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Water Heater Sizing

It’s easy to determine the correct size of the tankless water heater needed for your application by following some simple guidelines.

Some Basic Information

The five variables to be considered in sizing and selecting the unit include:

  • The volume of water the unit is required to heat, measured as flow rate of Gallons Per Minute (GPM).
  • The temperature of the cold water entering the unit.
  • The desired temperature of the hot water exiting the unit.
  • Location of your unit, “Outdoor” or “Indoor” installation.
  • Fuel Type to be used, “Natural Gas” or “Propane Gas”.

Tankless water heaters are rated differently from that of a traditional “tank style” water heater. Unlike the standard total gallon capacity that a “tanked” water heater is rated in, a tankless water heater is rated in (“GPM”) (gallons per minute) which measures water flow. Water flow rate is based on the unit’s ability to heat water at a given GPM which is based on the incoming cold water temperature and the requested or required hot water temperature.

Correctly sizing a tankless water heater for an application can be done in the following simple steps.

1. First, estimate the total amount of hot water that you may be using at one given time by adding the projected output of all the fixtures you might run at the same time.

General Guideline Water Flow Table for Fixtures

 Average Full Flow Water Rates for Standard Items
 Bathtub  Shower  Kitchen / Bathroom Sink  Laundry Sink  Washer

 2.0 - 3.0 gpm

 1.5 - 2.5 gpm

 1.0 - 1.5 gpm

 1.5 - 2.0 gpm

 1.0 - 2.8 gpm

For example, if you wanted to be able to run a standard shower and a washer at the same time, you would have a total usage of about 2.5 to 5.3 gallons per minute depending on the water flow rate of your shower and washer fixtures.

Please keep in mind that most fixtures use a mixture of both hot and cold water which means that hot water output is slightly reduced. For example, for normal use, a single shower head’s water flow, where the total flow rate of the shower calls for 2.5 gpm, is actually a mixture of both hot and cold water made up of 2.125 gpm of hot (85%) and .375 gpm of cold (15%) water. (Washing machines on hot water cycles do not require an adjustment because hot water flow is at the maximum temperature). For those fixtures where normal use requires a mixture of hot and cold water output, a slight adjustment may be taken to reflect reduced hot water flow.

Try to be as realistic as possible on your hot water demands when doing this. If you have a total of 3 showers in a home but truly only use two at one time, then calculate your demand accordingly.

Now add up your total demand based on your needs at any one given time and keep that number. This total is known as your (“Total GPM water demand flow”).

2. Second, find your area's average groundwater temperature on the map below.

After you have identified your listed areas average groundwater temperature, subtract it from 120°F degrees (most tankless manufacturers set their default temperature at 120°F), or (the required hot water temperature you would like).

This difference in temperature between the two numbers is called the “Temperature Rise”. (Note: 120°F is hot enough for most applications. However if hotter water is needed, a higher number can be substituted.)

3. Final Calculation - Now you should have two key numbers.

1. “Total GPM water demand flow” of hot water your system application requires
2. “Temperature Rise” that your systems unit will work under to achieve it.

Based on these findings, you should have a realistic guideline of what size unit your application requires, its location and its fuel type.

1. Your total hot water demand flow in GPM _?_ at the Temperature Rise _?_.
2. Location of unit (indoor or outdoor) _?_.
3. Fuel type (natural gas or propane) _?_.

With the above information you can now refer to a tankless model unit’s performance data sheet and correctly select your unit.

Rinnai Tankless Water Heater Flow Chart Navien Tankless Water Heater Flow Chart Noritz Tankless Water Heater Flow Chart

Please note: As a wholesaler, we understand that whether you are a contractor, building owner, management company or a homeowner, this is a large investment. If you are unsure of your sizing requirements, please feel free to contact our customer service dept. by calling 1-800-414-8575 or emailing us at We will be happy to assist you with your selection.