How to Size a Tankless Water Heater – For Small, Medium & Large Family

A tankless water heater is a device that works automatically for heating water directly without storing it in a tank.

Tankless water heaters are increasingly popular nowadays due to the instant flow of water, less power consumption, low maintenance expenditures, long life span, and easy installation.

If you are ready to install a tankless water heater at home but facing trouble making the right choice about the size of a tankless water heater, then no worries at all. We’re going to help you find out the perfect size of tankless water heater for your home.

Factors to Consider Before Sizing a Tankless Water Heater

An infographic about How to size a tankless water heater
  • Your Home size
  • Rate of water flow in your home
  • Household size
  • Temperature rise
  • Power Source

Before making a purchase, just go through these mentioned steps for the accurately sized tankless water heater.

Step 1: Calculate GPM by Measuring Hot Water Flow Rate

“GPM is a unit for measuring how many hot water gallons could run through fixture per minute.”

You can calculate it by adding the flow rate of each fixture running at your home that uses hot water. For example, you run a hot water faucet with a flow rate of almost 0.85 gallons per minute and a shower head with a flow rate of 2.6 gallons per minute. The demand of flow rate through a water heater will be 3.45 gallons per minute.

With a quick Google search, you can locate information regarding GPM estimate charts.

If you don’t want to face a shortage of hot water then you have to overestimate the number of fixtures running in the home at the same time.

Step 2: Find Out the Temperature Rise

After finding the amount of water needed to use, you should also consider the need for temperature rise for warming water.

For this, first you have to determine the groundwater temperature that is needed to be risen.

To calculate the temperature rise, subtract the incoming groundwater temperature from the desired water temperature.

Let’s suppose, your groundwater temperature is 45 degrees Fahrenheit, and your target water temperature is almost 106 degrees Fahrenheit.

Temperature Rise= Desired water temperature – groundwater temperature

106-45 = 61

So, a 61 degrees Fahrenheit of temperature will be the temperature rise for your tankless water heater.

tankless water heater flow rate in winter
Tankless water heater flow rate in winter

Step 3: What’s Your Power Source?

The next thing to consider for your water heater is the type of power source that can either be gas or electric and this depends totally on your preference.

Electric tankless water heaters suit best when there is short demand for water at home. It is mostly preferred in areas having warm groundwater temperatures and small GPM and heating requirements.

While Gas water heaters are perfect to choose for homes with a high demand for water.

For all this process, you should first consider the availability of power sources. Like do you have the facility of electricity or gas? This can help you to make mind but the other thing is the price of gas or electric, either any of it is affordable or not.

Gas tankless heaters can produce large volumes of hot water faster as compared to electric. It is also less expensive due to gas price but this price can vary in different areas.

Electric tankless water heaters are affordable and very easy to install. Its simple installation reduces the labor cost. If your area does not have shortage of electricity every hour, it will be best choice for you while in other case you’ve to install the battery backup.

you can get a piece of well-researched information about electric water heaters here

What Size of a Tankless Water Heater a Family of Four Needs?

In an average-sized family of four, we need to consider how many water appliances are running hot water at the same time.

To find this, first of all, the calculation of maximum GPM will be done by adding up the flow rate of each fixture.

Suppose the following appliances are running at the same time to supply hot water:

  • Dishwasher (1.5GPM)
  • Sink (2.5GPM)
  • Two Showers running at same time (2GPM)

A total of 6GPM will be efficient for the family of four.

For calculating temperature rise, let’s consider the groundwater temperature is 70 degrees Fahrenheit while the desired temperature is almost 120 degrees Fahrenheit. The rise in temperature for it would be 50 degrees Fahrenheit.

The Size of Tankless Water Heater should be enough to replace 50 Gallon Storage Tanks:

A tankless water heater should have enough power to fill the need for a lot of hot water, especially for multiple showers. So, a tankless water heater provides hot water on demand instead of storing it when you need.

How Much Sized A Tankless Water Heater I Need?

If you want to calculate the size of your own tankless water heater, just follow these steps to calculate it, these are some steps as mentioned above:

  1. Find out GPM by adding fixtures installed and being used in your home.
  2. Calculate the temperature rise by the help of subtracting desired temperature from ground water temperature.
  3. When you have calculated these two important aspects for selecting a tankless water heater, the only thing you will have to decide the type of power source according to your needs and choice.

The choice you have to make is between a gas tankless water heater and an electric tankless water heater. You can go through their specifications thoroughly and then decide accordingly. Gas or electric availability and pricing range help a lot in taking a final decision to get a properly sized tankless water heater.

About Zarmish Fakhar

Zarmish has been working as a freelance research writer for 5 years. She is a Zoologist and social media activist as well. She has worked for many reputable newspapers including Dawn. She is a valuable member of Aqua macho as her passion for clean water and eco-friendly products are booming day by day.

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