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

Tankless water heaters are very popular because they are space- and energy-efficient.

Unlike traditional tank-style water heaters, tankless units heat water only when needed, so you don’t need a large storage tank. However, one of the most frequently asked questions is, “What size tankless water heater do I need?”.

Choosing the proper size can be daunting, as it requires a thorough understanding of your household’s hot water needs. This guide will explain hot water demand as one aspect of tankless water heater sizing.

Understanding the sizing of tankless water heaters

Unlike traditional tank water heater units, which have a fixed capacity based on their size, tankless water heater units are determined based on the amount of water they can heat per minute. This can be measured through gallons per minute (GPM) and refers to the amount of hot water the heater can supply at one time. 

The most important thing to consider when choosing the right tankless water heater for your house is peak demand or the maximum amount of hot water you’ll need at one time. This can vary based on household size, number of bathrooms, and usage habits.

Factors to consider while searching for a tankless water heater

Household size

The number of residents in your household directly impacts the tankless water you need. Larger households with more occupants will need a unit with a higher flow rate to meet the increased demand for hot water.


The climate in your region also influences the size of the tankless water heater needed. Colder climates require a unit with a higher capacity to provide sufficient hot water, while warmer areas may require a smaller unit.

Hot water demand

Understanding the peak hot water flow demand is essential for correctly sizing your tankless water heater. Consider the number of devices used simultaneously by your family in your home to determine the required flow rate.

How do you determine which size tankless water heater you need?

So, let’s focus on the factor “hot water demand.” 

Determining your hot water demand is one way to find the proper size of tankless water heater for your home

You can do this by:

  • Assess your household’s hot water usage patterns.
  • Count the number of hot water appliances (such as showers, washing machines, dishwashers, and bathroom sinks), plumbing fixtures, and bathrooms you have.
  • Consider simultaneous usage of multiple hot water outlets.
  • Estimate your peak hot water demand, the maximum amount needed at any time.

GPM (Gallons per minute) calculation

The primary metric used to determine the size of a tankless water heater is the GPM, or gallons per minute, rating. This measurement reflects the unit’s capacity to deliver hot water at a specific flow rate. Understanding your household’s peak hot water demand, expressed in GPM, is essential for choosing the right size. 

To calculate the demand in your house, you can use the following formula:

Total Hot Water Demand (gallons per minute) = Sum of Hot Water Needed by Each Plumbing Fixture/Appliance

Here’s an example of how to calculate the demand for hot water for a typical household:

BTU requirement and flow rate calculation

In addition to hot water demand, consider your tankless water heater’s BTU (British Thermal Unit) rating and flow rate. The BTU rating indicates the unit’s heating capacity, while the flow rate specifies the maximum hot water it can deliver simultaneously.

To calculate the BTU requirement, determine the temperature rise (difference between incoming and desired water temperature) and the maximum flow rate. Use this formula:

BTU Requirement = Flow Rate (GPM) x Temperature Rise (°F) x 8.3

For example, if your maximum flow rate is 5 GPM and you need a temperature rise of 70°F (from 50°F to 120°F), the BTU requirement would be:

BTU Requirement = 5 GPM x 70°F x 8.3 = 2,905 BTU

The flow rate requirement is determined by the simultaneous use of hot water appliances and fixtures in your home. Add up the flow rates of all fixtures and appliances used simultaneously to ensure your tankless water heater meets peak demand.

Sizing guidelines for tankless water heaters

To help you navigate the process of selecting the appropriate tankless water heater size based on the information we discussed above, consider the following general guidelines:

  1. Small Homes or Apartments (1-2 Bathrooms): Tankless water heaters with a capacity of 140,000 BTU or less, typically delivering 3-5 GPM, are often suitable for small homes or apartments with one to two bathrooms.
  2. Medium-Sized Homes (2-3 Bathrooms): Tankless water heaters with a 180,000-199,000 BTU capacity and delivering 5-7 GPM are generally recommended for medium-sized homes with two to three bathrooms.
  3. Larger Homes (3+ Bathrooms): To meet the increased hot water demand, homes with three or more bathrooms require a tankless water heater with a capacity of 199,000 BTU or more, capable of delivering 7-10 GPM or higher.

It’s important to remember that these are general guidelines, and the specific hot water needed may vary based on the factors discussed earlier. Always consult a professional plumber to ensure you select the correct size of tankless water heater for your home.

Common mistakes to avoid in sizing a tankless water heater

When choosing a tankless water heater, avoid these common mistakes:

  1. Underestimating Hot Water Demand: Failing to calculate the household’s hot water demands accurately can result in a unit that’s too small.
  2. Ignoring Temperature Rise: Not considering the required temperature rise can result in a heater that doesn’t provide sufficient hot water.
  3. Overlooking Simultaneous Usage: Forgetting to account for multiple appliances simultaneously can result in an undersized heater.
  4. Choosing the Cheapest Option: Selecting the least expensive heater without considering capacity and efficiency may increase long-term costs and cause performance issues. It’s essential to invest wisely to ensure not only immediate satisfaction but also prolonged efficiency and reliability.
  5. DIY Installation: Installing without proper knowledge and permits can be dangerous and lead to improper operation or safety hazards.

Professional sizing and installation recommendations

While this guide offers a comprehensive overview of choosing the proper size of a tankless water heater, it’s advisable to consult a professional plumber or tankless water heater service specialist. They can assess your needs, recommend the appropriate size, and ensure proper installation for optimal efficiency and performance. If you’re uncertain, call the Plumb-Tech expert team to seek professional guidance tailored to your home. 

Ready for reliable hot water? Call Plumb-Tech to get started!

Optimizing your tankless water heater size is crucial for maintaining a consistent hot water supply and enhancing energy efficiency in your household. Consider factors like GPM and household usage patterns to make a well-informed decision.

Remember, there’s no one-size-fits-all solution for tankless water heaters. Take the time to assess your household’s hot water needs thoroughly, seek professional advice, and anticipate future changes or expansions.

Ready to upgrade your hot water system? Contact Plumb-Tech today for expert consultation and begin your journey toward a more efficient and reliable hot water solution.

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