What Size Tankless Water Heater Do I Need? Find Out Here!

To determine the size of the tankless water heater you need, calculate the flow rate and temperature rise required based on your household’s hot water usage and the incoming water temperature. A tankless water heater provides hot water on demand without a storage tank, making it an energy-efficient option for residential use.

With the right size, it can efficiently supply enough hot water for your household’s needs. Several factors influence the size of the tankless water heater you require, including the number of people in your home, hot water usage patterns, and incoming water temperature.

In this article, we will guide you on how to calculate the flow rate and temperature rise required to find the appropriate tankless water heater size for your household.

What Size Tankless Water Heater Do I Need? Find Out Here!

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Factors To Consider When Determining Tankless Water Heater Size

Tankless water heaters are an excellent innovation that has revolutionized the way we heat our water. Unlike traditional heaters, they do not store hot water, but instead heat it as required, resulting in less energy consumption and cost savings. However, when purchasing a tankless water heater, it’s essential to consider the factors that will determine the size of the unit to suit your household’s hot water needs.

Here are some crucial factors to keep in mind when deciding what size tankless water heater you need.

Average Hot Water Usage Per Day

The amount of hot water you use daily is a crucial determinant of your tankless water heater’s ideal size. To get an accurate estimate, monitor your household’s hot water usage for several days using a flow meter. Typically, the average household uses about 10-20 gallons of hot water per day for each person living in the house.

Therefore, a family of four will need a unit that heats roughly 40-80 gallons of hot water per day.

Peak Hot Water Usage

In addition to the average daily hot water usage, it’s crucial to determine the maximum hot water demand you need. Try to identify the busiest time in your home when everyone is using hot water simultaneously. This could be when everyone is taking showers or when your dishwasher and washing machines are running.

Based on this, you can estimate your peak hot water usage and get a unit that can meet these demands.

Number Of People In The Household

The number of occupants in your household plays a significant role in deciding the tankless water heater’s size you need. Generally, larger families will need a water heater with a higher gallon-per-minute (gpm) rating, which measures the amount of water heated per minute.

For households with three or more people, a tankless water heater with a gpm rating of at least 7 is recommended.

Flow Rate And Temperature Rise Required

The flow rate is the amount of water that passes through your tankless water heater per minute. This rate varies from one household to another, depending on their hot water needs. The temperature rise needed refers to how much the unit has to increase the water’s temperature as it passes through the unit.

To determine the ideal tankless water heater, you will need to consider the required flow rate and temperature rise, which are influenced by the size of your household and the number of appliances you have.

Size And Type Of Appliances That Use Hot Water

Different appliances have different hot water needs, which should be considered when deciding on the size of your tankless water heater. For example, a dishwasher may use 2-3 gallons of water per minute, while a showerhead may use up to 2.

5 gallons per minute. Consider all the appliances that require hot water and the rate at which they consume hot water. This will help determine the unit’s ideal size needed to avoid running out of hot water during peak usage times.

By considering factors such as average hot water usage per day, peak usage, number of people in the household, flow rate and temperature rise, and appliances’ size and type, you can decide on the right tankless water heater size for your household.

It is essential to choose a size that will meet your hot water demand without wasting energy, resulting in significant savings on your energy bills.

Calculating The Right Size Tankless Water Heater For Your Home

Tankless water heaters have become an incredibly popular replacement for traditional tank-style water heaters. Unlike traditional heaters, these innovative devices heat water on demand, helping save energy, money, and space in the home. However, when it comes to selecting the right-sized tankless water heater, things can get complicated.

Formula For Calculating Flow Rate And Temperature Rise

To calculate your ideal tankless water heater size, you will need to calculate two crucial elements: flow rate and temperature rise. The flow rate refers to the amount of water the heater needs to deliver and the temperature rise is the difference between the incoming temperature of the cold water and the outgoing temperature of the hot water.

The formula for calculating flow rate and temperature rise is as follows:

  • Flow rate = gallons per minute (gpm)
  • Temperature rise = desired temperature – incoming water temperature

Example Calculation For A Standard-Sized Household

Suppose you live in a standard-sized household with four family members and would like to determine what size of tankless water heater you need for your bathroom and kitchen. You will require at least 4 gallons of hot water per minute (gpm) to run both places without any interruption.

If the incoming cold water temperature is about 50 degrees f and the desired hot water temperature is 120 degrees f, then the temperature rise is 70 degrees f (120°f – 50°f = 70°f). Thus, your demand is 4 gpm at a temperature rise of 70°f.

Benefits Of Using A Sizing Calculator

Using a sizing calculator is highly recommended as it allows for a more accurate calculation of the right-sized tankless water heater for your home. This will eliminate guessing and assist in determining the exact flow rate and temperature rise needed.

The advantages of using a sizing calculator are:

  • No guesswork – the calculations are based on factual and accurate information
  • Saves money – buying the right-sized tankless water heater means you won’t overspend on a larger unit than necessary
  • Energy-efficient – the correct size tankless water heater will be more energy-efficient than an oversized one, helping to save both money and energy

By carefully following the above guidelines, you can calculate the perfect sized tankless water heater for your home and enjoy the benefits it offers.

Common Mistakes To Avoid When Sizing A Tankless Water Heater

When it comes to sizing a tankless water heater, there are several common mistakes that homeowners tend to make. Here are some of the most crucial ones to avoid:

Overestimating Hot Water Usage

One of the common mistakes that homeowners make is overestimating their household’s hot water usage. This can lead to purchasing a tankless water heater that’s too large, which results in wasted energy and increased costs. To avoid this mistake, it’s essential to determine your home’s exact hot water needs.

Bullet points:

  • Calculate the peak hot water demand
  • Consider the number of people in the household
  • Estimate the amount of hot water used in daily tasks

Underestimating Peak Hot Water Usage

On the flip side, homeowners may underestimate their peak hot water usage, leading to a tankless water heater that’s too small. This issue results in inadequate hot water supply, particularly during heavy usage times such as baths, laundry, or dishwashing.

Hence, it’s crucial to identify peak hours and their hot water needs to steer clear of this error.

Bullet points:

  • Do not forget “busy” hours
  • Identify the largest hot water demand
  • Calculate enough hot water for simultaneous hot water use

Ignoring The Effects Of Cold Climate On Tankless Water Heaters

Ignoring how cold climate affects tankless water heaters is another common mistake. Tankless water heaters work efficiently in warmer climates, but in colder regions, its performance declines, leading to additional costs. It’s important to consider the impact of climate on tankless water heaters, the temperature rise, and flow rate required to maintain hot water supply.

Bullet points:

  • Know your incoming water temperature
  • Explore the need for a condensing tankless water heater in colder regions
  • Check the minimum flow rate to activate the water heater in winter

Choosing A Tankless Water Heater Based On Price Alone

A homeowner must not base the decision of purchasing a tankless water heater solely on price. Poor-quality water heaters are likely to malfunction early, causing more downtime and increased costs. It’s equally necessary to consider the features, energy efficiency, hot water output, and warranty period when selecting a water heater.

Bullet points:

  • Consider the lifespan of the water heater
  • Know the energy savings of a tankless water heater
  • Choose established brands or models with a good warranty

Making these mistakes can cause long-term problems when it comes to furnishing hot water for your household needs. By avoiding these mistakes, you can select the right size tankless water heater that will offer efficiency while reducing costs.

Frequently Asked Questions For What Size Tankless Water Heater Do I Need

What Is A Tankless Water Heater And How Does It Work?

A tankless water heater heats water only when needed, without storing it. It uses a heat exchanger and a heat source (usually electricity or gas) to quickly heat water as it passes through the unit. This makes them more energy-efficient than tanked water heaters.

How Do I Determine The Size Of A Tankless Water Heater I Need?

Several factors can affect the size of a tankless water heater you need, including the number of people in your household, the number of appliances that use hot water, and your home’s climate and water hardness. A professional installer can use these factors to recommend the right size unit for your needs.

What Are The Benefits Of A Tankless Water Heater?

Tankless water heaters have several advantages over traditional tanked models. They can save energy and money on utility bills, take up less space, and provide homeowners with an endless supply of hot water. They also tend to last longer than tanked models and are easier to maintain.

Are There Any Drawbacks To Using A Tankless Water Heater?

While tankless water heaters have many benefits, there are also some drawbacks to consider. They can be more expensive to purchase and install than tanked models, and they may not work as well in homes with low water pressure or hard water.

Additionally, multiple units may be required in larger homes to meet the demand for hot water.

How Often Does A Tankless Water Heater Need Maintenance?

Most tankless water heaters require annual maintenance to keep them running efficiently. This usually involves flushing the unit to remove any mineral buildup and checking the components for signs of wear and tear. It’s important to follow the manufacturer’s recommendations for maintenance to keep the unit working properly and to prevent problems in the future.


After reading this article, you should have a clear understanding of what size tankless water heater you need for your home. It is essential to consider the size of your home, family needs, and hot water usage. You do not want to flush your money down the drain by purchasing a unit that fails to meet your hot water needs, nor do you want to go overboard by buying a unit that is too large.

Consulting a professional plumber or an hvac technician can help ensure that you purchase a unit that is the right size and meets your home’s specific hot water needs. Remember to research the different tankless water heater options based on size, efficiency ratings, and fuel source before making a purchase.

Ultimately, choosing the right size tankless water heater can save you money, improve energy efficiency, and provide an endless supply of hot water in your home.