What Size Tankless Water Heater Do I Need for My Boise Home?

July 31, 2019

In the market for a new tankless water heater?

The easiest (and most accurate) way to size your tankless water heater is to have a plumber calculate the size for you. That way, you don’t have to spend time crunching numbers.

But, if you want more context about how tankless units are sized and want to give it a try yourself, we’ll give you a step-by-step guide.

First, let’s go into more detail about why size matters when it comes to your tankless water heater. 

Want a FREE quote (and sizing help) for a new tankless unit? We can help!

Why the correct size matters

The “size” (heating capacity) of a tankless water heater is measured by flow rate. Flow rate refers to how many gallons of hot water a tankless unit can provide per minute (measured in gallons per minute, or gpm). 

The higher the flow rate, the more hot water a tankless unit can provide at one time.

It’s important to get a tankless water heater that’s sized appropriately for the following reasons:

  1. A tankless unit that’s too small can’t provide enough hot water for all of your appliances.
  2. A tankless unit that’s too big means you’ll spend extra money on a unit you don’t actually need.

Now that you know why size is important, let’s look at how to size a tankless unit correctly.

How to size a tankless water heater

Follow the steps below to calculate what size tankless water heater is right for your home.

Step #1: Estimate your home’s flow rate

First, you’ll need to calculate your household’s total flow rate.

To do this, start by listing all of the hot water appliances your family uses at one time. Then, add up each appliance’s average flow rate. 

How do you know the flow rate for each appliance?

Here is a list of common hot water appliances with their average flow rates:

  • Kitchen sink: 2–4 gpm
  • Dishwasher: 1–3 gpm
  • Washing machine: 2–4 gpm
  • Shower: 1–3 gpm
  • Tub: 3–5 gpm

To get precise flow rates for each appliance, you can look at the fixture itself or contact a professional for help measuring the exact flow rate.

Example: Let’s say your household often runs the shower and kitchen sink at the same time. If that were true, your household’s average flow rate would be 3–7 gpm.

Step #2: Note the temperature rise you need

After you’ve calculated your household’s average flow rate, you’ll need to account for the temperature rise.

Temperature rise is the difference between the incoming cold water temperature and your normal hot water temperature.

In Boise, the average groundwater temperature is 45° F.

Most water heaters are set to 120° F. So, the minimum temperature rise you’ll need is the difference between 45 degrees and 120 degrees, 75° F. 

Step #3: Search for a unit that matches your flow rate and temperature rise

Now that you know your home’s average flow rate and temperature rise, you can start looking for a tankless unit that matches your needs.

If you’re looking online, you can look under the product specifications to find a tankless water heater’s flow rate (gpm) and the temperature rise (see image below).

If we were to use the average household flow rate example in Step #1, this Navien tankless water heater would likely work for that home. (This tankless unit provides 5 gallons of hot water per minute at a 75° temperature rise.

Want a professional to help you find the right size tankless unit for your Bosie home?

We can help you calculate what size tankless water heater is best for your home. We can also give you a FREE quote on the cost to install the tankless unit.

For more information about what to expect when you hire us, visit our tankless water heater installation page.


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