How To Calculate Tankless Water Heater Size

How to Select the Right Size Tankless Water Heater

Tankless water heaters are classified according to the highest temperature rise that may be achieved at a given flow rate. For this reason, in order to calculate the appropriate size of a demand water heater for your home, you must first estimate the flow rate and temperature increase that will be required for its application (either the entire house or a distant use, such as a bathroom). It is vital to note that you should never attempt to save money by purchasing a tankless water heater that is undersized.

Step 1:

In order to determine the maximum temperature rise achievable for a given flow rate, tankless water heaters are rated according to this parameter: For this reason, in order to calculate the proper size of a demand water heater for your home, you must first estimate the flow rate and temperature increase that will be required for its application (either the entire house or a distant application, such as a bathroom).

Please keep in mind that you should never attempt to save money by purchasing a tankless water heater that is too small.

Step 2:

Tankless water heaters are rated based on the highest temperature rise that can be achieved at a given flow rate. As a result, in order to properly size a demand water heater, you must first establish the flow rate and temperature increase that will be required for its application (either the entire house or a distant use, such as a bathroom) in your home. It is critical to remember that you should never try to save money by purchasing a tankless water heater that is too small.

Step 3:

Example of sizing: An typical shower will be between 104 and 106 degrees Fahrenheit and utilize 2.6 gallons of water. Assuming that the water temperature entering your home is 40° and that you wish to create enough hot water to run two showers at the same time, what temperature rise would you need to produce to achieve this goal? Answer:You’ll need to boost the temperature of the entering water from 40 degrees to 105 degrees. The ability to heat a minimum of 5.2 gallon of water will be necessary.

Flow Rates

Is there a limit to how much hot water you may use at once? You need to operate two showers at the same time, or a shower and a pair of sinks, or anything similar. The figure below illustrates the range of water consumption ranges as well as the typical water temperatures for a variety of fixtures. In order to determine your total simultaneous water requirements, we recommend that you use the following reference points: 2.5 gpm for showers and 1.0 gpm for bathrooms.

Fixture

Tub 4.0 GPM 102°F
Shower 2.5 – 3.0 GPM 104°F
Washing Machine 2.0 GPM 120°F
Dishwasher 1.5 GPM 110°F
Kitchen Sink 1.5 GPM 110°F

Consider the following scenario: If you are taking two showers at the same time, you will require 5 gallons of hot water per minute from your tankless water heater. A shower and the washing machine would each use 4.5 gallons of water per minute from the water heater, thus you would need to turn on both at the same time. In any of these scenarios, you’ll want to make sure that the unit you choose is large enough to accommodate or surpass the amount of hot water you’ll require at the same time.

It is important to note that tankless water heaters are designed to heat only potable (drinking) water, and that the water entering a tankless device should not be pre-heated.

Other Sizing Notes

Gas tankless water heaters have the ability to create a greater temperature rise per gallon of water than electric tankless water heaters. The majority of demand water heaters are rated for a wide range of water temperature inputs. An average flow rate of 5 gallons per minute through gas-fired demand water heaters and 2 gallons per minute through electric water heaters allows for a 70°F temperature rise in the water. Increased flow rates or decreased intake temperatures can occasionally result in a reduction in the temperature of the water at the furthest faucet.

Recommended Reading:

  • Learn how to use a washer dryer combo
  • Learn about three common misconceptions about washer dryer combos
  • Learn all you need to know about wall heaters in this comprehensive guide
  • And more. Infrared Heaters 101: Your Complete Guide to Understanding Them

About Our Team

Jeff Flowers is just a person who is plagued by a chronic case of curiosity and who frustrates everyone around him with his rambling nonsense. In his journey from beer to house living, Jeff is simply attempting to hack his way through life while also writing a few notes about his experiences along the way. You can read his rants here, or you can listen to him whine about Austin traffic on Twitter at @Bukowsky. You can also follow him on Facebook.

What Size Tankless Water Heater Do I Need? (With Calculator)

Once you’ve made the decision to acquire a tankless water heater, the following step is determining what size tankless water heater you’ll need for your household. This helpful tankless sizing calculator will assist you in determining the appropriate tankless size for your house. Simply answer the questions, and the program will calculate the results for you!

Tankless Sizing Calculator

To establish your flow rate and temperature rise, please use ourSizing Calculator.

Where are you located?

We wish we could give you a simple answer, but you will need to spend some time working through the calculator to discover your specific requirements. It is important to consider where you reside, how many fixtures will be in use during your peak hour, and whether or not you will be doing laundry or other appliances when deciding the appropriate tankless size. However, it’s fair to suggest that you’ll need to invest in a bigger tankless water heater. In the majority of circumstances, a tankless heater with a flow rate of at least 7 GPM will be required.

What is a Tankless Water Heater?

As much as we wish it were a simple response, determining your specific requirements requires you to go through the calculator. It is important to consider the location of your home, the number of fixtures in use during your peak hour, and whether or not you will be running laundry or other appliances when choosing the appropriate tankless size. However, it is reasonable to assume that you will require a bigger tankless heater. A tankless heater with a flow rate of at least 7 GPM will be required in the vast majority of situations.

What Does it Mean toSize a Tankless Water Heater?

In the tankless water heater market, sizing a tankless water heater refers to the process of estimating the quantity of hot water your family uses in order to acquire a tankless appliance that will fulfill your hot water requirements.

It is necessary to understand the following factors in order to appropriately size a tankless water heater:

  • TemperatureRise (this is decided by your geographic location)
  • PeakHoursofUse (the time of day when you’ll need the most hot water)
  • PeakHourofUse (the time of day when you’ll need the most hot water)
  • Demand (the number of hot water fittings that are utilised during peak hour)

The answers to these questions will be entered into our tankless water heater size calculator, which will calculate the GPM required for your household’s demands. Your new tankless water heater may be purchased using the GPM that you have obtained from the manufacturer.

Why is Where I Live Important?

Simply said, the location of your home is critical in determining the necessary temperature rise. The temperature of the ground changes depending on where you live, and the temperature of the ground impacts the temperature of the entering water. The temperature rise is one of the most important factors to consider when determining the right size of a tankless water heating system. Suppose the incoming water temperature is 50°F and your tankless water heater is set to heat the water to 110°F. The temperature rise will be 60°F.

Because of this, you’ll need to invest in a more powerful tankless water heater in order to provide the amount of hot water required by your home.

What Does Peak Hour Mean?

In order to provide adequate hot water to your home when the demand is the highest, tankless water heaters must be capable of doing so. When sizing a tankless water heater, the word peak houris used to denote the time of day when you’ll be using the most hot water (usually in the morning). In the majority of situations, the busiest time is in the morning. Take a look at your morning (or peak hour) routine to get an idea. Is anyone taking a shower? Wash you do your own laundry? Do you want to run the dishwasher?

All of these tasks necessitate the use of hot water.

A tankless water heater that is not properly sized (or that is underpowered) can supply lukewarm water, making your shower less than enjoyable.

What is GPM?

It is the flow rate of a tankless water heater that is measured in gallons per minute (GPM), which is an abbreviation for gallons per minute. The flow rate is defined as the number of gallons per minute (GPM) of hot water that the tankless water heater can heat in one minute. The higher the GPM, the greater the amount of hot water that the tankless can provide. A tankless water heater that has a flow rate of 8 GPM will be able to supply more hot water than one that has a flow rate of 5 GPM, to put it another way.

It’s vital to remember that you just need to pay attention to the amount of fixtures that are in use during the peak hour at the moment.

If you have two showers but only use one during peak hour (or if they are not both in use at the same time), you will only use one in your calculations if you only take one shower during peak hour.

Should I Purchase a Gas or Electric Tankless Water Heater?

Once you’ve made the decision to go with a tankless water heater, the next important question is what kind of power source you’ll use. Both gas and electric tankless water heaters have advantages and disadvantages, and you should do your homework before making a decision. Read our post, Gas versus Electric Tankless Water Heater: Which is Better, for more information on this topic. This page analyzes and contrasts the two fuel sources in detail. Operational expenses, installation costs, maintenance needs, and other critical variables are all taken into account in this report.

When compared to a traditional tank-style water heater, an electric tankless water heater is easier to install, requires less maintenance, and has a lower initial cost.

Which Brand of Tankless Water Heater Should I Buy?

There are many high-quality tankless water heater brands available on the market today, but there are also a handful that are less than fantastic. In the end, the manufacturer you pick will be determined by the model that best matches your requirements. With a tankless water heater from Rinnai, Rheem, or Stiebel Eltron, it’s difficult to go wrong with your choice. EcoSmart offers a wide range of models that are both functional and affordable. Nortiz, Navien, and Bosch are among well-known and recognized names in the industry.

It is therefore essential that you choose a reliable manufacturer when purchasing your water heater.

  • Read our article on Gas Tankless Water Heaters for more information. We provide our top recommendations, as well as an in-depth buying guide, on this page. Read our article on Electric Tankless Water Heaters for more information. A buying guide is also included, in addition to our best picks:

What Size Tankless Water Heater Do I Need?

Note from the editors: We receive a commission from affiliate links on Forbes Advisor. The thoughts and ratings of our editors are not influenced by commissions. Unlike traditional tank water heaters, tankless water heaters give continuous hot water to the kitchen and bathroom. For many homes, installing tankless water heaters represents a welcome break from the deplete-heat-wait cycle associated with traditional tank-style water heater installation. However, reducing the tank also implies that the tolerance margins will be reduced to a bare minimum.

Properly sizing the tankless water heater ensures that you will never be without hot water—and that you will not be forced to purchase a system that is either too large or too expensive.

Testimonials from customers Exceptionally well-written Performance Platinum 9.5 GPM Liquid Propane Water Heater by Rheem Performance Platinum 9.5 GPM Liquid Propane Water Heater by Rheem 3Rinnai RU199iP RU Model Series 3Rinnai RU199iP RU Model Series 3Rinnai RU199iP RU Model Series 3Rinnai RU199iP RU Model Series 3Rinnai RU199iP RU Model Series Testimonials from customers Exceptionally well-written 3Rinnai RU199iP RU Model Series 3Rinnai RU199iP RU Model Series 3Rinnai RU199iP RU Model Series 3Rinnai RU199iP RU Model Series 3Rinnai RU199iP RU Model Series 4Rinnai V53DeP V Model Series 4Rinnai V53DeP V Model Series 4Rinnai V53DeP V Model Series 4Rinnai V53DeP V Model Series Model Series: Rinnai RL75eP RL Model Rinnai RL75eP Testimonials from customers Exceptionally well-written Model Series: Rinnai RL75eP RL Model Rinnai RL75eP (Please keep in mind that all information and pricing are current as of publishing and are subject to change.) If you’re considering purchasing a propane tankless water heater, you may feel overwhelmed by the number of options available.

Price, customer rating, maximum GPM, heating capacity (in BTUs), and Energy Star certification were all taken into consideration while compiling this top-five ranking.

What a Tankless Water Heater Does

Traditional tank-style water heaters, in contrast to tankless water heaters, heat 40 to 50 gallons of water using a gas or electric burner to heat the water. The burner helps to maintain that temperature by turning on and off intermittently when the water temperature lowers. Maintaining the temperature of a pot of water on the stovetop by turning the burner on and off as needed is analogous to this. Tankless water heaters do not store hot water in a reserve tank like traditional water heaters.

  1. They are energy efficient.
  2. The majority of homes have one or two tankless water heaters, which are often positioned in the basement, mudroom, utility room, or hallway of the house.
  3. Customers with tank-style heaters experience the same transient temperature difference as those who use faucet-style heaters because of the distance between the water heater and the faucet.
  4. The installation of supplemental heat recirculators, which circulate the water in a continuous loop between the faucet and the heater, is an option for certain households.
  5. Some tankless water heaters are equipped with built-in heat recirculation systems.
See also:  How To Install Fridge Water Filter

How to Calculate the Right Size of Tankless Water Heater

Temperature rise is defined as the difference between the groundwater temperature in your location during the winter, or the coldest time of year, and the recommended set temperature of your tankless water heater. To find out what the typical winter groundwater temperature in your location is, look at a map showing average winter groundwater temperatures. Alternatively, during the coldest time of year in your location, you may use a thermometer to monitor the temperature of your water as it is drawn directly from the ground (usually, at an exterior faucet).

Determine Peak Hot Water Demand

When it comes to peak hot water demand, this is the greatest amount of hot water that your home may require at any given time. It is not intended to be a practical figure; rather, it is intended to ensure that your water heater is capable of meeting theoretical peak demands. Bathroom Utilities 2 gallons per minute for the clothes washer Estimate and include the maximum number of services that might be consuming hot water at any given time, such as the following: Shower = 2 gallons per minute 1.5 gallons per minute (gpm) in the kitchen sink Dishwasher = 2 gallons per minute Total flow rate: 5.5 gpm Alternatively, in a household with a large number of individuals who need hot water: Shower = 2 gallons per minute 2 gpm in a tub 1.5 gallons per minute at the bathroom sink 1.5 gallons per minute (gpm) in the kitchen sink Dishwasher = 2 gallons per minute Total flow rate: 9 gpm

Choose a Tankless Water Heater

Tankless water heater manufacturers provide calculation tables or online calculators to assist you in determining the best model for your needs based on peak hot water demand and the temperature rise in your location. For example, if the temperature rise in your location is 60 degrees, your residence may have a peak hot water demand of 6 gpm at its highest point. This may direct you to a few of models that meet the bill in this situation. However, if the temperature rises over 40 degrees Celsius, the manufacturer may propose totally other models.

You must compare and contrast the two sets of information in order to determine the most appropriate tankless water heater for your home.

Tankless Water Heater Pros and Cons

  • When compared to huge tank models, little units take up less room. There is no need to wait for the hot water to cycle because there is continuous hot water. Because of the elimination of standby hot water, there will be no unnecessary heating of water.
  • There is no hot water stored in reserve in case the electricity goes out
  • When all aspects are taken into consideration, it does not cost less than tank models. Excessive scaling as a result of the extremely hot burner, resulting in a larger requirement for routine maintenance

Compare Quotes From Top-rated Water Heater Installers

Estimates are provided without obligation.

What Size Tankless Water Heater Do I Need? (For Family Of 2,3,4,5,6)

When it comes to tankless water heaters, one of the most common mistakes is purchasing a device that is not powerful enough to meet all of our hot water demands. You don’t want a tankless heater that’s too little, nor do you want one that’s too large and would waste energy unnecessarily heating your home. The size of your tankless water heater should be as close to your household’s hot water requirements as feasible. In what size tankless water heater do I need to invest my money? In order to determine how many GPM tankless water heaters I require for the gas unit and how many kW I require for the electric unit, I must first determine how many GPM tankless water heaters I require for the gas unit.

  1. What is the greatest amount of hot water you require? It’s important to know how much water per minute (measured in gallons per minute, or GPM) a particular tankless water heater can heat up, as well as how many degrees it can heat it up by.

It is necessary to establish a preliminary estimate of our maximal hot water requirements at any given point in order to properly design the tankless water heater. From 9 p.m. to 11 p.m., most families have the greatest demand for hot water. That is the time of day when we shower, brush our teeth under a hot faucet, and perhaps even have the dishwasher on. We need to keep track of how much hot water we’re using. Here’s a handy table that shows how many GPMs are required by different types of water fixtures:

Fixture Gallons Per Minute (GPM)
Shower 2.0 – 3.0 GPM
Faucet (kitchen, bathroom) 1.0 – 2.0 GPM
Dishwasher 1.5 – 2.0 GPM
Washing Machine 2.0 – 2.5 GPM

A tankless water heater with a minimum flow rate of 5 GPM and a maximum temperature of 110 F will be required if you are having a shower (with 100 percent flow and 110 F hot water) and concurrently using two faucets (with 100 percent flow and 110 F hot water). It is possible to get anything from 2 GPM to 12 GPM of hot water using a tankless heater. How many gallons per minute do you require? The ones with a flow rate of 5-10 GPM are the most suitable for the majority of houses. As previously stated, the cost of a tankless water heater rises in direct proportion to the capacity of the unit.

Keep in mind that electric tankless hot water heaters are best suited for small water demands up to 8 GPM. If you have a larger need (8 GPM or more), you should choose one of the finest gas tankless hot water heaters available.

Difference Between Maximum Water Flow And Realistic Maximum GMPs

When comparing the specifications of different tankless heaters, you will see that they all list the maximum GPMs. When it comes down to it, the highest GMP that your tankless heater will truly reach might be far lower. What is the source of the discrepancy? Because the maximum water flow in GMP is calculated by heating water to 77 degrees Fahrenheit, The inlet temperature of the water that is currently in your pipes is quite important. For example, in south Texas, the inflow water temperature is 77 degrees Fahrenheit.

  1. That represents an additional 40 degrees Fahrenheit differential that a tankless water heater must overcome.
  2. Because the input temperature is 77 degrees Fahrenheit in Texas, we can really obtain 10 GPM of 110 degrees Fahrenheit water.
  3. In Minnesota, on the other hand, the inlet water temperature is 37 degrees Fahrenheit.
  4. You don’t come from Minnesota or Texas, do you?
  5. An additional example based on the infographics shown above is as follows: If you reside in Florida (inlet temperature of 77 degrees Fahrenheit), the Rinnai RU160iP SE+ Series tankless heater will have a maximum water flow of 7.1 GPM at its maximum temperature.
  6. If you reside in New York, on the other hand (with an intake temperature of 52 degrees Fahrenheit), the same tankless water heater may deliver a maximum water flow of 4.5 GMP.
  7. In New York, the heater must contend with an additional 25 degrees Fahrenheit.
  8. It’s important to consider the operating costs as well, especially with larger units.
  9. Another useful piece of information about propane units is how much propane is consumed by these on-demand hot water heaters.

What Size Tankless Water Heater Do I Need For A Family Of 2, 3, 4, 5, Or 6?

When it comes to tankless water heater sizing, one of the most often asked topics is how much of a unit you need for a household of multiple people. Obviously, a tankless water heater designed for a family of three will be smaller than one designed for a family of five. But what are the specific GPM (for gas-powered engines) or kW (for electric-powered engines) figures? Because of the changing temperature of the water entering the tankless hot water heater, determining the correct size of the tankless hot water heater is difficult.

A table containing estimations may be seen below. Unfortunately, the confidence ranges are extremely wide. Those in the Northern United States will require larger units than homes in the Southern United States, for example, as follows:

Number Of Family Members: Gas Tankless Heater Size (GPM) Electric Tankless Heater Size (kW)
What size tankless water heater do I need for afamily of 2? 6-8 GPM 10-18 kW
What size tankless water heater do I need for afamily of 3? 7-9 GPM 15-23 kW
What size tankless water heater do I need for afamily of 4? 8-10 GPM 20-28 kW
What size tankless water heater do I need for afamily of 5? 9-11 GPM 25-34 kW
What size tankless water heater do I need for afamily of 6? 11+ GPM 34+ kW

These data are provided just as a point of reference. The size of your tankless water heater is determined by a number of crucial criteria, such as the temperature of the water entering the tank and the amount of hot water you use on a regular basis.

Looking At Specifications Sheets

It is common to find manufacturers specifying a maximum water flow number in GMP or a maximum electric power number in kW on specification documents for their products. The GMP number for gas-powered tankless water heaters is often found on the product label, whereas the kW number for electric tankless water heaters is found on the product label. As we’ve shown, the maximum GMP is a function of context. It is dependent on where you live in the United States (because that affects the inlet water temperature).

  1. Comparing the maximum wattage of different tankless heaters (as we have done in the table of the best tankless heaters below) allows us to determine how powerful they are in comparison.
  2. Check here to see if investing in an energy-efficient tankless water heater is truly worth it (we performed some calculations).
  3. For example, if you want to replace your current 50-gallon water heater, the first question you should ask is what size tankless water heater you need.
  4. What size tankless water heater would be appropriate for a household of five, for example.

What Size Tankless Water Heater Do I Need To Replace A 50 Gallon Water Heater? (Example1)

Here’s how things work in this situation: You now have a tank-style water heater that holds 30, 40, 50, or even 80 gallons of water and wish to upgrade to a tankless water heater. The most significant distinction, of course, is that a water tank provides, say, 50 gallons of hot water, but a tankless water heater provides water heating on demand. For example, during a typical 10-minute shower, you use around 10 gallons of hot water on average. Taking 3 showers, running a few of faucets, running a dishwasher, and so on will easily deplete those 50 gallons in no time.

Instead of storing hot water, the tankless heater’s strong heating exchanger warms the water as it is needed, up to a particular maximum GMP limit, depending on the model.

  • If you live in the northern region of the United States, you should have a 10 GPM gas tankless water heater or at least a 27 kW electric tankless water heater. If you live in the southern region of the United States, you should have a 7 GPM gas tankless water heater or at least an 18 kW electric tankless water heater.

As a result, Rinnai, the world’s leading manufacturer of gas tankless heaters, provides a broad range of models ranging from 7 GPM to 11 GPM: Rinnai’s gas tankless versions are available in a variety of sizes. As previously said, they are considered to be the top gas tankless water heater brand.

Please keep in mind that this is simply an approximate estimation. The prudent course of action is to get a tankless heater that is somewhat more powerful than the anticipated need. It is preferable to be safe than sorry.

What Size Tankless Water Heater Do I Need For A Family Of 5? (Example2)

If five individuals reside in the same house, they can use multiple faucets or showers at the same time. This must be taken into consideration while determining the appropriate size of a tankless water heater. Showers are the home hot water user that consumes the most hot water the fastest. 5 persons can also operate many hot water taps at the same time, as well as a dishwasher and do laundry, among other things. In short, if you live in the northern part of the United States, where the input water temperature is lower, you would require a 10 GPM gas tankless heater or a 27 kW electric tankless heater.

For those who live in the southern part of the country, the tankless water heater’s capacity can be reduced by up to 30 percent.

Keep in mind that, especially with larger units, tankless water heater circulation pumps can save you a significant amount of money on hot water.

You won’t have to wait for hot water to start flowing, squandering all of the cold water that would have been wasted in the meanwhile.

How Many Tankless Heaters Do I Need?

This is a rather typical topic, especially when it comes to larger homes. Here’s how it works: In the majority of situations, one tankless heater is sufficient to heat an entire house. For those who live in really large homes (2 or more bathrooms), even the largest Rinnai gas tankless water heater with a flow rate of 11 GPM will not be adequate to meet all of their simultaneous hot water demands. It makes logical in these situations to install two tankless water heaters. The most common combination is as follows: The largest gas unit (11 GPM, 199,000 BTU) and the largest electric unit The large tankless water heater meets the majority of the household’s hot water requirements.

Alternatively, you might utilize two units for different parts of the home, one for one section and another for the other section of the house.

I hope this has been of assistance.

What Size Tankless Water Heater Do I Need? (+ Sizing Calculator)

This page may contain affiliate links, so please keep that in mind. If you purchase a product or service after clicking on one of these links, we will get a commission at no additional cost to you. See our product review method in further detail, or read our FTC affiliate disclosure for more information. Tankless water heaters allow on-demand access to an infinite supply of hot water. Furthermore, they can reduce your utility bills by more than 30% due to the elimination of standby energy losses, they are about the size of a suitcase and therefore take up less space, and they have a lifespan of up to 20+ years, which is nearly twice as long as the lifespan of a traditional tank-based unit.

See also:  Why Does My Water Filter Smell Like Fish

A system that is too tiny will not be able to deliver enough hot water to suit the demands of your entire household, so don’t skimp on this purchase.

This is why it is critical to have the right size before making a purchase. This purchasing guide will lead you through the process of sizing a tankless water heater and will also throw light on other issues that you should consider when shopping for a tankless water heater. Contents

  • This website may contain affiliate links, so please keep this in mind. You will not be charged any additional fees if you purchase a product or service after clicking on one of these links. See our product evaluation method in further detail, or examine our FTC affiliate disclosure for more information. Tankless water heaters allow on-demand access to an unlimited supply of hot water. Apart from that, they can lower your utility bills by more than 30% as a result of the elimination of standby energy losses, they are about the size of a suitcase and therefore take up less space, and they have a lifespan of up to 20 years, which is nearly twice as long as the lifespan of a conventional tank-based unit. In contrast, the amount of water that tankless water heaters can heat at any given moment is restricted. A system that is too tiny will not be able to supply enough hot water to suit the demands of your entire home, so don’t skimp on this investment. If you go too big, you’ll end up paying too much money. In order to avoid this, correct size should be performed before to purchasing any clothing items. This purchasing guide will lead you through the process of sizing a tankless water heater and will also throw light on other things that you should consider when shopping for a new tankless water heater. Contents
  • Tankless water heater size calculator
  • Frequently Asked Questions
  • Tankless water heater installation

How to Size a Tankless Water Heater

To put it another way, tankless water heaters, also known as on-demand water heaters, are rated according to their maximum output water flow rate at a given temperature rise. So, in order to properly size a tankless water heater, you must take into account two factors:

  1. How many gpm (gallons per minute) of hot water you’ll use during peak consumption periods is calculated. Keep in mind that tankless water heaters do not store water, but rather heat it as it passes through them. The needed temperature rise, which is determined by the parameters for the input water temperature and the output water temperature

What happens if you are undersized? That was a bad idea! Most water heaters will automatically reduce the supply of hot water if the demand for hot water exceeds the maximum capacity, even if it is just for a brief period of time. As a result, there will be less hot water available at each outlet, resulting in a temperature and/or pressure reduction. Furthermore, a tankless water heater that operates at maximum capacity all of the time is more likely to fail prematurely. What happens if you go a little too far?

The main drawback is the excessive up-front expense.

Step 1: Flow Rate – How Many GPM for a Tankless Water Heater

Begin by calculating your peak hot water use requirements. This may be accomplished by referring to the chart below. It is a list of the typical flow rates of several types of water outlets available for purchase in the United States. On the other hand, you may seek up your individual fixtures and other items on the internet or in their product manuals. Simply decide which devices you want to be able to operate at the same time and how many of them you want to be able to run at the same time.

In the case of two showers and one kitchen faucet operating at the same time, your necessary maximum water flow is as follows: 2.5 gpm plus 2.2 gpm equals 4.7 gpm.

It’s just that simple!

  • By anticipating the highest demand during peak hours, such as the morning, you can assure that you will always have hot water available, no matter what happens. An further benefit is the fact that a tankless water heater that does not have to operate at full capacity all of the time is likely to survive far longer. At the same time, with a little forethought, you will be able to drastically cut peak demand. It entails taking turns in the shower, allowing the dishwasher to do its work while everyone is away from the house, and preparing your meals before or after the showers are completed. You must obviously consider not just the amount of bathrooms in your home, but also the number of people that will be living under one roof while making this decision. Even in a home with five bathrooms, two persons can only use two showers at the same time
  • Consider the following questions: Will the water heater service your complete home or only portions of it? It is important to note that the flow rates mentioned below represent total water production, which includes both hot and cold water. As a result, showering with a 2.5-gpm shower head does not guarantee that the water will be hot during the showering session. It is more likely that you will need to mix in some cold to get the required temperature level. As a result, the real demand for hot water is slightly lower
  • It is possible to limit flow rates by installing low-flow aerators or fixtures.
Water Outlet Standard Flow Rate
Hand washing sink 0.5 – 1.5 gpm
Shower head 2.5 (2.0*) gpm
Bathroom faucet 2.2 (1.5*) gpm
Bathtub faucet 3.0 – 4.0 gpm
Kitchen faucet 2.2 gpm
Washing machine 23+ gallons per load, gpm hard to determine
Dishwasher 6 gallons per load, gpm hard to determine

*Products that are WaterSense certified Please keep in mind that older fixtures will most likely have greater flow rates. Please note that we did not include flow rates for washing machines and dishwashers as you can see in the table above. This is due to the fact that we found it quite difficult to obtain credible information on this topic. Some sources state 2 to 3 gpm, while others state 1.5 gpm. Instead, you might run each appliance independently and keep an eye on the timer and your water meter for any anomalies.

As a matter of fact, to get a more scientific perspective on your peak water flow, you may fill a 1-gallon bucket with water and time how long it takes your shower head and other fixtures such as kitchen faucets, bathroom faucets, and other fixtures to fill it up.

Use this formula instead of filling an entire gallon per outlet if you don’t want to squander a full gallon each outlet: Flow rate = 15 / Number of seconds necessary to fill a 14-bucket container

Step 2: Temperature Rise

The following step is to calculate the temperature rise that is necessary. All you have to do is subtract the temperature of your input water from the temperature of the desired output water in this situation.

Output water temperature minus feed water temperature equals required temp raise. What is the best way to determine the temperature of your feed water? There are two alternatives available to you:

  1. Measure using a thermometer
  2. Use our fantastic groundwater temperature map for the United States

Please keep in mind that these are approximations of typical temperatures. The actual temperature varies depending on the season and weather. As you can see, the location of your home in relation to the average groundwater temperature has a significant impact on the temperature of the water. The temperature will naturally be greater in warmer areas in the south, reaching up to 77 degrees Fahrenheit in southern Florida. On the other hand, groundwater temperatures may drop to as low as 37 degrees Fahrenheit in Alaska, sections of North Dakota, Minnesota, Wisconsin, and other northern states with colder climates.

Assume you reside in Michigan, where the average feed water temperature is 40 degrees Fahrenheit.

105 to 115 degrees Fahrenheit output water temperature is considered ideal for everyday household use, so 110 degrees Fahrenheit minus 40 degrees Fahrenheit equals 70 degrees Fahrenheit.

Showering at 105 degrees Fahrenheit is regarded to be the top end of the temperature range that is most pleasant.

Step 3: Putting It All Together

Okay, you know how much hot water you’ll need during peak hours, as well as the temperature spike that will be necessary. In order to complete this process, you must go out and seek for a tankless water heater that satisfies all of the standards. Almost all manufacturers include sizing charts with their goods, which state maximum flow rates for a specific temperature rise or vice versa, depending on the product. Some manufacturers additionally provide flow rates for various input and output water temperatures.

Keep in mind, though, that manufacturers tend to advertise their products by highlighting the best-case situations, so you should treat the information with caution.

Are you ready to take the next step?

Video

Do you prefer video? Take a look at this:

Size Chart

If you look at a few size charts, you will immediately discover that the use of gas or electricity makes a significant difference. In general, tankless gas water heaters are more powerful than electric water heaters, which means that they can produce more gallons per minute (gpm) at the same temperature increase.

As an illustration: If you want a temperature rise of 70 degrees Fahrenheit, a big tankless gas water heater can provide 5.0 – 5.5 gallons per minute at that temperature. The biggest electric unit (36 kW) produces a maximum flow rate of little more than 3.0 gpm.

Example Size Chart

For your convenience, the following is an illustration of a tankless water heater size chart: 6.6 gpm at a temperature rise of 50 degrees Fahrenheit and 4.8 gpm with a temperature rise of 70 degrees Fahrenheit for this specific heater.

Popular Tankless Water Heaters

Here are some real-world examples of tankless water heaters that are widely used: (Tip for mobile users: Swipe to scroll.)

Model Fuel Type, Power Temp Rise Max GPM of Bathrooms
Rinnai V75iN Natural gas, 180,000 BTU 70 °F 4.3 gpm 1 – 2 bathrooms
50 °F 6.0 gpm 2 – 3 bathrooms
Rinnai RU199iP Propane, 199,000 BTU 70 °F 5.5 gpm 2 bathrooms
50 °F 7.6 gpm 3 bathrooms
Stiebel Eltron Tempra 36 Plus Electric, 36 kW 70 °F 3.5 gpm 1 bathroom
50 °F 4.75 gpm 1 – 2 bathrooms
Rheem RTEX-18 Electric, 18 kW 65 °F 2.0 gpm 1 bathroom
55 °F 2.0 gpm 1 bathroom
EcoSmart ECO 11 Electric, 11 kW 68 °F 1.1 gpm 1 bathroom
48 °F 1.56 gpm 1 bathroom

Frequently Asked Questions (FAQ)

Which size tankless water heater do I need to provide hot water for a family of five? It’s virtually hard to determine from the information provided. It is dependent on the number of bathrooms and water-using appliances you have, as well as the temperature rise necessary. Follow the steps 1 through 3 in our sizing guide, or use our tankless water heater sizing calculator, and you will receive a satisfactory solution to your concern – guaranteed. I need to replace my 50-gallon water heater, but what size tankless water heater do I need to do so?

  • It is dependent on the number of bathrooms and water-using appliances you have, as well as the temperature rise necessary.
  • 3.
  • The number of tankless water heaters you require is determined by your peak hot water consumption and the temperature rise that is necessary (check sizing guide above).
  • Electric tankless water heaters are ideal for households with fewer people or for tiny flats.
  • Furthermore, even though it is more expensive up front, putting two smaller units in series can often be more cost effective than installing a single large unit at a single location.
  • Thank you for your time!
  • She is the head of content production and has completely immersed herself in the home water treatment sector, resulting in her becoming an expert in the field herself.
  • Find out more about.

How To Size A Tankless Water Heater

In the market for a new water heater, you may want to investigate tankless hot water heaters as an option to examine. This type of hot water heater may provide you with a number of various advantages. In addition, you will need to understand how to properly size a tankless water heater.

What Is A Tankless Hot Water Heater?

It is a type of water heater that does not contain a storage tank, as opposed to standard water heaters. As soon as you turn on your faucet, cold water is instantly turned to hot water, and vice versa.

You will save money on your utility costs because, unlike standard water heaters, you will not be required to heat the water on a continuous basis. Tankless water heaters are also more space-efficient than typical water heaters, taking up significantly less space.

Sizing Your Water Heater

When compared to standard water heaters, a tankless hot water heater does not have a tank to hold hot water. As soon as you turn on your faucet, cold water is instantly turned to hot water, saving you time. Your utility expenses will be cheaper since, unlike standard water heaters, you won’t have to heat the water on a continuous basis. Water heaters using tankless technology are also more space-efficient than typical water heaters, taking up significantly less space.

Sizing Example

You will need to have a shower temperature of between 104 and 106 degrees to get the most out of it. When you take a shower, it will use 2.6 gallons of water every minute. A minimum of a 60-degree increase in temperature is required, and your tankless hot water heater should be capable of producing 5.2 gallons per minute in order to guarantee that you have the appropriate water temperature. You will need to pay close attention to this since sizing your system incorrectly can result in you not having enough hot water to complete the tasks that you need to do.

  • Suppose you want to take two showers at the same time, or you want to run many sinks at the same time.
  • The water from your tankless water heater will be insufficient if you are running two different showers at the same time.
  • You’ll want to be certain that the heater you choose is the appropriate size.
  • If you want a higher temperature rise per GPM of water, you may want to look into buying a gas tankless hot water system here.
  • In order to have a better understanding of How To Size A Tankless Water Heater, contact the Original Plumber right now.
  • They may sit down with you and go through your use rates to evaluate if it would be more cost effective for you to purchase an electric tankless hot water heater or a gas tankless hot water heater.
  • The installation of a water heater may be complex and risky, therefore it is always a good idea to leave it to the experts.
See also:  About How High Can Water Be Theoretically Lifted By A Vacuum Pump At Sea Level

How To Size A Tankless Water Heater: Use Our Sizing Calculator

In the event that you are just getting started on your quest to get a tankless water heater, you are most likely asking yourself, “What size tankless water heater do I need?” When it comes to purchasing a tankless water heater, this is perhaps the most crucial question to ask. People are accustomed to thinking in terms of capabilities. For example, a hot water tank for 5 persons will necessitate the purchase of a tank that is at least 60 gallons in size. However, because tankless water heaters provide limitless hot water, you won’t have to worry about running out of hot water.

Flow rate is a measure of how much hot water you will want in a certain amount of time, and it is measured in Gallons Per Minute (GPM) (GPM).

In this post, I’ll go over flow rate and other important concepts so you’ll know exactly how to size your tankless water heater and save money. So let’s get this party started!

How Many Gallons Per Minute (GPM) Do I Need?

For the purpose of calculating your GPM, you must first identify which fixtures you can reasonably expect to be running at the same time. Then add up how much hot water (in terms of flow rate) is consumed by each individual fixture. Allow us to use the morning as an example because it is the time of day when most individuals are showering or doing laundry. Consider the following scenario: you have a two-bedroom, two-bathroom home with four people. You have two showers going in the morning, and someone has placed a load of clothes in the washer at the same time.

  1. A shower has a flow rate of how many GPM?
  2. Alternatively, how about a washing machine?
  3. Alternatively, you may refer to the useful cheat sheet provided below to get an idea.
  4. By the end of the week, you will have identified some trends and will be able to determine when your peak water demand occurs and how many GPM your on demand water heater will require to meet that demand accurately.

Tankless Water Heater Sizing Calculator

After you’ve written down the maximum number of appliances you use on a typical week, enter the information for those fixtures into our unique tankless water heater sizing calculator to get the appropriate size.

Fixture Type Flow Rate (GPM) Qty Total for Fixture Type
0 GPM
0 GPM
0 GPM
0 GPM
0 GPM
0 GPM
Total Flow Rate 0 GPM

You should have the whole GPM that you require for a tankless water heater at this point in time. Having discussed GPM, let’s move on to the considerations that go into determining what size inline water heater you require. First and foremost, you must comprehend what temperature rising entails. Temperature increase is just the difference between the temperature of the entering water and the temperature you desire to achieve. Your faucets should be set to a temperature of 110 degrees Fahrenheit in most cases.

When you see the GPM rate of a tankless water heater, you are looking at the best case situation.

You will have an infinite supply of lukewarm water if the GPM you want is greater than what the tankless water heater is capable of producing.

Determining Your Temperature Rise

To figure out how much your temperature will climb, you must first know the temperature of the water that is entering your home. In addition, this is based on groundwater temperatures, which will differ significantly depending on where you reside in the United States. See the groundwater temperature map to obtain a broad sense of the temperatures you may expect based on where you reside in order to plan ahead of time. If you want real-time temperature information, you might check this interactive map from the United States Geological Survey.

Simply take the temperature that you anticipate your incoming water to be based on the graph above and multiply it by two.

This represents an increase in your body temperature. Then you may look at the specification sheet of the tankless water heater you’re considering purchasing and see what the GPM rating is based on the temperature rise you calculated.

How Big of a Tankless Water Heater Do I Need?

The size of the tankless water heater you require is determined by the GPM of your peak hot water demand. In addition, your temperature rise is governed by the temperature of your entering water, which is deducted from your hot water setting. I’ll give you an example of what came out for me as a result of this exercise. Because I live in Boston, the temperature of my groundwater is around 47°F. Because I have little children at home, I keep the temperature of my water about 105°F. This is a safe temperature, and it will also help me save money on my gas bill.

  • I determined my GPM to be 6 since our peak water use is caused by a shower, a washing machine, and a faucet all running at once.
  • I produced a fairly extensive essay analyzing the top gas tankless water heaters for different sized houses that you might find useful.
  • Anyway, I’m wondering what size tankless water heater would be the most appropriate for a family of four.
  • Despite the fact that my variables are not the best case situation, the flow rate is reduced to 6.7 GPM from 9.8 GPM on the label.
  • For the calculation, you may use a Rinnai tankless water heater sizing calculator to identify the most appropriate model for your needs and budget.
  • If you live in a cold environment, the type of tankless water heater you choose will be influenced significantly by this issue.

How Big of a Water Heater Do I Need for Radiant Heat

When it comes to radiant floor heating and domestic hot water demands, water heaters are a popular choice among homeowners. The question is, though, how much of a water heater do you require for radiant heat. When you have a radiant floor installed, keep in mind that the tankless water heater will have to operate continually to keep up with demand. In addition to your typical home water use, you may want to think about upgrading to a greater GPM. Furthermore, because your water heater is expected to survive just 5-10 years if it is being used for radiant heat, we believe it is prudent to consider installing an aboiler in conjunction with your hydronic radiant heating system.

In order to figure out how much of a water heater you’ll need for radiant heating, you’ll need to figure out how much heat you’ll lose.

It actually depends on your specific scenario, such as how old your house is, and other factors.

The water heater listed below is one that we suggest for radiant floor heating.

It is a model with 11 GPM and 199,000 BPU, which is suitable for the majority of families. And don’t forget that radiant heating thermostats will assist you in keeping your system in good working order. Rinnai Water heater with a flow rate of 11 GPM

  • Limitless Hot Water: Take use of an unlimited supply of hot water throughout the house
  • Efficiencies in terms of energy and space: A space-saving design helps to preserve energy while also conserving space. A strong, efficient hot water flow rate of up to 11 GPM is achieved by optimal water pressure. Instant Heating: ThermaCirc360 technology gives quicker hot water with a single turn of the water heater. Guaranteed to Last: 15-, 5-, and 1-year home heat exchanger warranties are available.

How to Choose a Tankless Water Heater

There is a lot more to consider than simply the size of the tankless that you should be aware of. To make your selection, there are several options. Each has a certain function for which it has been conceived and constructed. Here are a few things to think about before making a purchase.

Gas vs Electric

In my opinion, the vast majority of houses should make use of a gas tankless water heater. Because they produce the greatest GPM, gas tankless water heaters are the best choice for most houses. In order to provide hot water to the entire house with only one unit, this is the most efficient option to consider. When you don’t have access to natural gas or propane, electric units are an excellent alternative. The initial cost of electric tankless heaters is a significant advantage. They are far less expensive to purchase than a gas-powered variant.

How Much BTU?

The British Thermal Unit (BTU) is a unit of measurement for the amount of energy required to heat water. As an example, consider the following formula: 1 BTU is equal to the amount of energy required to elevate one pound of water by one degree Fahrenheit. The greater the BTU rating, the greater the amount of water that can be heated. To ensure that the water is heated to the proper temperature, you must have a high BTU for a high water demand ratio. If you need to heat water for the entire house and your peak water demand is significant, you’ll need a boiler with a capacity of up to 200,000 BTU.

A modest family with a low water demand may get by with anything in the neighborhood of 140,000 BTU.

Whole House vs Point Of Use

The majority of individuals will choose a single unit that will offer hot water for the entire house. This makes the most sense in a variety of situations. Although there is a strong argument for having numerous tiny tankless water heaters at each point of usage, this is not always the best option. Example: If you have two bathrooms, you may install a unit under each sink and it will offer hot water on demand for the sinks and showers in each bathroom, as well as for the washing machine in the second bathroom.

These point-of-use tankless water heaters may deliver a few GPM, which is sufficient for the majority of applications, and they are not nearly as expensive to purchase or install as conventional tankless water heaters.

Installation

Costs associated with installing a whole-house gas tankless heater can be prohibitively expensive, and this factor may influence the decision on which type of tankless heater to purchase. The distance between the heater and the fixture, as well as the location of the venting, are all elements that influence the cost of the project. In other circumstances, you may have to drill through a brick wall, which raises the expense of the project. You can find out all you need to know about your installation choices by clicking on that link.

Because the heating element is a form of anode, there is no need for an exhaust.

Is it possible to install a tankless water heater on your own?

In the vast majority of circumstances, you most certainly can. It’s simply a matter of following the directions provided. However, it may be beneficial to contact a professional to ensure that you do not invalidate your guarantee by installing the product incorrectly.

FAQ About Tankless Water Heaters

Is it possible for a tankless heater to run out of hot water? Technically, no, it is not possible for it to run out of hot water. When you have a tankless water heater that is the proper size for your hot water need, you will have an unlimited supply of hot water. Accordingly, if your tankless heater cannot keep up with your demand, you will have an infinite supply of tepid water since it will have to reduce the temperature to maintain the required level of heat output. This is why it is critical to get the most appropriate equipment for your requirements.

  • They do not, in fact.
  • A hot water recirculating pump, on the other hand, will be required if you wish to have immediate hot water.
  • With proper care and annual cleaning of the heat exchanger, you can expect your tankless water heater to last for at least 20 years or more.
  • Is it possible to use a tankless hot water heater in conjunction with a well?
  • Additionally, when dealing with hard water from a well, a filter is recommended.
  • The Takagi T-H3-DV-N, which provides a massive 10 GPM at the lowest possible price, is by far the most affordable option for the highest GPM.
  • If you intend on operating at least two showers and a faucet during peak hours, a household of five in the Southern United States would require a 9 GPM gas tankless heater.
  • It’s always better to be a bit too big than it is to be a little too little.

Are Tankless Water Heaters Worth It?

It is possible that after sizing a tankless water heater and considering your requirements, you will conclude that a tankless water heater is not the best option for you. After all, they are not suitable for everyone. When the circumstances are favorable, a tankless water heater is the most cost-effective option. If you have read poor reviews, it is likely that you are dealing with someone who did not adequately examine their demands and ended up with a product that did not provide the GPM they need.

One of the first questions you should ask yourself is what size tankless water heater you will require.

Nick Lopresti is the creator of YourH2Home and a well-known specialist in the field of home renovation. He has years of expertise writing on a wide range of home improvement issues, the most of which are related to plumbing and water systems.

Leave a Reply

Your email address will not be published.