What Size Tankless Water Heater Do I Need Calculator

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

There is one more thing to consider before making your final selection, and it is a very important one. When choosing a water heater, should you go with a typical storage hot water tank or switch to a tankless water heater? Tankless water heaters are a type of water heater that does not require a tank to be filled with water. Flow rates are used to determine the capacity of a tankless water heater rather than the number of gallons it can hold. Water heaters with high flow rates should be installed in your home if you expect your family to use the system at its peak.

Compile each of the flow rates and add them all together!

Many versions have a maximum flow rate of 3.5 gallons per minute, which is sufficient for a sink faucet and a shower head.

According to the information provided, changing a hot water tank provides you with some alternatives you may not have considered previously.

We’re here to assist you.

We’ll go through the numbers with you and make sure you have a hot water heater that’s the right size for your household.

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?

Due to the fact that they only heat water when there is a need, tankless water heaters are sometimes referred to as on-demand water heaters. When a hot water faucet is opened, the tankless appliance is activated when cold water begins to flow through the water heater, causing the tankless appliance to switch on. A heat exchanger heats the water to the desired temperature as it circulates through it. As the water becomes hotter, it enters your domestic plumbing system as it makes its way to the open hot water faucet.

A tankless water heater, in contrast to a standard tank-style water heater, which is restricted by the size of its tank, may provide an unending supply of hot water.

Interested in learning more about the differences between tankless water heaters and regular water heaters? Continue reading. Please Visit This Site

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? (+ Sizing Calculator)

Learn more about gas tankless water heaters in this post. Besides providing our top recommendations, we also provide a thorough buyer’s guide. Check out our post on Electric Tankless Water Heaters for more information on this subject. A buying guide is also included, in addition to our best selections ;

  • In the case of a tankless water heater, the first step is to determine the flow rate. Putting it all together
  • Step 2: Temperature increase
  • Step 3: Putting it all together Video
  • 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

The amount of hot water you require during peak usage (in gpm, or gallons per minute). Keep in mind that tankless water heaters do not store water, but rather heat it as it runs through the system. The needed temperature rise, which is calculated by the parameters for the input water temperature and the output water temperature.

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.

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 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.

See also:  Where Can I Buy A Hot Water Heater

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. You want something about 110 degrees Fahrenheit at your kitchen sink.

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.

Gene enjoys reading books on philosophy and social topics, producing music, and going on hikes when he is not at BOS. Find out more about.

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).

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. 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. 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.

How Big of a Tankless Water Heater Do I Need?

To figure out how much your temperature will climb, you must first calculate the temperature of the water that is entering your house. Groundwater temperatures, which can vary significantly depending on where you reside in the United States, are taken into consideration. To get a basic sense of what temperatures to expect based on where you reside, have a look at the groundwater temperature chart shown below. If you want real-time temperature information, you can check this interactive map from the United States Geological Survey website.

Simply take the temperature that you anticipate your incoming water to be based on the graph above and multiply it by 100.

You’re experiencing a spike in temperature.

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.

See also:  How To Insulate Outdoor Tankless Water Heater

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.

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.

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°.

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

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 hot water heater. 4.5 gallons per minute would be required from the water heater if a shower and the washing machine were both being used at the same time. Any of these scenarios necessitates the purchase of a hot water heater that is large enough to accommodate or surpass the amount of hot water you require simultaneously. The intake water to a tankless water heater should not be preheated since tankless water heaters are meant to heat only potable (drinking) water.

Recommended Reading:

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  • 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

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What Size Tankless Water Heater Do You Need? (Calculate GPM Needed)

When it comes to tankless water heaters, you’re not alone in feeling overwhelmed by the variety of alternatives available. For many homeowners, the question “What size tankless water heater do I need?” is a source of frustration. Our guide will assist you in determining the appropriate size tankless water heater for your needs. Here, we’ll guide you through the exact calculations in a step-by-step fashion so that you can choose a tankless water heater that provides the necessary quantity of hot water to your house while also ensuring that you never run out of hot water.

How to Size a Tankless Water Heater

The first consideration in selecting a tankless water heater is determining the size of your existing hot water heater. Unlike typical tank water heaters, which are sized according to the number of people in the household, tankless water heaters are sized according to the number and types of fixtures or appliances they will service. The resulting value is referred to as “GPM” (Gallons Per Minute), or “flow rate” in certain cases, and it is used to estimate the size of your hot water heater. Walk through the process of calculating and selecting the appropriate size tankless water heater.

Step 1: Calculate How Many GPM for Your Tankless Water Heater

If you sum up the flow rates of each individual fixture that utilizes hot water in your home, you may establish your own maximum GPM. This is just a method of determining how many gallons of hot water may be passed through the fixture in one minute by monitoring the flow rate. It is possible to determine the real flow rates for your individual fixtures by consulting your owner’s handbook or looking for the model number on the internet. Individual hot water appliance GPM rates that are running at the same time equals total GPM required.

Make use of GPM estimation charts to find this information, which may be found with a short internet search.

The sample chart below will assist you in determining the appropriate size tankless water heater for your home.

Write down each GPM amount, whether it is the exact GPM of your fixture or an estimate, and then add them all together. Take into consideration any fixtures that would be running at the same time, but only if they would be using hot water.

Appliance Flow Rate (GPM)
Bathroom Faucet 0.5 GPM
Kitchen Faucet 0.5 GPM
Shower 2.5 GPM
Dishwasher 1.0 GPM
Clothes Washer 1.5 GPM

(Source) Keep in mind that if you don’t want to run out of hot water, you should overestimate the number of fixtures that might be running simultaneously!

Step 2: Determine Temperature Rise Needed

It is necessary to consider not just the volume of water that has to be moved, but also how much of that water needs to be heated. Another calculation is the temperature rise, which is the length of time it takes to raise the temperature of groundwater to the desirable “hot” temperature you want it to be. Typically, this temperature is approximately 120 degrees Fahrenheit, although this is simply a personal preference. Using a ground temperature map, you may quickly determine the typical groundwater temperature in your local area.

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The map you choose to use when sizing a tankless hot water heater is entirely up to your personal taste and needs.

This is another another specification that will be used to determine the size of the water heater you require.

Step 3: Determine Your Power Source

Choosing a power source for your tankless water heater is the next step to take into account. There is no simple answer to the question of whether gas or electric tankless water heaters are preferable, as it is dependent on your individual circumstances and tastes. Generally speaking, electric tankless water heaters are better suited for point-of-use applications and for usage in houses with lower water consumption. Electricity is often favored only in situations when incoming water temperatures are higher and heating and GPM needs are lower.

  1. First and foremost, you should evaluate the availability of the power source you intend to employ; does your home currently have access to natural gas or electrical power, for example?
  2. If this is not the case, there are a few other considerations to consider when selecting a power supply for your tankless water heater.
  3. If the cost and availability of natural gas and electricity are comparable in your area, you may want to think about which characteristics of a tankless water heater are most essential to you before making your final decision.
  4. They are also often less expensive to operate because of the low price of gasoline, however this may vary depending on your location.
  5. The less complicated installation will also lower labor expenses, resulting in even greater savings for the consumer at the time of purchase.

A power outage will result in the loss of hot water if the system is not connected to a battery backup. Let’s look at some real-world examples now that we’ve gained a better knowledge of key terms and considerations when selecting a tankless water heater.

What Size Tankless Water Heater Does a Family of Four Need?

A family of four living in an average-sized house would need to evaluate how many people would be using hot water at the same time in order to determine how much hot water they would require. To begin, we will total up the flow rates for each fixture in order to determine their maximum GPM capacity. Our hypothetical household may find themselves in the position of needing to operate the following appliances at the same time:

  • Washing machine (2 GPM)
  • Kitchen sink (3 GPM)
  • Dishwasher (2 GPM)
  • Up to two showers (each with a flow rate of 1.5 GPM) can be used at the same time.

Based on the low end of the ranges of each fixture on our GPM table above, and assuming the family purchased energy-efficient appliances, this would be 6.5 GPM on average. We would then compute the temperature rise caused by the tankless water heater based on the location of the customer. For the sake of illustration, let us choose the state of Hawaii. To maintain the normal 120 degrees Fahrenheit hot temperature, they will need to raise the average groundwater temperature for the region by 43 degrees Fahrenheit from the typical 77 degrees Fahrenheit.

6.5 GPM RISE at 43 degrees Celsius will be required for this couple’s needs.

What Size Tankless Water Heater For a Couple In a Condo?

However, a couple that lives in a Florida condominium will have a whole different set of requirements. Their condo has only one bathroom and no on-site laundry or dishwasher, which is a disappointment. However, they are aware that their brand new kitchen faucet has a flow rate of 1.5 GPM, but they employ generic figures from the GPM calculation table to get a flow rate of just 5 GPM. They’ll simply have to elevate the temperature of their water from the ordinary 68 degrees to the ideal 110 degrees to achieve this.

The affordableRheem RTEX-36 Electric Tankless Water Heaterwould be an excellent solution for this couple, as it could easily meet their water needs while also saving them money on energy and storage space costs.

What Size Tankless Water Heater Does a Family of Five Need?

Our final scenario has a family of five living in a spacious house that has access to natural gas connections. This home contains all of the same appliances as our first family, as well as an additional half bathroom, which adds another bathroom faucet with a 1.5 GPM flow rate to the mix. Given our suspicions that they may be running two showers, along with the dishwasher, washing machine (and maybe the half-bathroom faucet), we multiply their GPMs by two to get 9.5 GPM. At their house in Oklahoma, the groundwater temperature is 57 degrees, and the family loves to have their hot water at a temperature of 120 degrees.

Having answered the issue, “What size electric tankless water heater do I require?” this family now just has to choose a decent tankless water heater to complete their set up.

One choice is the Rheem RTGH-95DVLN 9.5 GPM Indoor Direct Vent Tankless Natural Gas Water Heater, which is available through Amazon. As long as the family does not exceed the estimated water use, the system will deliver enough water while maintaining an efficiency rating of 94 percent.

So, How Big of a Tankless Hot Water Heater Do I Need?

If you want to calculate the size of your own tankless water heater, simply follow the instructions outlined above.

  1. Adding up all of the fixtures in your house will give you the GPM you need to know. Calculate your temperature rise based on your target temperature and the temperature of the groundwater

Once you’ve decided on the size of your tankless water heater, the only thing left to decide is what sort of power source you’ll choose. Make a decision between a gas tankless water heater and an electric water heater, and then start shopping! On the heater’s specification plate or packaging, as well as on the majority of vendors’ and manufacturers’ websites, you may find specifications that correspond to your calculations. Consider it similar to buying for a car based on horsepower and miles per gallon.

Today, have a look at our assessment of the top tankless water heaters available!

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.

  • They are energy efficient.
  • 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.
  • 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.
  • 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.
  • Some tankless water heaters are equipped with built-in heat recirculation systems.

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

  • Many tankless water heater manufacturers include calculation tables or online calculators that can assist you in determining the best model for your needs depending on peak hot water demand and temperature rise in your location. For example, if the temperature rises by 60 degrees in your location, your home’s peak hot water consumption may be 6 gpm. A few models that match this description could be suggested by this. In contrast, if the temperature rises over 40 degrees Celsius, the manufacturer may propose totally other models. You may also need to investigate alternative kinds of tankless heaters if the temperature rise remains constant but the use rate fluctuates. Finding the right tankless water heater for your house requires a comparison of the two sets of information.
  • 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

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