How To Size A Tankless Gas Water Heater?

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.
  • Instead, consider in terms of the pace at which water flows.
  • 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.
  • You’ll need to find out how many gallons per minute each of those fixtures consumes and then add them all together.
  • A shower has a flow rate of how many GPM?
  • A dishwasher has a flow rate of how many GPM?
  1. Alternatively, how about a washing machine?
  2. It’s best to look at the fixtures’ specifications on the specification page provided in your guidebook.
  3. Alternatively, you may refer to the useful cheat sheet provided below to get an idea.
  4. Take a week or two and make a list of the occasions when you had many fixtures running at the same time, as well as which ones they were and when they occurred.
  5. 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
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.
  • The temperature increase is the method by which you will determine the real GPM of the tankless water heater you are using.
  • When you see the GPM rate of a tankless water heater, you are looking at the best case situation.
  • In certain cases, you will not get that water flow, but the water will still be hot as it comes out of the faucet.
  1. 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.
  2. This is why selecting the proper size tankless water heater is so critical.

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.View this groundwater temperature map to get a broad sense of what temperatures you may anticipate in different parts of the world based on where you reside.

  • If you want real-time temperature information, you can check this interactive map from the United States Geological Survey website.
  • As you can see, the temperatures change greatly depending on where you are.
  • Simply take the temperature that you anticipate your incoming water to be based on the graph above and multiply it by two.
  • Then deduct that amount from the desired temperature of your water.
  • This represents an increase in your body temperature.
  1. 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.
  • My temperature rises to 58° at that point.
  • 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.
  1. Because I reside in Boston, I should consider purchasing a gas tankless water heater, which performs best in cold areas and is more energy efficient.
  2. If you’re interested, I prepared a fairly extensive essay analyzing the best gas tankless water heaters for different sized houses that you might find useful.
  3. Anyway, I’m wondering what size tankless water heater would be the most appropriate for a family of four.
  4. It appears that the Rinnai RUC98iN will be the most appropriate choice for my requirements.
  5. 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.
  6. This will most likely increase in the summer because the temperature of my entering groundwater will also be a little higher in the summer.
  • For the calculation, you may use a Rinnai tankless water heater sizing calculator to identify the most appropriate model for your needs and budget.
  • Interested in a Rheem water heater?
  • Here is a link to their simple sizing calculator, which you can use to determine what size unit you need.
  • If you live in a cold environment, the type of tankless water heater you choose will be influenced significantly by this issue.

To learn more about how to operate a tankless water heater in a cold region, check out my entire post, which includes eight easy guidelines.

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 using a boiler for your hydronic radiant heating system. There are several factors that influence how long your radiant floor system will endure, so make sure you do your homework before purchasing one. 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. You may expect a heat loss of up to 50-60 BTUs per square foot in colder conditions and 15-25 BTUs per square foot in warmer situations on average. It actually depends on your specific scenario, such as how old your house is, and other factors. However, the table below has some preliminary figures that may help you determine how many BTUs you will require for your floor heat. 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 Endless Hot Water: Take advantage of unlimited hot water flow throughout the house
  • Energy and Space Efficient: The space-saving design conserves energy as it
  • Endless Hot Water
  • Optimal Water Pressure: Up to 11 GPM hot water flow rate for a powerful,
  • Instant Heating: ThermaCirc360 technology offers quicker hot water with a
  • Powerful,
  • Instant Heating: ThermaCirc360 technology provides faster hot water with a
  • 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.
  • It is possible to choose a lower BTU since it will spend less energy if you just require a handful of low flow fixtures at the same time.
  • 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.
  • After that, a larger one may be installed in the kitchen to give hot water for the sink and dishwasher there as well.
  • 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.
  • The following link will take you to a buyer’s guide to the best point-of-use tankless water heaters, which you can read for free.


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 learn everything you need to know about your installation choices by visiting this page.
  • Because electric units do not require venting, they are significantly less expensive to install.
  • Because the heating element is a form of anode, there is no need for an exhaust.
  • You will, however, require the assistance of an electrician because they must be hard connected.
  • Is it possible to install a tankless water heater on your own?
  1. In the vast majority of circumstances, you most certainly can.
  2. It’s simply a matter of following the directions provided.
  3. 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.
  • Is it true that a tankless water heater provides immediate hot water?
  • They do not, in fact.
  • If your unit is located close to your fixtures, you will have hot water more quickly than if your tank system is located far away from your fixtures.
  1. A hot water recirculating pump, on the other hand, will be required if you wish to have immediate hot water.
  2. How long does a tankless water heater have a life expectancy?
  3. 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..
  4. If, on the other hand, you are utilizing your tankless water heater to heat your radiant floor, you should anticipate a reduced life duration.
  5. Is it possible to use a tankless hot water heater in conjunction with a well?
  6. While it is possible to use a tankless water heater with well water, a sediment filter will be required to ensure that the heater does not become destroyed.
  • If you have hard water from a well, you need also use a filter to cope with it.
  • What is the finest water heater with a flow rate of 10 GPM?
  • The Takagi T-H3-DV-N, which will provide you with a huge 10 GPM at the lowest possible price, is without a doubt the best deal on the market for the highest GPM.
  • For a family of five people, what size tankless water heater do I need?

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.In the Northeast, a family of five would require a gas tankless water heater with an 11 GPM capacity.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.
  • If they had properly sized their tankless heater, they would have chosen the model with a greater GPM and, more than likely, would have been extremely delighted with their purchase.
  • One of the first questions you should ask yourself is what size tankless water heater you will require.
  • I hope that this post on how to size a tankless water heater was informative and that you will be able to choose the finest tankless water heater for your needs.
  • Nick Lopresti is the creator of YourH2Home and a well-known specialist in the field of home renovation.
  1. 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.

How to Size a Tankless Water Heater

Tankless water heaters are excellent appliances, but their capabilities are sometimes overstated when they are advertised.When it comes to excellent ideas, the marketing hype may frequently run ahead of the technology, resulting in salespeople over-promising and under-delivering on the capabilities of the product.As a consequence, the consumer is dissatisfied, money is squandered, and the appliance performs inadequately.

Factors That Effect Tankless Water Tank Performance

  • Take a look at any tankless water heater advertisement, and you’ll most likely find a feature like ″Provides up to 4GPM″ being pushed. That rating is most likely based on the BEST CASE scenario for that product, and it may not reflect the degree of performance you may reasonably expect. Why? In order to properly size a tankless water heater, three factors must be considered: the temperature of the water entering the unit (groundwater temperature), the desired temperature of the water exiting the unit (together, these two items determine the temperature rise required for the tankless water heater), and the required hot water flow rate (determined by the number of shower heads and faucets in the home).

An Example Using a Colder Climate

Consider the following scenario, which takes place in a cooler environment such as Boston, Massachusetts.The temperature of Boston’s groundwater is around 47 degrees Fahrenheit.A regular 105-degree shower requires a 58-degree temperature rise (105-47=58), which suggests you have a 58-degree temperature rise.

  • The GPM flow rate of the tankless water heater must be considered as a result of the 58-degree temperature spike, as outlined above.
  • Let’s take another popular product, the Bosch 1600P-NG tankless water heater, as an example of how to do things well.
  • The manufacturer claims that this machine has a flow rate of approximately 4 gallons per minute (GPM).
  • However, if you are still using an older model shower head, the flow rate of the older shower head may be more than the capacity of the tankless water heater you are using.
  • When using an earlier model shower head (pre-1992), the flow rate can be as high as 6 to 8 gallons per minute (GPM).
  1. A shower head installed after 1992 will typically utilize 2.2 GPM of water.
  2. However, let’s pretend that the Bosch 1600P-NG tankless water heater is just being utilized as a supplemental water heater in this scenario.
  3. What about the company’s claim of 4 GPM?
  4. Well, this device only offers 105-degree hot water with a temperature rise of 45 degrees Fahrenheit, which is typical of warm southern climes.
  5. The device is rated at approximately 3.3 GPM at the 58 degree rise necessary in Boston, Massachusetts.
  6. The only way this Bosch unit will function as a tankless water heater is if it is solely utilized to give hot water to the shower or the bathroom where the shower is situated, and if a low flow 2.2 GPM shower head (made after 1992) is being used to supply the water.
  • Additionally, if this device is being used as your home’s primary water heater and is anticipated to deliver hot water to more than one bathroom, it will be undersized and will not be able to function as a whole house tankless water heater because it only has a 3 or 4 gallon per minute flow rate.
  • A whole house application necessitates the use of a bigger unit or a group of smaller units, depending on the size and location of your home.

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:

Make a decision on the maximum number of devices that you wish to run and the overall flow rate of those devices.Then total their flow rates together (gallons per minute).This is the intended flow rate that you’ll need for the demand water heater that you’ve purchased.Consider the following scenario: you anticipate to be able to operate a hot water faucet with a flow rate of 0.75 gallons per minute while also running a shower head with a flow rate of 2.6 gallons per minute concurrently.It would be necessary for the flow rate via the demand water heater to be at least 3.26 gallons per minute.Installing low-flow water fixtures can help to lower flow rates.

Step 2:

Calculate the temperature rise that is necessary.To calculate the temperature rise, subtract the entering water temperature from the desired output temperature and multiply by 100.Assume that the incoming water temperature is 50 degrees Fahrenheit unless you know better.You may rest assured that you will not undersize your tankless unit if you use the low temperature assumption).If you reside in a warm climate, the temperature of your water will most likely be significantly higher.For the majority of applications, water should be heated to around 105–115°.

  1. A demand water heater that delivers a temperature increase of 55° would be required in this scenario.

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.As a result, you’ll need a tankless water heater that can provide at least a 60-degree increase in temperature while also dispensing 5.2 gallons per minute of water.

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.


Average Flow GPM

Average Temp.

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 only designed to heat potable (drinking) water, and that the water entering a tankless device should not be pre-heated before use.

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 via gas-fired demand water heaters and a flow rate of 2 gallons per minute through electric demand water heaters will result in a 70°F increase in water temperature in most cases.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.
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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.Alternatively, you can listen to him rant about Austin traffic on Twitter at @Bukowsky, where you can follow his musings as well.

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.The installation of tankless water heaters represents a pleasant break from the cycle of water depletion, heating, and waiting that is common with tank-style water heaters.However, reducing the tank also implies that the tolerance margins will be reduced to a bare minimum.Tank-style water heaters have the luxury of heating extra water, whereas tankless water heaters must heat only the appropriate quantity.

  1. 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.
  2. Additionally, you may be interested in the tankless water heaters available at Home Depot.
  3. 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.

The following top-five list was compiled using parameters such as price, customer rating, maximum GPM flow rate, heating capacity (in BTUs), and Energy Star certification as a guide.

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.When you use a tankless water heater, there is no hot water stored in reserve.These little devices, which are normally mounted on the wall, include an inbuilt heat exchanger that warms the water as it passes through it.They are energy efficient.

  1. Throughout the exchanger, the pipes are looped back and forth, allowing for a large amount of pipe to be compressed into a little amount of space It’s comparable to the way a garden hose full of water would heat up after being exposed to the sun for a period of time.
  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. Tankless heaters can be fuelled by either natural gas or electricity.

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.As a result, while the water can be heated continuously throughout the day, it is not instantaneous heat.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.There are similar systems in hotels that are designed to maintain hot water near the faucet and reduce waste water usage.Some tankless water heaters are equipped with built-in heat recirculation systems.

How to Calculate the Right Size of Tankless Water Heater

Calculate Temperature Rise

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).Consequently, if your recommended tankless set temperature is 120 degrees and the coldest groundwater temperature in your location is 50 degrees, the temperature rise is 70 degrees for your tankless installation.

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 sink flow rate is 1 gpm
  • shower flow rate is 2 gpm
  • tub flow rate is 2 gpm.

  • Sink: 1.5 gpm
  • Dishwasher: 2 gpm

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.Alternatively, if the temperature rise remains constant but the rate of consumption fluctuates, you will need to explore alternative tankless heater types.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

  • Advantages: Small troops take up less room than huge tank models
  • 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.
  • Cons 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.

How to Calculate the Size of a Tankless Water Heater

Tankless water heaters, as the name implies, do not have a storage tank.Therefore, determining the appropriate size for a home is a little more difficult than it is with tank water heaters.Although a smaller tankless water heater may be more economical, it may not be able to satisfy the water demands of your family.Larger water heaters, on the other hand, may prove to be an unnecessary expenditure.We’ll walk you through the process of determining the appropriate size tankless water heater for your home.

Step one: determine the number of devices and their flow rates

First and foremost, it is required to determine the maximum number of devices that will be using the water heater at any one time.After that, you must calculate their flow rate and combine the results together to arrive at a total.Water flow rate is measured in gallons per minute and refers to the rate at which water runs through a machine.Taking a bucket and seeing how long it takes to fill it up is one method of determining a device’s flow rate.The flow rate of a showerhead is two gallons per minute if it fills a five-gallon pail in two and a half minutes, for example.You can also monitor the flow rates of most devices by using the internet.

Step two: determine the temperature rise

The second phase involves determining the temperature rise (or the temperature of the water heater) that the unit is required to provide..To obtain this value, subtract the entering water temperature from the desired water temperature and multiply the result by 100.The optimal water heater for a residence with incoming water temperatures of 50 degrees Fahrenheit and a target temperature of 110 degrees Fahrenheit should create a temperature rise of 60 degrees Fahrenheit.Different internet tools may assist you in determining the proper incoming water temperature for your home based on the climate in your region of residence.

Step three: calculate the size

Make use of the information from the previous two stages to calculate the appropriate size of a tankless water heater.Please keep in mind the number of devices that will require hot water from the heater when planning your installation.Additionally, pay attention to your peak-hour water use.This is the period of year when the average home uses the most water.A good water heater should be able to handle the high demand for water during peak hours with ease.

Let the experts help

Because tankless water heaters do not require a storage tank, they are an ideal storage-saving solution for all types of households and businesses.Furthermore, they are energy-efficient since they do not always keep the water at a comfortable temperature (unlike tank water heaters).Whenever you want assistance in deciding the appropriate size of a tankless water heater for your family, don’t hesitate to seek expert assistance.They are capable of doing the necessary computations and recommending the appropriate size.In addition, they may provide useful maintenance advice.Before employing a firm, be certain that they are reputable.

  1. Look for internet evaluations or ask for suggestions from family, friends, or neighbors to narrow down your choices.
  2. The suitable organization should have years of expertise and a large number of satisfied customers.
  3. Water Wise Plumbing is a full-service home plumbing firm based in Las Vegas, Nevada, that serves the surrounding areas of Henderson, Paradise, Boulder City, Pahrump, and Northern Nevada.

Tankless water heater service, water softeners, water heater flushing, leak detection and repair, water line rerouting, pipe installation, hydro-jetting, sewer services, and more are all available from us.Contact us now for more information.We are a family-owned and run business that is licensed, certified, and insured.We provide flat-rate pricing as well as emergency assistance 24 hours a day, seven days a week.Call us at 702-597-9554 right now.

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

An expensive tankless water heater that is not powerful enough to meet our demands is one of the most common blunders people make when purchasing a tankless water heater.Tankless heaters should not be purchased based on their size.Neither should they be purchased based on their size, since this will waste energy.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. Here’s how it works: Before you can accurately answer the question of what size tankless water heater you require, you must first determine two things:
  1. What is the greatest amount of hot water you require?
  2. What is the maximum amount of water per minute (measured in Gallons Per Minute or GPM) that a particular tankless water heater can heat, and by how many degrees?

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 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

For example, if you’re taking a shower (with 100 percent flow and 110°F hot water) and concurrently using two faucets (both with 100 percent flow and 110°F hot water), you’ll need a tankless water heater with at least 5 GPM flow rate.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 grows in direct proportion to the capacity of the unit.It should be noted that electric tankless hot water heaters are suited for modest water demands up to 8 GPM.

  1. Choosing one of the top gas tankless hot water heaters from this list is recommended for larger requirements (8 GPM or more).

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.In Minnesota, for example, the temperature of the input water might be as low as 37 degrees Fahrenheit.

  1. That represents an additional 40 degrees Fahrenheit differential that a tankless water heater must overcome.
  2. Calculation in a few words: Consider the following scenario: we have a tankless heater with a maximum water flow of 10 GPM.
  3. Because the input temperature is 77 degrees Fahrenheit in Texas, we can really obtain 10 GPM of 110 degrees Fahrenheit water.

The heater must heat water from 77 degrees Fahrenheit to 110 degrees Fahrenheit, a difference of 33 degrees Fahrenheit.In Minnesota, on the other hand, the inlet water temperature is 37 degrees Fahrenheit.In order to heat water to 110 degrees Fahrenheit in Minnesota, a tankless heater must overcome a temperature differential of 73 degrees Fahrenheit, rather than the 33 degrees Fahrenheit difference in Texas.You don’t come from Minnesota or Texas, do you?Here’s an infographic developed for the Rinnai RU160iP SE+ Series 9 GPM tankless water heater that will give you an idea of what the maximum water flow rate is in your state (legal for the United States of America).

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.The water pressure is sufficient to run numerous showers at the same time.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.

That is a direct outcome of the temperature differential between the input and outlet.In New York, the heater must contend with an additional 25 degrees Fahrenheit.You will be able to operate two or three showers at the same time using the same heater and the same amount of energy consumption.It’s important to consider the operating costs as well, especially with larger units.You can find out how much power larger electric tankless water heaters consume by visiting this page.The amount of propane that these on-demand hot water heaters consume is another useful piece of information regarding propane units to read.

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.

  1. 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 a family of 2? 6-8 GPM 10-18 kW
What size tankless water heater do I need for a family of 3? 7-9 GPM 15-23 kW
What size tankless water heater do I need for a family of 4? 8-10 GPM 20-28 kW
What size tankless water heater do I need for a family of 5? 9-11 GPM 25-34 kW
What size tankless water heater do I need for a family 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).Power, on the other hand (measured in kW), is absolute.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.

  1. All things considered, you must also consider certain financial calculations, for example.
  2. It is advised that you check here to see if a tankless water heater is actually worth the investment (we did some calculations).
  3. Let’s take a look at two real-life situations.

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.Second, the number of people who live (and utilize) hot water in your home is taken into consideration.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. When it comes to tankless water heaters, though, things are a little different. 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. You would, roughly speaking, require the following items to replace a 50-gallon water heater: a 10 GPM gas tankless heater or at least a 27 kW electric tankless water heater if you live in the northern part of the United States
  • a 7 GPM gas tankless heater or at least an 18 kW electric tankless water heater in the southern part of the United States
  • and a 50-gallon water heater replacement kit.

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: 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 summary, if you live in the northern portion 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.The tankless heater needs to work extra hard to raise the water temperature to 110°F or 120°F, depending on the situation.

  1. For those who reside in the southern part of the country, the tankless water heater’s capacity might be lowered by up to 30 percent.
  2. As a result, for a family of five in the southern United States, a 7 GPM gas tankless heater or an 18 kW tankless heater should be more than adequate to meet all of their hot water demands.
  3. Keep in mind that, especially with larger units, tankless water heater circulation pumps can save you a significant amount of money on hot water.

These pumps come in handy when you need hot water from a faucet or shower in a short amount of time.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.During times when we require a large volume of hot water, the electric tankless water heater is activated to provide assistance.

  1. 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.
  2. Specifically, the plumbing for each units is separate in this instance.
  3. I hope this has been of assistance.

How To Size A Tankless Water Heater

If you are in the market for a new water heater, tankless hot water heaters may be an option to investigate. This type of hot water heater can provide you with a number of different 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.When you turn your faucet on, cold water is instantaneously turned into hot water.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

You will need to determine the appropriate size of your water heater.If you choose the incorrect size, it may not be able to adequately heat the water in your house.Particularly problematic is the situation in which you are attempting to utilize various restrooms or appliances at the same time.It is possible to determine How To Size A Tankless Water Heater by following the methods outlined below: Step 1: Take some time to sit down and figure out how many devices you’ll need to have hot water for, as well as the flow rate for each.The flow rate is the number of gallons per minute of water flow that you will require.If you want to run your shower and faucet at the same time, you’ll need to figure out how much water each of them can handle.

  1. When it comes to hot water faucets, a flow rate of around 0.75 gallons per minute is recommended.
  2. The flow rate of the shower head will be 2.6 gallons per minute.
  3. Your hot water heater should be able to deliver at least 3.26 gallons of hot water per minute, whichever is higher.

Low-flow water faucets and shower heads are available for purchase and installation if you wish to reduce your flow rates.2.Determine the temperature rise that will be required for your equipment in step two of this procedure.You will need to subtract the temperature of the incoming water from the temperature of the output water.If you do not know the exact temperature of your entering water, you should always presume that it is 50°F or above.

This will assist to guarantee that you do not choose a tankless hot water heater that is too small.It is crucial to understand that if you reside in a hotter climate, the temperature of your entering water will be greater.You will want to choose a temperature range of 105 to 115 degrees.

If this is the case, you will need to ensure that the tankless water heater has the ability to offer a temperature rise of 55 degrees or greater.

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.Determine the flow rates that you will require from your hot water heater in order to guarantee that you can complete the tasks that you have set for yourself.Suppose you want to take two showers at the same time, or you want to run many sinks at the same time.

  1. In order to meet your bathroom water requirements, it is advised that you utilize 1.0 GPM.
  2. The water from your tankless water heater will be insufficient if you are running two different showers at the same time.
  3. Water consumption will be around 4.5 gallons per minute if you need to operate both your washing machine and shower at the same time.

You’ll want to be certain that the heater you choose is the appropriate size.Calculating your water use rates can assist you in selecting the most appropriate tankless water heater for your needs.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.These gas hot water heaters have the ability to swiftly raise the temperature of the water.For more information on How To Size A Tankless Water Heater, contact the Original Plumber right away.

They will be able to tell you what size water heater you require and will show you the many alternatives that are available.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.Hiring a professional plumbing firm eliminates the need to worry about the installation of the hot water heater or ensuring that you have the proper size.

The installation of a water heater may be complex and risky, therefore it is always a good idea to leave it to the experts.

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

  • Last updated on August 25, 2021. Written by Gene Fitzgerald 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. More information about our product review methodology may be found here, as well as our FTC affiliate disclosure. 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. Tankless water heaters, on the other hand, are restricted in the amount of water they can heat at any given moment. 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. If you go too large, you will end up paying too much. 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. Table of ContentsHow to calculate the size of a tankless water heater Step 1: Calculate the flow rate (in gallons per minute for a tankless water heater)
  • Step 2: Calculate the temperature rise
  • Step 3: Put it all together
  • Video
  • Size chart
  • Example size chart
  • 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.
  2. 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?

  1. Oversizing isn’t a major concern in this situation.
  2. The main drawback is the excessive up-front expense.

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

  • Begin by determining your peak hot water consumption 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. Then sum up all of their flow rates together. 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. The necessary flow rate for WaterSense-certified goods is 2.0 gpm plus 1.5 gpm, which equals 3.5 gpm. It’s just that simple! But, before you get started, consider the following suggestions: 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 te

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