How to Size a Tankless Water Heater: The Ultimate Guide.

To size a tankless water heater, calculate your household’s peak water demand and required temperature rise, ideally with the help of a professional installer. A tankless water heater is a high-tech system that heats water on demand, rather than storing hot water in a tank.

Choosing the right size for your household is critical to ensure that you have enough hot water when you need it without wasting energy. In this article, we will guide you on how to size a tankless water heater and provide some key factors to consider.

By following our tips, you’ll be able to determine the best size for your needs, which will help you save both money and energy in the long run. Let’s dive in!

How to Size a Tankless Water Heater: The Ultimate Guide.


Why Is It Important To Size A Tankless Water Heater Correctly?

Tankless water heaters are becoming increasingly popular in residential homes due to their energy-saving potential and unlimited hot water supply. However, to get the full advantages of these heaters, it’s imperative to size them correctly. We will discuss the importance of sizing a tankless water heater correctly and the benefits of having one that is appropriately sized.

We will also look at the problems that may arise with an improperly sized water heater.

Benefits Of Having A Properly Sized Tankless Water Heater

  • Efficient performance: A tankless water heater that is correctly sized can meet the household’s hot water demand without working too hard or wasting energy. As a result, the heater can work seamlessly and efficiently, providing hot water on demand to each faucet or showerhead without any delay or fluctuation.
  • Save on energy bills: A tankless water heater that is sized correctly can significantly reduce energy bills. Since it operates only as needed without wasting energy heating up a large storage tank of water, it’s the most energy-efficient option available today. The correct sizing means it does not have to overwork to keep up with the hot water needs of your home, further reducing energy costs.
  • Longevity: According to experts, a well-sized tankless water heater can last up to two decades. If the system is undersized, it could overwork, leading to frequent breakdowns and poor performance. As a result, a correctly sized water heater can lead to a more extended lifespan, ensuring that you get your money’s worth.

Problems That May Arise From Improper Sizing

  • Insufficient hot water: If a tankless water heater is undersized, it won’t be able to meet the household’s hot water demand. As a result, it can lead to insufficient hot water, particularly during peak hours such as mornings or in large households, where simultaneous usage can occur.
  • Overworking: If a tankless water heater is oversized, it could also lead to problems, as it may continually turn off and on, increasing wear and tear on the system. This could lead to frequent breakdowns, higher maintenance costs, and a shorter lifespan.
  • Higher energy bills: An oversized water heater can also lead to higher energy bills, as the system works harder than it needs to, wasting both energy and money.

The importance of sizing a tankless water heater correctly can’t be overstated. It can save energy, reduce bills, enhance performance, and extend the system’s lifespan. Conversely, there are potential problems if the system is sized incorrectly, which can lead to insufficient hot water, higher energy costs, and more frequent breakdowns.

Therefore, it’s essential to consult with a professional plumber or hvac technician to determine the appropriate size of a tankless water heater for your household’s hot water needs.

Factors To Consider When Sizing A Tankless Water Heater

Tankless water heaters have become increasingly popular in recent years for their energy-saving and space-efficient features. However, when it comes to sizing a tankless water heater, it’s important to consider a few factors to ensure optimal performance and efficiency. In this guide, we’ll cover the key factors to consider when sizing a tankless water heater to help you make an informed decision.

Water Demand

One of the critical factors to consider when sizing a tankless water heater is the amount of hot water your household requires. The water demand in your home depends on various factors, such as the size of your household, daily activities that consume hot water, and the number of hot water appliances in your home.

You can determine your household’s hot water demand by considering the following:

  • The number of people in your home
  • Number of bathrooms and the number of fixtures in each bathroom, including showerheads and faucets
  • Kitchen and laundry room hot water consumption

Climate And Regional Considerations

Another crucial factor to consider when sizing a tankless water heater is your location’s climate, altitude, and groundwater temperature. The groundwater temperature affects how much hot water your tankless water heater can produce and maintain, ultimately impacting the heater’s size you need.

For example, homeowners located in colder regions where groundwater temperatures are lower will require a larger tankless water heater to meet their hot water needs, while those living in warmer climates may need smaller-sized heaters.

Flow Rate

The flow rate is how much water your household consumes at any given moment. You’ll need to consider the maximum flow rate required for your household to identify the appropriate tankless water heater size. Here’s how to determine your flow rate:

  • Determine the number of fixtures that will use hot water simultaneously
  • Check the flow rate of each fixture, such as showerhead or faucet
  • Add up the flow rate of each fixture to determine the total flow rate

Temperature Rise

Temperature rise is the difference between the cold groundwater temperature and the required hot water temperature for your home. The required temperature depends on your household needs and preferences. For instance, some households may require a lower or higher temperature than the standard 120°f.

Energy Efficiency

Finally, it’s crucial to consider energy efficiency when selecting a tankless water heater size. The energy efficiency rating determines how effectively the heater converts energy into hot water. Opt for a high energy-efficient tankless water heater model to avoid excessive energy bills.

The above factors are vital considerations when you want to size a tankless water heater to meet your water needs. It’s advisable to consult a professional plumber to help you identify the best tankless water heater size for your home.

How To Calculate The Required Size Of A Tankless Water Heater

If you’re looking for a way to save money on your energy bills, switching to a tankless water heater is an excellent solution. But before you take the leap, it’s crucial to know how to size your new system correctly.

In this guide, we’ll cover everything you need to know about how to calculate the required size of a tankless water heater.

The Flow Rate Formula

The first step in sizing a tankless water heater is to determine your household’s hot water flow rate. This measurement is expressed in gallons per minute (gpm) and represents the amount of hot water used at peak times. To calculate your flow rate, follow this formula:

*step 1*: identify the maximum number of hot water devices you would use simultaneously.

*step 2*: find the flow rate of each of those devices in gallons per minute. You can usually find this information on the device or in the manufacturer’s documentation.

*step 3*: once you have the flow rates, add them together to get the total flow rate in gallons per minute.

An Example Calculation

Suppose your family likes to take warm showers in the morning while running the dishwasher. You have two showerheads in the house, each with a flow rate of 2 gallons per minute, and a dishwasher with a flow rate of 1.

5 gpm. Your total flow rate would be:

2 gpm (showerhead 1) + 2 gpm (showerhead 2) + 1. 5 gpm (dishwasher) = 5. 5 gpm

Evaluating The Calculated Answer

Once you have the calculated gpm, you can choose a tankless water heater size that is appropriate. In general, you should choose a model with a flow rate that matches or exceeds your calculated gpm. However, keep in mind that some tankless water heaters may only work with a minimum flow rate.

Always review the manufacturer’s product specifications before purchasing your tankless water heater.

By following these simple steps, you can correctly size your tankless water heater. With the right size, you can enjoy endless hot water while reducing your energy costs.

Determining The Type Of Tankless Water Heater Needed

Tankless water heaters are a popular choice for many homeowners because they offer several advantages over traditional tank water heaters. One of these benefits is the ability to select a specific size and type of heater to meet your unique needs.

In this guide, we will explore how to determine the type of tankless water heater that best suits your needs.

Different Types Of Tankless Water Heaters

There are two primary types of tankless water heaters: gas-powered and electric-powered.

Gas-Powered Tankless Water Heaters

Gas tankless water heaters are more powerful than their electric counterparts, and are better suited for homes with high hot water demands. They use a gas burner to heat the water as it flows through the unit. Some of the main benefits of gas-powered units include:

  • Higher flow rates and temperatures
  • Lower cost of operation, when compared to electric models
  • They can still operate during power outages

Electric Tankless Water Heaters

Electric tankless water heaters do not require a flue or vent, making them a more convenient option when installing in tight spaces. However, they are generally less powerful than gas models and are best suited for small to medium-sized homes.

Some of the main benefits of electric tankless water heaters include:

  • Lower upfront cost, when compared to gas models.
  • More energy-efficient, with an energy factor of up to .99
  • No risk of carbon monoxide poisoning since there is no combustion.

Point-Of-Use Or Whole House?

When selecting a tankless water heater, you’ll also need to decide whether you need a point-of-use unit or a whole-house unit.

Point-Of-Use Units

Point-of-use units are designed to be installed directly where hot water is needed, such as under the sink or near a shower. They are best suited for small spaces, and for those who have specific hot water needs in certain areas of the home.

Some of the benefits of point-of-use units include:

  • Lower upfront costs
  • More energy-efficient since they only heat water where it is needed
  • Faster hot water delivery times.

Whole-House Units

Whole-house units are designed to provide hot water to all areas of the home, and must be powerful enough to meet the demands of every fixture and appliance that requires hot water. They are best suited for larger homes with higher hot water needs.

Some of the benefits of whole-house units include:

  • The ability to provide hot water to every fixture and appliance in the home
  • Can be more power-efficient than storage tank units since there are no standby losses.
  • Can be more convenient than storage tank units, since the supply of hot water is unlimited.

When selecting a tankless water heater, it’s essential to consider your specific hot water needs, as well as the size of your home and your overall budget. With the information provided in this guide, you should now have a better understanding of how to determine the type of tankless water heater that is best for you.

Frequently Asked Questions Of How To Size A Tankless Water Heater

What Size Tankless Water Heater Do I Need For My House?

The size of the tankless water heater you need depends on the number of people in your home and the appliances you have. An average home with 2-3 people needs a heater with at least 5 gpm. Larger homes with more people will need a heater that can deliver more gpm.

How Do I Determine The Flow Rate Of Hot Water In My Home?

To determine the flow rate of hot water in your home, add up the flow rate of all hot water appliances and fixtures you plan to use at the same time. This can include showers, faucets, and appliances like dishwashers and washing machines.

What Is The Difference Between A Tankless Water Heater And A Traditional Water Heater?

A tankless water heater heats water on demand, unlike a traditional water heater which stores a set amount of hot water in a tank. Tankless heaters are more energy-efficient and take up less space. Traditional heaters are cheaper upfront but cost more to operate in the long run.

Can A Tankless Water Heater Supply Enough Hot Water For A Large Family?

Yes, a properly sized tankless water heater can supply enough hot water for a large family. Larger tankless heaters can deliver up to 11 gallons of hot water per minute, which is enough for even the busiest households.

Can I Install A Tankless Water Heater Myself?

It is not recommended to install a tankless water heater yourself. These appliances require special skills and equipment, and proper installation is crucial for safety and efficiency. Always hire a licensed and experienced professional for installation and maintenance.

How Can I Maintain My Tankless Water Heater To Prolong Its Lifespan?

To maintain your tankless water heater, flush it with vinegar once a year to remove mineral buildup. Check the air intake and exhaust vents regularly for obstructions. Perform regular maintenance checks and have a certified technician perform a yearly inspection.


As you can see, sizing a tankless water heater isn’t as complicated as it may seem at first glance. It’s important to remember that the size of your tankless water heater depends on the specific needs of your household, and there are several factors that need to be considered.

By following the steps we outlined in this post, you will be able to accurately determine the sizing requirements for your tankless water heater. Remember, choosing the right size can result in significant energy savings and improved efficiency for your household.

Don’t hesitate to reach out to a professional if you need further assistance with sizing or installation. Trust us, investing in a high-quality tankless water heater that is properly sized can make all the difference in the comfort and convenience of your home!