You accept the use of cookies as described in our Privacy Policy by clicking 'OK'.
Making sure you have a properly sized water heater is very important. If your water heater is too small, your home won't have enough hot water to meet demand, which is an obvious inconvenience, and overloading the unit may cause premature wear and tear.
There's no benefit to oversizing a tankless water heater. If your tankless water heater is too large it will never operate up to full capacity, and the system may cost more than necessary.
Tankless Water Heater Size Calculator
Our free Navien water heater sizing tool, NaviSizer, is especially effective in helping people find the right size water heater for their project.
NaviSizer uses information you provide, including number of people in your household, number of bathrooms, fuel type and region (for local groundwater temperature) to help you choose the right size model.
Video: How do you size a tankless water heater?
How Tankless Water Heater Sizing Works
There are two primary factors to consider when determining the right size water heater.
The first is the maximum flow rate required by the hot water system, measured in gallons per minute (GPM). Maximum flow rate is determined by the number of fixtures (e.g. washing machine, dishwasher, shower) that will be used at the same time, and the flow rate of each.
For example, if your home would use a dishwasher (2-3 GPM), a shower (2-3 GPM) and a washing machine using hot water (3-5 GPM) all at once, that would be a maximum flow rate of 7-11 GPM.
These are estimated flow rates - a licensed plumber can provide a more accurate estimate for the fixtures in your home. Your water pipe sizing, especially the incoming water supply, can also affect flow volume.
Temperature rise is the other critical factor. Temperature rise is simply the difference between the desired temperature setting of the water heater and the inlet water temperature.
For example, if the hot water temperature is set to 120°F and if the incoming water temperature is 55°F the resulting temperature rise is 65 degrees (120-55=65).
Typically, residential hot water temperature settings are 120°F or 125°F, and commercial temperatures can range from 140°F to 180°F, depending on the application.
Since incoming water supply temperature can vary based on the season, temperature rise should be calculated based on the coldest days of the year.
When you you know the required flow rate and the temperature rise, you can compare them to the tankless water heater output.
You can find this information on each Navien tankless water heater model page on our website, in the QuickFacts section under Temperature Rise. Maximum flow rate can also be found in the QuickFacts download for each model.
Domestic Hot Water (DHW) Capacity
Tankless water heater capacity is rated based on the gas energy input, which is given in BTU/h (British Thermal Units per hour). You’ll see this measurement when comparing different models of Navien tankless water heaters. The higher BTU, the greater the maximum flow a tankless unit can produce.
Finding Your Tankless Water Heater Size
To find the correct size tankless water heater, you need to use the maximum possible temperature rise and the highest required flow rate.
Choose a unit with a flow rate equal to or greater than the maximum hot water demand.
To help you get started, use these links to see the flow rate by temperature rise for popular Navien tankless water heaters:
Condensing:
- NPE-180A2
- NPE-210A2
- NPE-240A2
- NPE-2 Series QuickFacts PDF (all models)
Non-Condensing:
- NHW-160AI
- NHW-180AI
- NHW-199AI
- NHW-A Series QuickFacts PDF (all models)
Or explore all Navien tankless water heaters.
How to Quickly Determine Your Tankless Water Heater Size
To make the process of purchasing the correctly sized water heater easier, Navien has created NaviSizer. By providing basic information about your project, you can determine the correct size of water heater in as little as one minute.
If you're looking for the right size water heater for a family of 4 or a family of 5 (for example), or for a house with one or two (or more) bathrooms, simply enter that information to see recommended tankless water heaters with required BTU/h and GPM as well as estimated operating costs, energy savings and more.
What Size Tankless Water Heater Replaces Your Tank?
Sizing a tankless water heater is different from sizing a tank water heater. Tank water heaters are sized based on the number of gallons the tank can store (for example 40 gallon, 50 gallon, or 75 gallon). This limits how much water a tank-style water heater can provide.
Tankless water heaters are measured by hot water flow rate in GPM (gallons per minute). There’s no equivalent flow rate for storage tank capacity.
It’s not accurate to say that a tankless water heater with a specific flow rate will always replace a tank water heater of a certain size, since tankless operates differently by providing endless hot water.
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.
How Many Tankless Water Heaters Do You Need?
When sized properly, one tankless water heater can provide an endless supply of hot water meeting all your household needs.
For homes with extra high hot water demand, or commercial applications, Navien NPE-2 Series tankless water heaters can be cascaded, which means two or more units work together as flow demands.
When one unit reaches 80% capacity, the next unit will turn on to help meet hot water requirements. This not only generates more hot water, it balances out the workload to extend service life.