Sizing a Tankless Water Heater: What Tankless Size Do I Need?

Achieving the right fit for your household’s hot water needs requires careful consideration when sizing a tankless water heater. By taking into account various factors, you can ensure an efficient and reliable supply of hot water.

Sizing a tankless water heater differs from sizing tank-type heating appliances. The principle remains the same: you want your tankless unit to provide an ample supply of hot water at the minimum comfortable temperature during peak usage periods.

Sizing a tankless water heater
Tankless water heater (photo:

As with selecting a tank-type heater, it’s crucial to avoid purchasing an oversized or undersized tankless water heater. Doing so can lead to unnecessary issues such as higher upfront costs, increased operating expenses, and wastage of water and energy. These factors can significantly impact your budget and time.

Sizing a Tankless Water Heater: Things to Consider

To assist you in this process, here are essential steps to help you accurately size a tankless water heater:

Step 1: Determine Your Peak Hot Water Demand

Determine the optimal capacity for hot water by estimating the maximum demand during the peak period of the day, considering the following key factors:

  • Household size: Consider the number of people residing in the household, as it directly affects the overall hot water consumption. Larger households typically require more hot water.
  • How many appliances are in use: Assess the number of bathrooms, tubs, dishwashers, and washing machines that will be in use. Each of these fixtures contributes to the hot water demand. Additionally, take into account any planned additions or renovations that may increase the number of fixtures in the future.
  • Hot water needs of kids or teenagers: If you have children or teenagers in your household, their hot water usage tends to be higher due to activities like longer showers or additional laundry needs.
  • Usage patterns: Analyze the usage patterns within your household. For instance, if multiple people tend to use hot water at the same time (such as showering and running the dishwasher simultaneously), the peak demand will be higher.
  • Quantity and type of fixtures: Consider the number and type of fixtures in your household. Different fixtures, such as low-flow showerheads or energy-efficient appliances, may have varying hot water requirements.

Step 2: Calculate the Required Flow Rate

Estimate the flow rate of each hot water fixture/appliance in gallons per minute (GPM). Add up the flow rates of all the fixtures/appliances that may operate simultaneously to determine the total required flow rate.

How Much Hot Water Is Needed per Application

Here are consumption rates to serve as a reference when estimating the water requirements for specific applications and sizing a tankless water heater:

  • Shower: Standard showers typically have a flow rate of 1.5-2.5 GPM. High-pressure showerheads or multiple showerheads may have higher flow rates.
  • Sink faucet: Sink faucets generally have a flow rate of 0.5-1.5 GPM, depending on the aerator and water-saving features.
  • Dishwasher: Dishwashers typically require 1.5-3 GPM during the wash cycle.
  • Washing machine: The flow rate of washing machines can vary, but a typical top-loading machine may require 1.5-3 GPM, while front-loading machines may use 1-2 GPM.

All the above data takes into account an average temperature between 100 and 110 F.

Please note that these consumption rates are general guidelines and can vary based on specific fixtures, brands, and models. It’s essential to refer to the manufacturer’s specifications for precise flow rates of your appliances.

Calculate the Peak Demand for Hot Water

To calculate the peak demand for hot water, make a list of the number of devices you will be using simultaneously and add up their flow rates. If the tankless unit is being used to supply a shower and two sinks at the same time, assuming a peak demand of 2.5 gallons per minute (GPM) for the shower and 0.75 GPM for each sink, the total peak demand for hot water would be 4.0 GPM.

Whole-house electric heaters typically provide a flow rate of 2 to 4 GPM with a temperature rise of 75 degrees Fahrenheit, while gas heaters provide even higher flow rates. Choose a tankless water heater model that comes closest to meeting your calculated flow rate and temperature rise requirements.

Note: Standby heat loss, which refers to the energy wasted when hot water is not being used, is a significant factor to consider when determining tank size. However, in the case of tankless water heaters, standby heat loss can be completely eliminated due to the absence of storage tanks. Additionally, tankless models do not have a “first-hour rating” as they provide hot water instantaneously.

Step 3: Determine the Temperature Rise

Determine the temperature rise required by subtracting the temperature of the incoming water (usually around 50°F) from the desired hot water temperature (e.g., 120°F). The temperature rise indicates how much the tankless water heater needs to increase the water temperature.

Step 4: Check the Unit’s Flow Rate Capacity

Look for the flow rate capacity (measured in GPM) specified for each tankless water heater model you’re considering. Ensure that the unit can meet or exceed your required flow rate determined in step 2.

Step 5: Consider the Unit’s Energy Output

Tankless water heaters have different energy outputs, measured in British Thermal Units per hour (BTU/h). A higher BTU rating generally indicates a higher hot water production capacity.

Step 6: Account for Additional Factors

Consider any additional factors that may affect the sizing, such as cold climates (which may require larger units), future household expansions, or the availability of natural gas or electricity.

Step 7: Consult a Professional

While you can perform the calculations yourself, it’s advisable to consult a professional plumber or a qualified tankless water heater installer.

Sizing Guides From the Manufacturers

Other Important Factors to Keep in Mind for Proper Tankless Sizing

Multi-system. If the largest available tankless water heater cannot meet all your residential hot water needs, you can connect multiple tankless units into a single system (such as the Noritz multi-system with 24 units) to provide ample hot water for every application and meet the demands of any house.

Minimum flow rate. As cold water passes through the heat exchanger, it must maintain the correct flow rate to ensure proper heat transfer. If the flow rate is lower than what the manufacturer recommends, excessive heat from the gas burner can adversely affect the performance of the heat exchanger, leading to deformation or unit shutdown.

Many professionals consider flow rate to be the most critical factor when sizing a tankless water heater. The flow rate depends on the temperature of the incoming water. This is why it is beneficial to determine the climate area where you reside (use a map below if you are in North America) to determine the average groundwater temperature.

For instance, Rinnai, a manufacturer of tankless heaters, recommends using the RU90 model for 3-4 applications when the groundwater temperature is between 65 and 75 F. During the winter season, when the incoming water temperature is between 45 and 55 F, the same model would be suitable for two applications instead.

The flow rate can also be determined by calculating the temperature rise, which indicates how much you need to heat the water to reach the desired temperature settings (typically recommended at 120 F).

For example, if you set the heater temperature to 120 F and the groundwater temperature in your area is 60 F, there is a temperature rise of 60 F. The recommendation is to select a residential tankless heater based on the colder seasons to ensure you meet the peak demand for hot water.

Power. Gas input, heat power from heating elements, and the energy factor (EF) are also significant factors to consider when sizing a tankless water heater. For instance, Energy Star qualified residential models with an EF of 0.87 or higher can have a maximum gas input of 199,900 BTU/hr. Condensing units typically have even higher efficiency than the mentioned value.

Groundwater Temperature Chart

Since the temperature of the incoming water varies across North America and experiences fluctuations, it is important to consider three climate zones when sizing a tankless water heater:

  • Northern zone: This zone has an average groundwater temperature ranging between 37°F and 51°F.
  • Central zone: The central zone experiences an average groundwater temperature between 52°F and 61°F.
  • Southern zone: The southern zone has an average groundwater temperature ranging between 62°F and 77°F.


When sizing a tankless water heater, the water capacity of the unit becomes irrelevant since it operates on-demand, continuously supplying hot water.

If the tankless unit is not sized correctly, the output temperature will remain the same, but the flow will be affected. It is important to heat water to at least 120°F, with a minimum flow rate typically around 2 gallons per minute. If the flow rate is higher, the temperature will be lower.

It is recommended to calculate the combined flow rates of all the applications you will be using and consider the incoming water temperature. Lower temperatures require higher BTU output. If a single unit cannot provide enough hot water, multiple models can be installed using the multi-system option.

When choosing between gas tankless heaters and electric models, it is worth noting that gas units can generally deliver hot water at higher flow rates than electric models. Ensure that the flow rates can meet the peak demand at the desired temperature rise.

Proper sizing of a tankless water heater is crucial, as up to 20% of home energy usage is attributed to water heating. For homeowners new to tankless systems, the benefits include high energy efficiency, low energy consumption, on-demand supply of endless hot water, space-saving design, reduced pollution, and long lifespan.


What Size Tankless Water Heater Do I Need for a Family of 4?

The size of the tankless water heater for a family of four depends on various factors, including the desired flow rate, temperature rise, and hot water usage patterns in your household. However, as a general guideline, a tankless water heater with a flow rate of around 7-9 gallons per minute (GPM) should be suitable for a family of four.

What Is the Largest Size Tankless Water Heater?

The largest size of a tankless water heater for home use can provide flow rates of 10-11 GPM. These high-capacity units are designed to meet the hot water demands of larger homes or commercial applications where multiple fixtures may be used simultaneously.

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