Tankless Hot Water Heater Sizing Chart: Everything You Need to Know

Tankless Hot Water Heater Sizing Chart: Understanding the Basics

When it comes to providing a continuous supply of hot water while saving energy and space, tankless hot water heaters are the top choice for many homeowners. However, finding the right size can be a bit tricky. Fear not, as we have prepared this detailed guide to help you determine the perfect tankless hot water heater sizing chart for your household’s needs. Whether you’re replacing an old water heater or installing a new one, this article will equip you with all the information you need to make a well-informed decision.

Tankless Heater Sizing Guide

What is a Tankless Hot Water Heater?

Before diving into the sizing details, let’s understand what a tankless hot water heater is. Unlike traditional water heaters with a storage tank, tankless heaters heat water on-demand as it passes through the unit. This means you get hot water whenever you need it without wasting energy on constantly reheating stored water.

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Benefits of Tankless Hot Water Heaters

Let’s explore some of the key benefits of using tankless hot water heaters:

  • Endless hot water supply
  • Energy efficiency and cost savings
  • Space-saving design
  • Longer lifespan compared to traditional heaters

Factors Affecting Tankless Hot Water Heater Sizing

Determining the right size for your tankless hot water heater depends on various factors. Consider the following:

  • Hot Water Demand: Calculate the maximum hot water demand in your household by considering the number of bathrooms, appliances, and simultaneous hot water usages.
  • Flow Rate: The flow rate measures the gallons of hot water per minute (GPM) the heater can produce. It’s crucial to match the flow rate to your household’s peak demand.
  • Temperature Rise: This refers to the temperature difference between the incoming water and the desired hot water temperature.
  • Climate: In colder climates, the incoming water temperature will be lower, affecting the heater’s size requirement.
  • Water Hardness: Hard water may require a larger heater to compensate for mineral buildup.
  • Family Size: Larger households with more occupants will have higher hot water demands.
  • Water Fixture Types: Different fixtures have varying GPM requirements, so consider the types of fixtures you have.

Determining the Right Size for Your Tankless Water Heater

Sizing Chart for Tankless Hot Water Heaters

Household Size GPM Incoming Water Temperature Heater Size (GPM)
1-2 people
2-4 GPM
6-8 GPM
3-4 people
4-6 GPM
8-10 GPM
5+ people
6-10 GPM
10-12 GPM

Step-by-Step Sizing Guide

Finding the right size on the chart above is a great starting point, but let’s take a closer look at how to fine-tune your tankless hot water heater sizing.

  • Calculate Peak Demand: Determine the total GPM required during peak hot water usage in your household. Add up the GPM of all the fixtures that may be in use simultaneously (e.g., showers, washing machines, sinks).
  • Check Temperature Rise: Measure the difference between the coldest incoming water temperature and your desired hot water temperature. For example, if the incoming water is 40°F and you want hot water at 120°F, the temperature rise is 80°F.
  • Adjust for Hard Water: If your water is hard, consider increasing the heater size slightly to compensate for potential mineral buildup that can reduce efficiency.
  • Factor in Climate: If you live in a colder climate with lower incoming water temperatures, choose a heater with a higher GPM capacity to meet the demand.
  • Consider Future Needs: If you plan to expand your household or anticipate higher hot water usage in the future, opt for a larger heater to accommodate these needs.
  • Consult with a Professional: When in doubt, it’s always best to consult with a qualified plumber. They can conduct a detailed assessment of your hot water needs and recommend the right size accordingly.

FAQs about Tankless Hot Water Heater Sizing

Absolutely not! An undersized heater won’t be able to meet your hot water demands, leading to frustration and inconvenience. It may even reduce the lifespan of the unit due to overworking.

Yes, tankless hot water heaters are an excellent choice for large families. By selecting the appropriate size, you can ensure an uninterrupted supply of hot water for all household members.

While some homeowners may have the skills to install a tankless hot water heater, it’s best to leave this task to a licensed professional. Improper installation can lead to safety hazards and warranty issues.

Yes, tankless heaters need periodic maintenance to ensure optimal performance. Flushing the system regularly can prevent mineral buildup and extend the unit’s lifespan.

With proper maintenance, tankless hot water heaters can last around 20 years or more, making them a durable investment.

Yes, as long as you choose a heater that can handle the total GPM required for both activities combined.


Selecting the right tankless hot water heater size is essential to ensure your household’s hot water needs are met efficiently. By considering factors like hot water demand, flow rate, temperature rise, and climate, you can determine the perfect size for your home. Remember to consult with a professional plumber and use our sizing chart as a starting point. With the right tankless hot water heater, you’ll enjoy endless hot water and energy savings for years to come.

Water Heater Service Area in Atlantic County, NJ

Atlantic City
Corbin City
Egg Harbor Township
Estell Manor

Galloway Township
Hamilton Township
Hammonton Township
Margate City

Port Republic
Somers Point
Ventnor City

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