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What Size Tankless Water Heater Do I Need? (With Sizing Calculator)


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Purchasing a tankless water heater is a major investment. It'll increase the value of your home, lower your utility expenses, and provide your family with an endless stream of hot water. One of the biggest decisions you'll need to make is determining what size tankless water heater you need. Buy a tankless too small, and your showers will be lukewarm at best. Buy one too large, and you'll be spending  more money than necessary.

We've put together a tankless water heater sizing calculator to help you find the right size tankless for your household. In addition, this article covers the most commonly asked questions that new tankless buyers ask. You may also want to check out our sizing guide for tank-style water heaters.

Tankless Water Heater Sizing Calculator

Where are you located?

US Incoming Water Temperature Map

Use our  Sizing Calculator to determine your: 

Flow Rate  and Temperature Rise

What is a Tankless Water Heater

Tankless water heaters heat and deliver hot water when there's a demand for hot water. You may hear them called on-demand water heaters because they do not hold hot water in a tank, but rather heat the water only when there's a demand. 

When a hot water faucet is opened, cold water flows into the tankless water heater. The cold water is heated as it passes through the heat exchanger. When the hot water leaves the tankless water heater it travels through your household's plumbing to the hot water faucet.

Once the hot water faucet is turned off, the demand for hot water is over, and the tankless water heater shuts down.

Tankless water heaters can deliver an endless stream of hot water since they are not limited by their size of a tank, like a traditional tank-style water heater.

Read our Water Heaters Buying Guide: Tankless or Tank-Style to learn more about the differences between between these two types of water heaters.

Watch the Video

Five glasses with various levels of water

What Does it Mean to Size a Tankless Water Heater?

Sizing a tankless water heater simply means to find the right sized appliance to meet your hot water needs. 

There are several factors that need to be determined to properly size a tankless water heater:

  1. Peak Hour which is the hour of day when your household will need the most hot water.
  2. Demand which is the amount of hot water fixtures and appliances used during the peak hour.
  3. Temperature Rise is how hot the cold water needs to be heated. If the incoming water is 55°F, and your desired hot water temperature is 120°F, then the temperature rise will be 65°F (120 - 55). The temperature rise is determined by where you live.

Our Tankless Water Heater Sizing Calculator will guide you through the answers to all of these questions, and determine the Gallons Per Minute (GPM) your tankless heater will need to produce.

Once you know your GPM, you'll be ready to shop for your new tankless water heater.

Map of United States based on ground water temperatures

Why is Where I Live Important?

The ground water temperature is determined by where you live, which determines your incoming water temperature. Since the incoming water temperature impacts the temperature rise, where you live plays a significant role in selecting a tankless water heater that can deliver enough hot water during high demand periods. 

Temperature rise is difference between your desired hot water temperature and the incoming cold water. In other words, if the incoming water is 50°F and you want your tankless heater to heat your water to 115°F, the temperature rise will be 65°F (115°F - 50°F).

When purchasing a tankless water heater, manufacturers list the Gallons Per Minute (GPM) of hot water the unit can deliver at a variety of temperature rises. You'll need to find one that matches your required GPM at the temperature rise the unit will need to achieve.

If this sounds complicated, you don't need to worry, our Tankless Water Heater Calculator will ask the right questions so you'll know exactly how powerful of a unit to purchase.

Man checking his wrist watch

What Does "Peak Hour" Mean?

The term peak hour is used to determine the hour of day your household will use the most hot water. This is important because tankless water heaters need to be able to deliver enough hot water when the demand is the highest. 

In most cases, the peak hour is in the morning, but your household routines may be different. 

Once you know when your hot water peak hour is, pay attention to how many showers or baths are used. If the clothes washer or dish washer is running. Or is anyone shaving? 

Any activities that require hot water will need to be factored into your peak hour usage. 

You'll need to purchase a tankless water heater that's powerful enough to service any hot water demands that occur simultaneously

Close up of a shower head

What Does GPM Mean?

When you hear the term Gallons Per Minute (GPM), think flow rate

Traditional tank-style water heaters are sized by how many gallons of hot water can be stored in their tank.

But tankless water heaters are sized by their flow rate. 

The flow rate is simply how many gallons of water the tankless water heater can produce each minute.

GPM is the measurement used to determine the flow rate of a tankless water heater. The higher the GPM, the more hot water the unit can deliver.

A tankless heater with a flow rate of 8 GPM can deliver more hot water than a tankless with a flow rate of 6 GPM. 

To determine the GPM your household needs, you'll need to count the number of fixtures (showers, hand sinks, washing machines, etc) used during your peak hour. The temperature rise will also need to be taken into consideration.

Keep in mind, you only need to count the fixtures that are being used at the same time. If your family takes 3 showers during your peak hour, but only 2 showers are taken at the exact same time, you'll only need to count two showers.

Family of Five Sitting in their Front Yard

What Size Tankless Does a Family of 5 Need?

Unfortunately, there is no one-size-fits-all when it comes to tankless water heaters. Even when you compare the same number of family members.

There's a lot of things to consider, such as your climate, your peak hour of hot water usage, and the number of fixtures and appliances running. 

Working through the tankless sizing calculator will allow you to tailor your demand to your individual needs. So you can find the right sized tankless for your household.

With that said, you'll most likely need a tankless water heater capable of delivering at least 7 gallons of hot water per minute (GPM) to meet the needs of a family of 5.

Electric bolt and gas droplet on a scale

Gas or Electric: Which is Best?

Both gas and electric tankless water heaters have their advantages, and if you're lucky enough to be able to decide (sometimes you won't have a choice), you should definitely spend the time to research which is the best fit for your household.

Although, this topic is beyond the scope of this article, we highly recommend checking out our article: Gas vs. Electric Tankless Water Heater: Which is Best? 

In the article we compare 9 factors (such as operational costs, installation costs, maintenance requirements, etc.) head-to-head to help you make an informed decision.

As a general rule, you can expect that an electric tankless water heater will be easier to install, require less maintenance, and have a lower purchase price.

Where a gas tankless heater is capable of delivering a higher GPM, requires more maintenance, and is more expensive to install.

Crown with Top Brands printed on red ribbon

Which Brand of Tankless Water Heater is Best?

A quality tankless water heater that's properly maintained can last over 20 years, so choosing a reputable manufacturer is an important decision.

Unfortunately, there's some less than stellar tankless manufacturers, but there are also plenty of high quality companies that offer quality products, solid warranties, and excellent customer service. 

We recommend researching:

Each of these companies have been in the business for a long time and have multiple product lines.

EcoSmart is a newer player that's gaining in popularity. They focus on balancing quality and budget-friendly units to allow more homeowners to enjoy the benefits of owning a tankless water heater.  

If you're still not sure and want some advise, you may want to consider talking to a professional. They can help you assess your household hot water needs, give you an installation quote, and make a recommendation.

Man reading about tankless water heaters on his computer

Recommended Reading

Buying a tankless water heater can be overwhelming and confusing. We have plenty of articles that can help you make an educated buying decision and to help you  understand how to care for your new purchase.

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