How To Size A Tankless Water Heater

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  • Author Dimitri Svolos
  • Published April 16, 2007
  • Word count 578

The most important (and often neglected or misunderstood) aspect of purchasing and installing an electric tankless water heater is selecting the proper model

for a specific household demand.

So, how do one sizes a tankless water heater for optimal on-demand hot water delivery?

The process of sizing a tankless water heater involves six steps:

1.Temperature Rise

The entire operation of a tankless water heater is based on how hot you want your water to be when you use it. This is called the “temperature rise” and is a

simple subtraction of the incoming water temperature from the water temperature that your tankless water heater would be set to deliver. . A comfortable

shower temperature should not be much higher than 105°F (temperatures above 125°F are scalding). Lower temperatures will also prevent scale deposits from

forming, which may affect the operation of the heater in the long run.

  1. Flow Rate

After determining the temperature rise, you need to calculate how much water (in Gallons per Minute or GPM) your tankless water heater could deliver (if needed) at this temperature. This is called Peak Demand and is a simple addition of the flow rates of all the sources in your house that could be running


For your convenience, here is a table of average flow rates per source:

Fixture Type:0.5

Lavatory: 2.0-4.0

Bathtub Shower:1.5-3.0

Kitchen Sink:1.0-1.5

Pastry Sink:1.5-2.5

Laundry Sink:2.5-3.0



Many retailers sell units that will accommodate 110V, 120V, 208V, 220V, 240V, and 277V

4.Amperage/ Power

Different electric tankless water heaters will have various requirements in amp draw and power output. You will want to ensure that you can support the electrical demands of your Electric Tankless Water Heater. Typically, the power output of a Tankless Water Heater is calculated using the formula: Kw Rating = (GPM x Temperature Rise) / 6.83. For example, if you want a temperature rise of 45 degrees F and your Peak Demand is 4 GPM then you are looking for a tankless water heater model with power rating of 26 kW (45 x 4 / 6.83).

5.Circuit Breaker

You must ensure that you have a circuit or circuits that will support your Electric Tankless Water Heater. It may be necessary to put your Electric Tankless

Water Heater on its own circuit or circuits

6.Potisioning/ Piping.

Prior to installation and sizing, it is important to determine the position of your tankless water heater in respect to the delivery points. The further

the tankless water heater is located, the longer the piping needed. Longer piping means more heat losses in the long run, as well as, more possibilities of

fouling after years of use. If you are installing one tankless water heater, chose a central location that could serve all your points of use easier. If you are installing multiple tankless water heaters, position them close to the points of use they will serve.

There are a lot of tankless water heater models to choose from. The general

market tendency is “The bigger, the better”. By following these six simple steps outlined here, you will be able to determine fast and easy which tankless

water heater size and model will best serve your needs.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. Please ensure that you

consult a qualified technician prior to installing a tankless water heater or any suggestions put forward by e-SmartLiving. There are no warranties, either

expressed or implied, regarding the accuracy or completeness of this information.

Dimitri Svolos is Chemical Engineer that has spent years working as a process and controls optimizer for various manufacturing sectors. His website advocates energy efficient and healthy lifestyles.

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