise is 65 degrees (120-55=65). Typically, residential set temperatures are 120 or 125 degrees, and commercial temperatures can range from 140 to 180 degrees depending on the application. Since building supply temperature can vary throughout the year based on the season, temperature rise should be calculated based on the coldest days of the year.   
Knowing the flow rate required, and the temperature rise of the application, those factors can be compared to the output of the tankless water heaters. Tankless units are rated based on the gas energy input, which is given in BTU (British Thermal Unit). You’ll see this measurement when comparing different models of Navien Tankless Water Heaters.  The higher the gas BTU input, the greater the maximum flow a tankless unit can produce.

Finding Your Tankless Water Heater Size

To find the correct size of Navien tankless water heater for your application, you need to use the maximum possible temperature rise and the highest required flow rate. Comparing the required flow rate, and the system temperature rise, to the specifications of the tankless water heaters will determine the size required.  The higher the BTU input, the greater the flow the unit can produce, which must be equal or greater to the system demand.

how to find the right size tankless water heater

Tankless Water Heater Sizing Calculator 

To make the process of purchasing the correctly sized water heater for your needs, Navien has created NaviSizer. By filling in all the information about your tankless water heater application, you can determine the correct size of water heater you need in as quickly as one minute.

To try NaviSizer click here.


What Size Tankless Water Heater Replaces Your Tank? 

If you are replacing a traditional water storage heater with a Navien tankless water heater, there is not a way to select a tankless size based on the tank capacity, since they operate differently. Tank water heaters are measured by how much water they can store at once, but tankless water heaters are measured by hot water flow rate in gallons per minute (GPM). It's not accurate to say that a tankless water heater with a specific flow rate will always replace a tank water heater with a certain capacity. Licensed plumbing professionals can help you best determine the correct tankless water heater to replace your tank, and can also help explain the differences between them. A professional plumbing contractor can also highlight the other features available on a tankless water heater that can help improve the installation and operation of the unit.


What Size Tankless Water Heater for Your Household? 

Proper sizing is important prior to select and installing a tankless, but in general the average rules for sizing the correct water heater for a household are roughly: 

  • 3.5 GPM for 1-2 fixtures simultaneously 
  • 5 GPM for 2-3 fixtures simultaneously 
  • 7 GPM for 3-4 fixtures simultaneously 

Although these numbers can obviously vary per household, these values can be used as a preliminary estimate of what unit may be needed for most homes.


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