A tankless water heater attached to a bathroom wall.

How To Select the Right Size Tankless Water Heater

What Is a Tankless Water Heater and How Do They Work?

A tankless water heater (also often called a demand-type water heater or an instant water heater) is a type of water heater that you can install in your home that unlike the most conventional types of water heater, it comes without a storage tank. Without a storage tank, the way that these types of heaters heat up your water is a little bit different.

Tankless water heaters use either a gas burner or a heating element that takes the water that comes from your main water line cold and heats it, and then sends it through to your hot water faucet. Due to their special no-tank design, this type of water heater can give you unlimited hot water, while conventional types of water heaters can usually only provide as much hot water at once as it is able to fit in its storage tank.

After that, you would have to wait for the tank to fill back up until you can get hot water again, which does not happen with tankless models. There are several benefits to getting a tankless water heater. They are much more energy-efficient than traditional types, since they are only in use when there is a demand for hot water, and do not spend energy keeping the water hot in the storage tank while it is not in use.

This means they are more eco-friendly as well. They are also more long-lasting than other water heater models and require less maintenance. They are more space-saving than their tank-wielding counterparts. And finally, as it was mentioned above, it provides you with an endless stream of hot water.

How Can You Select The Right Size Tankless Water Heater?

Tankless water heaters typically have a little bit less power than storage water heaters, meaning that their water flow rate (which is measured in gallons per minute) is often lower since the water heating process happens on demand. This means that if you have a large home, you might need to get either one of the larger tankless models or multiple tankless units for different fixtures or different appliances.

Selecting the right size tankless water heater is not a difficult process. You might want to hire a professional if you are not completely comfortable with the process or if you simply do not want to purchase the necessary equipment to measure the things you need to measure in order to get the right size tankless water heater. Getting a professional for this process should not be an expensive endeavor. So without further ado, here is a step-by-step guide to getting the right size tankless water heater.

Gold faucets in a row.

You can’t know the right size for your water heater without first knowing how many water fixtures will be needing hot water.

Step 1 – Determine the Number of Fixtures

First and foremost, you need to figure out which faucets, which showerheads, and which appliances or devices (such as washing machines or dishwashers) you will be needing hot water for. It is important to know how many of them and what they are so you can move on to the next steps on this list.

Step 2 – Determine the Flow Rates

The water flow rate refers to the amount of water (measured in gallons) that will flow through your faucet over a given period of time (usually measured in minutes). This means that a water flow rate of 2 GPM (gallons per minute) will let you fill a 2-gallon container over the time span of one minute.

You should be able to determine the water flow rate of your fixtures by filling up a container for one minute and measuring the amount of water in it. This might be a little bit more difficult to do for appliances such as a washing machine or a dishwasher, however often times this information can be found on the instructions manual as well as the manufacturer’s web page.

You will need to do this for each fixture you want hot water for and add all of their water flow rates that you need working at the same time. Their total flow rate will be the flow rate you will require from your tankless water heater. In order to lower the flow rates, you may install low-flow water fixtures, or get more than one tankless unit.

Step 3 – Determine the Temperature Rise

The temperature rise refers to the difference between the incoming water temperature from the output temperature that you are aiming for. This means that if your incoming water temperature is 50°F (which is what you should assume it is unless you plan on measuring it) and you want to heat up your water to 110°F (most uses need water to be between 105°F and 115°F), then the temperature rise on your tankless water heater should be of 60°F.

Water pours out of the shower on the girl's hands

The flow rate and temperature requirement of each water source will tell you how big your water heater must be.

Step 4 – Determine How the Two Interact

Now it is important to see which of the fixtures would need to be working simultaneously. Would you need two showers to be working at the same time, or your kitchen faucet and one showerhead? Then you need to make sure you have added them all up and added the temperature rise. Here are the regular flow rates and temperatures for different activities you might do in your home:


Average Flow GPM: 2.5 to 3.0 GPM
Average Temperature: 104°F

Kitchen Sink:

Average Flow GPM: 1.5 GPM
Average Temperature: 110°F


Average Flow GPM: 4.0 GPM
Average Temperature: 102°F

Washing Machine:

Average Flow GPM: 2.0 GPM
Average Temperature: 120°F


Average Flow GPM: 1.5 GPM
Average Temperature: 110°F

There are also other things that you should take care of if you are choosing a tankless water heater. Make sure you give this list a read before you go out and get yourself one:

  • Type of heating process: you might prefer a gas burner since it will give you a higher flow rate, or a heating element, which is less expensive and more environmentally friendly
  • Anti-scale devices: if your tankless water heater comes with an anti-scale device (a sort of water softener device), it will require a great deal less maintenance
  • The amount of energy it will save you: different models will have different energy-saving capabilities
  • The cost of the product: you should make sure you are getting a good price-to-quality ratio
  • The warranty: different tankless water heaters will come with different types of warranties

Joshua Perkins

Joshua spent ten years as a water systems technician in California before settling down with his wife and two young children in Nashville in 2018. Through all of his experience, he learned the benefits and shortfalls of so many different types of water filtration systems, from pitchers to whole-house installations. He started Water Filter Authority in 2019 to empower other families to make the right decision for their long-term health and wellness.