What Tankless Water Heater Size Do I Need?

A water heater's job is to heat the water you use throughout your property. You might take yours for granted until you turn on the shower and feel cold water coming out of the faucet. Tankless water heaters use powerful burners to make instant hot water, transferring it directly to the faucets in your house without storing it in a tank. Are you considering one for your home? Do you know the best tankless water heater size to fit your daily needs?

If you need a new water heater for your house, consider tankless water heaters that offer convenience and compact design. Based on the space you have in your home and the gallons of hot water you use per day, you'll need to order the right-sized tankless water heater for your lifestyle. To determine which tankless water heater size works best for your living space, consider these factors:

  • The size of your home
  • The flow rate of your appliances and faucets
  • The number of people who live in your household
  • The temperature rise you'll need to heat the water
Proper Tankless Water Heater Size

1. The Size of Your Home

Tankless water heaters provide instant hot water the second you turn on the tap, so they have to be ready to bring heated water to your bathroom faucets, kitchen sink, shower, dishwasher and washing machine. You need a tankless water heater size that will accommodate your whole house, whether you have a small apartment or a 10-bedroom home. The more rooms you have, the more hot water you are likely to need.

Consider these factors to find out what tankless water heater size you should order:

  • The number of bathrooms: Full bathrooms come with showers, sinks and tubs. If you have two bathrooms, two people in your house could take separate showers simultaneously. Your family could also use the sink, which uses hot water, while someone is in the shower. Make sure you provide enough hot water for all the bathrooms in your house so that your family members and guests can have a comfortable experience taking showers, washing hands or soaking in the tub.
  • Type of kitchen and cleaning appliances: Kitchen sinks, dishwashers and washing machines need hot water to clean your belongings effectively. If you have a washing machine and dishwasher in your house, which each use several gallons of hot water per load, you may need a larger tankless water heater size to make sure you have enough capacity to sanitize your clothes and dishes. Keep in mind that you may need to run the dishwasher while your child is taking a shower or wash a load of laundry while rinsing off pots and pans.
  • Free space: While a tankless water heater is much smaller than a traditional water heater, you still need to be creative in where you want to store it. You should put your tankless water heater as close to the appliances as possible to heat the water for your sinks and appliances properly. You need to have free space in your house where you can store the water heater, whether you want to hide it for aesthetic purposes or keep it out in the open for easy access. Some smaller models can fit under a kitchen or bathroom sink, but they have limited use.

Don't Underestimate the Tankless Water Heater Size

Double-check that the heater you choose isn't too small. An inferior tankless water heater size may restrict your overall usage. You should have enough hot water to provide for all the appliances and faucets throughout your living space.

You need a large tankless water heater that can transfer hot water to all the faucets in your house if they all run together. If you have a small apartment or a mobile home, your rooms are smaller and closer together, so your tankless water heater doesn't have to work as hard to give you enough ready-made hot water.

2. Tankless Water Heater Flow Rate

Tankless water heaters heat the water flowing through the faucets and appliances in your house. You need hot water to be ready at any given time, especially for those busy mornings when multiple family members are showering as you're running the dishwasher and doing the laundry. To know what size tankless water heater you should order, you need to determine the incoming water rate of each of your appliances.

When you're trying to calculate the total flow rate you need for your tankless water heater size considerations, find out the flow rate of your water sources at their peak performance, measured in gallons per minute (gpm). You can look up the gpm of the various water spouts and appliances in your house by checking the manual or searching the product's model number on the manufacturer's website.

General Appliance Flow Rates

You can estimate the hot water per minute of the different water sources in your home by consulting this chart if you don't have the original paperwork of your appliances or faucets. Keep in mind that if your faucet is smaller or listed as energy-efficient, it would be on the lower end of the scale:

  • Showerhead flow rate is 1.25 gpm to 2.5 gpm.
  • Bathroom faucet flow rate is 1.0 gpm to 2.2 gpm.
  • Kitchen faucet flow rate is 1.75 gpm to 2.2 gpm.
  • Dishwasher flow rate is 4.25 to 10 gallons of water per cycle
  • Washing machines flow rate is 14 to 20 gallons of water per load.

After you determine the flow rate of your water sources, consider which appliances and faucets you would use at the same time. The number of gallons of hot water a household uses during peak times depends on their unique lifestyle. In the morning, your family might be piling into your bathrooms, taking showers and using the sinks, washing the dishes or running laundry. Perhaps your family is more energy-savvy and only uses one faucet at a time.

Once you know which appliances and faucets you use together, add the gpm of these appliances. Rather than including all the appliances and faucets you have, only include which water sources would be running simultaneously. The total water flow rate will help you determine which size of tankless water heater to buy. You can find the flow rate of your tankless water heater by looking at its specifications on the distributor's website.

Tankless Water Heater Size for Number of People in Your Home

3. Number of People in Your Home

Your household head count affects your tankless water heater size because everyone in your house uses hot water at some point throughout the day. To get an accurate idea of the water heater capacity you need for your house, you should always overestimate the amount so you'll have enough water for everyone.

Consider these factors for deciding your water heater's size according to the number of people in your home:

  • Your family's schedule: Consider your household's rhythm and flow before deciding how large or small your tankless water heater size should be. Your family members may use the water in your house throughout the day in different quantities. If you have children in sports, they may need to shower before and after school. The people living in your home might wash their laundry or rinse off dishes based on when they're available.
  • How often you host parties and holidays: Your tankless water heater size needs to handle occasional usage of individual faucets as well as high peak usage. Aside from the people living in your house, you should also consider how often you host large parties. If your house is where people gather for holidays, you'll need more hot water to provide for the extra cooking and cleaning you do during these large gatherings.
  • Who visits and at what times: If you have an open-door policy, you should have a larger water heater to accommodate people coming in and out of your house. Your living space may be where your kids' friends hang out after school. The tankless water heater in your home makes enough hot water for the people using your bathroom and kitchen. You should also consider how often you have overnight guests who will need to take showers and use your resources for a longer time.
  • What changes will occur in the future: You may also need to consider how your family will grow and change over the years if you just moved into your house. A tankless water heater will last for several decades, so you don't want to replace it any time soon once you get a new one. Find a water heating system that can accommodate the number of children you plan to have and the parties you'd like to host over time.

4. Temperature Change

Water heaters alter the groundwater temperature to make it comfortable for when you shower or wash your hands. The groundwater under and around your home is either warm or cold, depending on your local area. Your water heater bridges the gap between groundwater temperature and the desired temperature. The difference between the groundwater and desired temperature will affect how much heat your unit needs to generate enough hot water.

To determine the temperature rise of your tankless heater:

  • Find out the groundwater temperature: You can determine the natural temperature of your groundwater based on the climate region where you live. In general, if your property is in the northern region of the United States, your groundwater will be cooler than a spot in the southern zone. Also, the temperature of your groundwater may change based on the time of year because the weather is warmer or colder.
  • Think about why you need hot water: Depending on the appliance you're using, you need hot water for different reasons. Are you quickly washing your hands in the sink or are you taking a long shower? Your appliances, such as your dishwasher or washing machine, also require a hot temperature to sanitize your dishes and clothes. Think about your hot water usage and find out which one would use the hottest water.
  • Find out how hot your water needs to be: Each of your appliances can reach a maximum water temperature, depending on their use and water capacity. You can find the maximum temperature of your appliances and faucets by consulting the manufacturer's instructions or looking up the product's model number. For most applications, the water needs to reach a temperature of 120 F. Dishwashers without internal heaters may need the water heated at 140 F. You could also find a temperature based on your personal preference.
  • Find the difference: After determining the groundwater temperature and the maximum temperature of your hottest appliance, subtract your local groundwater temperature from your desired shower temperature. This number is the temperature rise that your tankless water system needs to accomplish to heat your groundwater.
Tankless Water Heater Power Source

5. Power Source

A residential tankless water heater will either use gas or electricity as their power source.

  • Electric tankless water heaters work best with point-of-use applications or in houses with smaller water needs. You may also use an electric water heater if your home has hotter incoming water with smaller gpm and heating requirements.
  • Gas tankless water heaters are excellent for whole-house use and households with larger water needs or residential properties with colder groundwater.

Power Priorities

Follow these tips for determining the power source you should choose for your tankless water heater:

  • Try to stick with the original design: If you're switching from a tanked system to a tankless one, you should keep the same setup to cut down on installation costs. You also know that your power source will work with your new tank if you use the same one. If you want to switch from gas to electric or electric to gas to save money, keep in mind that the installation cost will go up and the process may take longer.
  • Consider price and availability: While the upfront cost of one power source may be less expensive, you should also factor in the initial installation and maintenance price over the years. Gas or electricity might be more available in your area, so one power source may be more affordable for you. Your home might also already have gas or electrical access, which would make your tankless water heater easier to install. Before investing in a tankless water heater, research prices in your area to see which energy source is cheaper.
  • Go with what you want: Even though electric water heaters are typically for smaller homes, you may want it because it's a more affordable option. Keep in mind that they're more expensive to install, especially if your house already has a gas hookup. You may also choose gas water heaters because they're more reliable, even if the power goes out. Either way, you should pick the power source you want, especially if you're remodeling part of your home.

Find Your Perfect Tankless Water Heater Size Here!

At Ingram's Water & Air, we have a wide selection of tankless water heaters for your home, whether you're in a small apartment in New York City or a spacious home in Colorado. Browse through our collection of water heaters to find the best application for you and your property.

Along with our high-quality, reliable water heaters, we offer free shipping on most online orders and a price match if you find a lower price somewhere else. We also provide lifetime technical support on our products to help you take care of them for as long as you have them.

2 comments (view/add)
  • C Leitch
    C Leitch
    Posted on 1/13/2022

    Great explanation. So please provide the formula to answer the question “what size tankless water heater do I need”. Your answer was, “it depends “. I’ve already determined my needs, that doesn’t answer the question without providing a formula. Not helpful.

  • Rebekah Muller
    Rebekah Muller from Ingrams
    Posted on 1/17/2022

    The general estimate for sizing for residential use is 5 gpm per person at a temperature rise of 100F. If your know the maximum gpm required for your household at any one time you can use that and find a model which has the required capacity.

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