What Size Tankless Water Heater To Replace A 50 Gallon

What Size Tankless Water Heater to Replace a 50 Gallon – Size Guide

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Selecting the Right Size of Tankless Water Heater

The size of your tankless water heater is determined by the amount of water your family consumes. If you have a small household or only want a water heater for a single faucet, a compact unit is the best choice. If you have a large family of three to four people, or if you want to utilize a single water heater for the entire house, investing in a larger unit is the best option. A unit that is smaller than required results in water that is lukewarm. The facility is unable to meet the demand due to its limited capacity.

As a result, it is important to select the appropriate size water heating unit that will meet the hot water needs of your entire household.

Factors Determining the Size of a Tankless Water Heater

Are you considering upgrading your outdated water heating unit? It is critical to upgrade appliances that are used on a regular basis. Throughout the year, an excessive amount of energy is consumed by a water heating device. Installing a unit that is more energy efficient and cost-effective is unquestionably a positive and significant move. The most crucial elements to consider when determining which water heater would perform best for your household are two of them.

  • Total hot water demand for the entire house
  • Capacity of the water heating unit in gallons per minute

The GPM water need of all fixtures must be added together in order to compute the total amount of hot water required by the entire home at any one moment. The majority of faucets use between 1 and 2 liters of water per minute on average. Following that, the water requirements of various fixtures are stated.

  • Shower flow rate: 3 GPM
  • Laundry machine flow rate: 5 GPM
  • Dishwasher flow rate: 1.5 GPM
  • Kitchen faucet flow rate: 1 GPM
  • Bathroom faucet flow rate: 1 GPM

The anticipated water delivery of each faucet might differ from one residence to the next. For example, if you do more dishwashing than the typical household since all of your meals are prepared at home, you should add this into your calculations. Tankless water heaters have a capacity ranging from 2 to 12 gallons per minute. For modest households, 8 GPM is sufficient flow rate. When building a larger house, a water capacity of more than 8 GPM will be required.

Realistic Maximum GPM

The maximum GPM of a water heating unit is shown on the front side of the appliance. The maximum GPM attained in practice, on the other hand, differs from the figure quoted. A possible explanation for this disparity is because the GPM flow rate of a unit is computed by assuming an increase in temperature of 77 degrees Fahrenheit. As a result, the temperature of your groundwater is quite important. The temperature of the water is already high in hotter climes, thus it requires less time and energy to bring it up to the needed level in these environments.

The flow rate of the unit is determined by the temperature of the intake water.

The Appropriate Size Depending on the size of your family One of the most often asked inquiries is how many GPM units are required for a household of four.

If you have a household of three people, a compact apartment will enough for your needs.

If you install a single unit for a large household, the water will take a long time to heat up and the temperature will not be as hot as you need it to be. Take a look at these more resources:

  • Tankless water heaters for large families, RV tankless water heaters, and point-of-use tankless water heaters are all excellent choices.

Temperature Rise

For large families, the best tankless water heaters are: best RV tankless water heaters, best point of use tankless water heaters, etc.

Replacing the 50-gallon Traditional Water Heater

In order to replace a typical heater with a capacity of 50 Gallons, you must take the following factors into consideration.

  • A gas unit with a capacity of 10 GPM or an electric device with a capacity of 27 kW If you intend to place the unit in a cold area, you should consider the following: If the climate is warm and the groundwater temperature is greater, a 7 GPM gas tankless water heater or an 18 kW electric unit should be used.

Similarly, you may estimate the size of the tankless water heater you’ll need to replace your 60, 70, or 80-gallon water heater by measuring the capacity of your current water heater. The estimated estimate provided above will give you an idea of the size of water heater you will require. In order to be more precise about the size, consult with a local contractor for a better advice. Because of their extensive knowledge, they will provide you with an exact number, which you can then purchase from any nearby store or online.

I’ve been working in the water heater and plumbing industries for several years.

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What Size Tankless Water Heater Do I Need?

Over the course of several years, I’ve worked in the water heater and plumbing sector. Providing relevant information regarding tankless water heaters is my goal, and I’m here to assist my readers in doing so. Adam’s most recent posts (see all)

  • The square footage of your home
  • The pace at which water flows through your home
  • Temperatures are rising
  • The number of people in your family

In this essay, we’ll go into further depth about each of these aspects. However, before we go any further into the subject, it is critical that you understand that you should never purchase a tankless water heater that is lower in capacity than you actually want simply because of price disparities. Before we get started, let’s have a look at a brief chart.

Sizing Guide for Residential Tankless Water heater

Household Occupants Usage Capacity Required (Measured in Gallon)
Electric Gas
1-2 people Regular/ Low 30 30
High 40-50 40-50
2-3 people Regular/Low 40 40
High 50 40-50
3-4 people Regular/Low 50 40
High 80 50-75
5+ people Regular/Low 80 50
High 80+ 75

1. The Size of Your House

Tankless water heaters are available in a variety of sizes. Modern water heaters, on the other hand, have the ability to completely steam your whole home’s water supply. Inefficient versions may be obtained by purchasing smaller models, which are limited to heating the water that flows into specific bathrooms in your house. While traditional storage tank heaters can only store and heat the quantity of water that you anticipate using, tankless heaters can store and heat as much water as you are able to flow through them.

2. The Rate of Water Flow in Your House

We’ve established that tankless water heaters are different from traditional tank heaters in that they steam the water as it enters your home, rather than heating it. This implies that the rate at which water is released from your faucets is governed by the flow rate supplied by your water heater, not the other way around. Taking this element into consideration allows you to operate more than one faucet or shower at the same time without your faucet slowing down. Given that the typical flow rate of one shower head is 1.8 gallons per minute, it is possible that another shower will begin to run alongside the first shower at a rate of almost the same as the first shower.

Adding up the flow rates of the two showers will give you the flow rate that you will need for your tankless water heater.

We’ve included some average flow rates below to assist you in estimating the flow rates of the majority of your faucets. The actual flow rates for your home and its many appliances may differ from one another.

  • Bathroom faucet flow rates range from 0.5 to 1.0 GPM
  • Showerhead flow rates range from 1.5 to 2.0 GPM
  • Kitchen tap flow rates range from 2.0 to 6.0 GPM
  • Dishwasher flow rates range from 1.5 to 2.5 GPM
  • And laundry washer flow rates range from 2.5 to 3.5 GPM.

3. Temperature rise

Following the calculation of the flow rate of each faucet in your home, you must decide how much heat your heater must create in order to provide you with the necessary heat in your water outlets. The needed temperature rise will be determined by taking into account the temperature of the water that enters your house and that of the water that you wish to have flow out of your tap when calculating the required temperature rise. We will suppose your home’s incoming water supply is 45°F and your shower (which is connected to your tankless heater) will deliver water at 105°F; the temperature raise necessary is 60°F.

To obtain the appropriate temperature in your shower, your heater must heat 1.8 gallons of water by 60 degrees Fahrenheit every minute, assuming that the shower flows at a rate of 1.8 GPM.

Some states are expected to have temperatures as high as 75 degrees, while others will have temperatures as low as 35 degrees, according to estimates.

If you’re not sure what it is, you may use the assumption that the temperature is 40 degrees to make sure you don’t underestimate the amount of temperature rise that will be required.

4. Your Household Size

This is yet another important consideration to bear in mind when installing a tankless water heater in your house. Your tankless water heater should be capable of managing occasional usages of hot water coming from specific taps without causing damage to the system. An additional point to keep in mind is when the most people are utilizing the service. What exactly does this imply? Every home suffers a spike in hot water use when they have visitors around, are organizing a wedding, or any other event of this nature.

Even though you may not be utilizing hot water all of the time, you must take into account the periods when your use spikes and use it as a starting point when determining your tankless water heater specs and requirements.

  • 2 showerheads with a total flow rate of 3.6 GPM
  • 2 kitchen faucets with a total flow rate of 4 GPM
  • 2 laundry machines with a total flow rate of 6 GPM

We’re looking at an estimated flow rate of around 13.6 gallons per minute, which may be higher than the average rate of flow in our area. Tankless heaters are designed to provide smooth heating operation at peak periods as well as during periods of average consumption. When sizing your tankless heater, consider the highest potential simultaneous usage.

Gas Tankless Water heater or Electric Tankless Water Heater?

Aside from the size, the other component that influences the operation of a tankless water heater is the size of the tankless water heater. For the most part, gas tankless water heaters can heat up to 5 gallons of water to 70 degrees Fahrenheit in a minute, but electric tankless water heaters can only heat 2 gallons of water to 70 degrees Fahrenheit in the same amount of time. We picked 70 degrees Fahrenheit since that is the temperature rise necessary for normal groundwater at 40 degrees Fahrenheit to reach a sweltering 110 degrees Fahrenheit.

Tankless gas water heaters have a noticeable advantage over their electric equivalents in terms of power, albeit the amount of power varies depending on the brand and type. Before making a purchase, always double-check the product specs for GPM and temperature increase ratings.

Tankless Water Heater Size FAQs

You may still have some queries about the size of the tankless water heater you require; we will attempt to answer them in this section.

Which sizes last longer?

You might still have some queries about the size of the tankless water heater you require; we will attempt to address them in this section.

Where can I find the right model size?

Departmental plumbing stores are likely to carry all of the available model sizes. For your convenience, you may want to try purchasing them online; however, make sure you carefully read the requirements to avoid any misunderstandings.

What size tankless water heater can I install myself?

Because you will require the skills of a skilled installation, this is not a do-it-yourself project. The installation procedure is time-consuming and difficult. It entails gas connections, electrical circuits, propane units, and other such things. In such instances, the business will always recommend a tech person to customers.

See also:  What Is A Quick Recovery Water Heater?

What size tankless water heater is easier to maintain?

In comparison to bigger models, it seems obvious that smaller model sizes would be less difficult to keep up with. Regardless, you will want the services of a professional to do routine maintenance on your tankless water heater at regular intervals. This involves checking for leaks, inspecting the air vents, and cleaning the burner, among other things. If you reside in a hard water area, you should consider flushing your tankless water heater with vinegar every three weeks to prevent sediments from blocking the heat exchanger, especially if the tankless water heater is tiny.

What are the prices for different tankless water heater sizes?

Tankless heaters are priced differently depending on their size and model. A small gas-fired variant may be purchased for roughly $180, while larger models can be purchased for over $2,000. For $1,000, you may also acquire a model that is of medium size. Electric tankless heaters range in price from $90 to $900.


Following the calculation of your water flow rate and needed temperature rise for typical hot water consumption, make sure to account for peak usage periods so that your heater can provide you with hot water at all times. You can let us know if there is anything further you need to know after reading over all of the information we’ve provided in the comments section below.

What Size Tankless Water Heater Should You Buy: Sizing For Your Family Needs

Tankless water heaters are the way of the future when it comes to hot water usage in residential and commercial buildings. Tankless water heaters are characterized by their small form and ability to provide hot water on demand. On the other hand, on-demand does not always imply continuous service. Flow rate is the amount of water that may be produced in one minute by a tankless water heater (both electric and natural gas). If the demand for hot water in your household exceeds the capacity of your water heater, you may experience a lack of hot water.

  • Gallons per minute (GPM) is the unit of measurement for tankless water heaters (GPM).
  • During peak hours, the average family consumes around 6.5 GPM.
  • A tankless water heater with a capacity of 4 GPM can deliver enough water for one shower and one appliance to run at the same time.
  • When a tankless water heater is undersized, it will not be able to provide enough hot water to fulfill the needs of your household.
  • A tankless water heater with a flow rate of 4GPM will be unable to heat enough water to fulfill this demand for hot water.
  • A lar ge tankless water heater has greater installation and operating expenses than a traditional tank water heater.
  • For example, a 4 GPM tankless water heater will cost between $400 and $700 plus installation, and an 8+ GPM tankless water heater would cost between $900 and $2000 plus installation.
  • Hot water on demand is defined as having continuous hot water output for the duration of time that the hot water tap is open.
  • If you have too many hot water faucets open at the same time, the water will become lukewarm if the tankless water heater’s output capacity is surpassed.
  • Installation costs are in addition to the cost of the tankless water heater.

We’ve put together this complete shopping guide to assist you in making the best decision for your family when it comes to tankless water heaters. The following section will lead you through the many various considerations you should make while shopping for tankless water heaters.

Gallons of Hot Water You Need for a Tankless Water Heater

The first thing you’ll need to think about is how much hot water you’ll require from your tankless heater. In most cases, we refer to this as “sizing,” although you will not be using dimensional measures to determine the appropriate size for your tankless water heater. Instead, you’ll need to pay attention to two separate metrics: the flow rate and the temperature rise (or decrease).

Tankless Water Heaters GPM Flow Rate Explained

It is the time of day when you use the most hot water at one time that is referred to as peak demand. Tankless water heater size is determined by the flow rate, which is measured in gallons per minute (GPM). The majority of tankless water heaters are capable of producing between 2 and 10 gallons of water per minute on average. During peak demand periods, it will be necessary to total up all of the flow rates of the fixtures and appliances in your home that may be using hot water at the same time in order to establish your water flow rate need during those times.

However, the following list of normal flow rates for popular fixtures and appliancesshould assist you in making an estimate:

Fixture or Appliance Usage: AverageGallons per Minute(GPM) Used
Bathtub 4 GPM
Rainhead shower head Up to 5 GPM
Standard shower head 2.5 GPM
Standard dishwasher 2.5 GPM
High-efficiency dishwasher 0.5 – 1.5 GPM
Standard clothes washer 2.5 GPM
High-efficiency clothes washer 1.0 GPM
Sink faucet 1.5 GPM

Using the expected GPM production as a guideline. There are some fixtures and appliances that have a larger output than others. Using a normal shower head (2.5 GPM) consumes approximately half the amount of hot water as using a rainhead shower head, as seen in the following table (5.0 GPM). The kitchen faucet consumes approximately the same quantity of hot water as other sink faucets, regardless of where they are installed. It is uncommon for many sink faucets to function in tandem for extended periods of time.

What is a Tankless Water Heater’s Temperature Rise

Once you’ve determined the amount of water flow your tankless unit will require during peak demand periods, you’ll need to figure out how much “temperature increase” it will require. The temperature rise refers to the amount of degrees your hot water heater will need to raise the incoming average groundwater temperature in order to supply enough hot water for your household. For example, suppose the temperature of the entering cold water is 52 degrees Fahrenheit. If you want hot water that is 110°F, you’ll need a temperature raise of 58°F.

Putting Flow Rate and Temperature Rise Together

The flow rate and temperature rise of a water heater are both measured in the ratings. If you want to appropriately size your water heater, you must take into consideration both the flow rate necessary and the rise in temperature required.

Ground Water Temperature Expected Gallons per Minute Output
37 F 3.9 GPM
42 F 4.7 GPM
47 F 5.5 GPM
52 F 6.3 GPM
57 F 7.1 GPM
62 F 7.8 GPM
67 F 8.6 GPM
72 F 9.4 GPM

Another example is based on a tankless water heater with a 9.5 GPM output and a temperature rise of 35 degrees. Consider the following scenario: you have four gadgets that may all demand hot water at the same time:

  • 1 GPM / 110° for the hot water faucet
  • 1 GPM / 110° for the dishwasher
  • 2 GPM / 120° for the washing machine.

Furthermore, let’s say that the entering cold water temperature is 50°F. A tankless water heater with a flow rate of 5.0 GPM and a temperature increase of 70° would be required to run all of these devices at the same time. The total flow rate of 5.0 GPM is equal to the sum of all device flow rates combined. The temperature increase of 70 degrees Celsius is obtained by subtracting the entering water temperature (50 degrees Celsius) from the highest device water temperature (120 degrees Celsius).

If you don’t intend to use all of your hot water equipment at the same time, you may save money by purchasing a less costly heater with a lower flow rate instead of an expensive one.

Are Tankless Water Heaters Worth the Cost

Furthermore, let’s say that your incoming cold water temperature is 50° F. If you want to run all of these devices at the same time, you will need a tankless water heater with a flow rate of 5.0 GPM and a temperature increase of 70° F. The total sum of all device flow rates equals the 5.0 GPM flow rate. By deducting the entering water temperature (50°) from the maximal device water temperature (120°), a temperature increase of 70 degrees is achieved. Having said that, just because the aggregate flow rate of your devices totals 5.0 GPM does not always imply that you require a water heater with a flow rate of 5.0 GPM.

  • Tankless water heaters have a lifespan of up to 20 years, whereas tank water heaters have a lifespan of as short as eight years. Due to the fact that tankless water heaters do not have a tank that might malfunction and leak, the danger of water damage is reduced
  • Tankless water heaters do not require any maintenance because they do not have a tank. Tank models require yearly flushing maintenance to maintain the tank’s integrity, which is commonly neglected, resulting in a shorter tank’s lifespan as a result. Tankless water heaters have the ability to produce hot water on demand when needed. A 50-gallon tank water heater can get depleted in as short as 30 minutes of continuous usage and requires a minimum of one hour of recovery time
  • However, this is not the case.

Electric vs. Gas Tankless Water Heaters

Once you’ve calculated the appropriate flow rate and temperature rise, you’ll need to choose between electric and natural gas tankless water heaters for your application.

Electric Tankless Water Heaters Cost Less to Buy and Install

It is one of the key advantages of purchasing an electric tankless water heater over purchasing a gas-powered tankless water heater because they are less expensive. Tankless electric water heaters for the entire house normally cost between $500 and $700, but whole-house tankless gas water heaters cost between $1,000 and $1,200 or more. Aside from that, the cost of installing an electric tankless heater is much lower than the cost of constructing a gas-powered tankless heater. Following your confirmation that your home’s wiring is suitable with the type you’ve chosen, installing the electric heater should be a reasonably simple and quick task to complete.

In order to work, some electric water heaters require up to four 220v breakers, which dramatically increases the expenses of installation and operating.

For starters, you’ll need to build a ventilation system to safely exhaust any fumes that your water heater may produce.

Electric Tankless Water Heaters are Easier to Maintain

Because electric units are simpler devices than gas-powered units, they are less expensive than gas-powered units. Because there are fewer components within, there are fewer moving parts that might break down and necessitate a costly repair job. The cost of fixing a propane-powered device is often cheaper than it would be if the equipment were fueled by gas. The majority of tankless water heaters will beep to alert you when tankless water heater maintenance is required.

Electric Tankless Water Heaters are More Energy-Efficient

The most efficient gas-powered tankless heaters have an energy efficiency of around 85%. When compared to most electric vehicles, which have an average energy efficiency of 98 percent, an 85 percent efficiency rating is considered inadequate. So even though natural gas is less expensive than electricity in your location, choosing an electric unit may result in you saving money on energy expenditures in the long run. Electric heaters are more ecologically friendly than other types of heaters.

The production of electricity is already less detrimental to the environment than the combustion of natural gas, and the greater energy efficiency of electric vehicles further adds to the disparity between the two modes of transportation.

Electric Tankless Water Heaters Can Require Significant Electrical Upgrades

The most important drawback of electric tankless water heaters is the difficulty in installing them on the premises. A separate 240v breaker is required for each burner on an electric tankless water heater. For large GPM models to work properly, three or four 220v breakers must be installed, which means you may need to install an electrical subpanel or make other electrical changes. If you choose an electric tankless water heater with a high output, you may need to build a new subpanel to supply electricity to the water heater.

You’ll need eight breaker slots to accommodate four 240v breakers.

According to industry standards, one heating element is required for every 2GPM of water production in an electric tankless unit.

See also:  How To Know If Hot Water Heater Is Leaking?

Gas Tankless Water Heaters Produce More Hot Water

A variety of natural gas and propane types are available for tankless gas water heaters. Some purchasers choose a tankless gas water heater over an electric water heater because gas versions can handle higher flow rates than electric water heaters. The best electric units have a maximum flow rate of 8GPM, but some gas ones have a flow rate that is substantially greater. In the case of a big home or an industrial application, you may be forced to choose a tankless heater that is fueled by natural gas.

The majority of on-demand tankless water heaters are capable of operating at a variety of intake temperatures.

In an electric model, the same rise in temperature of 70 degrees Fahrenheit (70 degrees Fahrenheit) will result in around 2 gallons of hot water per minute in hot water, which is a considerable drop in production.

Natural Gas is Cheaper than Electricity in Some Areas

Given the price disparity between natural gas and electricity, depending on where you reside, a gas-powered device may be more appealing than an electric model. However, even if natural gas is much less expensive than electricity, the gain in efficiency associated with electric models, as well as the anticipated increase in natural gas pricing, may still make turning electric the most cost-effective alternative.

Indoor or Outdoor Installation Options

In contrast to a standard tank-style water heater, which must be installed indoors, tankless water heaters may be installed either indoors or outdoors. Installation of your tankless water heater outside has the primary advantage of eliminating the need to install a venting system inside. Because electric units do not require ventilation, placing them outside provides no meaningful advantage in terms of energy savings. In warmer areas, it is possible to put a tank-style water heater outside; however, you will still need to construct an enclosure around it.

The majority of outdoor tankless water heaters include built-in freeze protection that can withstand temperatures as low as -30 degrees Fahrenheit.


Although you may not give it much thought, you use your hot water heater on a regular basis. Providing hot water whenever you need it is the one item that helps keep things clean and operating smoothly in any household! The question is, how do you choose the appropriate size tankless water heater for your household? Take into consideration the appliances that will be used at the same time. When two persons living in an average-sized home need to shower every day at different times of the day (for example, because they work opposing shifts), a 2 GPM model would be appropriate.

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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°. 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.

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.


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.

Other Sizing Notes

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

Recommended Reading:

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

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

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

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

  • 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

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

The main drawback is the excessive up-front expense.

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

If you are undersized, what should you do? I made the wrong choice! Most water heaters will automatically reduce the supply of hot water if the demand for hot water exceeds the maximum capacity, even if this 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. As an added bonus, a tankless water heater that operates at maximum capacity all of the time is more likely to fail early. If you take on too much, what happens then?

The main drawback is the excessive up-front cost of the system itself.

  • 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.
See also:  How To Flush A Gas Water Heater
Water Outlet Standard Flow Rate
Hand washing sink 0.5 – 1.5 gpm
Shower head 2.5 (2.0*) gpm
Bathroom faucet 2.2 (1.5*) gpm
Bathtub faucet 3.0 – 4.0 gpm
Kitchen faucet 2.2 gpm
Washing machine 23+ gallons per load, gpm hard to determine
Dishwasher 6 gallons per load, gpm hard to determine

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

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

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

Step 2: Temperature Rise

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

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

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

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

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

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

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

Step 3: Putting It All Together

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

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

Are you ready to take the next step?


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!
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What Size Tankless Water Heater Do I Need For My Home [Calculator]

Make your way into the groundwater Great! You’re aware of the two most crucial factors to consider when selecting the finest tankless water heater for a two- (or three-, or four-) bathroom home. You may now just navigate to our main ranking and select a model that best meets your needs. Please keep in mind that the flow rate you see in the chart represents the maximum load that a tankless can manage under ideal circumstances. Make use of the table to guide you, but make certain that the type you pick can provide adequate hot water at the temperature rise you anticipate.

Consider the following scenario: you have a one-bathroom home and you want to operate one shower and one other appliance at the same time.

As a result, if you have a one-bathroom home, you will require a tankless water heater that has a capacity of at least 3 gallons of hot water per minute. Rinnai or Ecosmart are two brands that we suggest.

Best Tankless Water Heater For 2 Bathroom House

Once again, everything is dependent on your specifications. It’s possible that my proposal isn’t the best option in your case. As a starting point for further study, if you have a two-bathroom (or two-bedroom) house and you want to run two showers at the same time, you will need a tankless water heater that can generate at least 5 gallons of hot water per minute, according to the manufacturer’s specifications. Rinnai V65iN is the model I recommend:

Rinnai V65iN Indoor Gas Tankless Water Heater

The following are the features and specifications of the Rinnai V65iN gas indoor tankless water heater:

  • Efficiency rating: 0.8
  • Product dimensions (inches): 13.9 8.8 23.6 inches
  • Flow rate (inches): 6.5 GPM
  • Activation flow rate (inches): 0.26 GPM
  • Energy Star® certified.

The Rinnai V65iN is capable of providing outstanding performance for its pricing range of 6.5 GPM. It is the ideal tankless water heater for a two-bedroom house with two bathrooms. Ideally suited for middle-sized homes in areas with a warm or mild climate, this design is a good choice. A more powerful model from this series would be appropriate for households in the northern states, in my opinion (click the link to check it out). For those who live in areas where natural gas is not an option and who want an electric unit (although I usually prefer gas/lp over electric), I would propose the following model:

EcoSmart ECO 27 Electric Tankless Water Heater

EcoSmart ECO 27 Electric Tankless Water Heater: EcoSmart ECO 27 Electric Tankless Water Heater:

  • Maximum Flow Rate:5 GPM
  • Activation Flow Rate:0.25 GPM
  • Dimensions:17″H x 17″W x 3.75″D
  • Maximum Flow Rate:5 GPM
  • Requirements: 113 amps (3 double pole x 40 amp breakers)
  • 99.8 percent energy efficiency

Best Tankless Water Heater for 3 Bathrooms

In a three-bathroom house (or three-bedroom house? ), you’d normally want to take at least two showers at the same time. But don’t forget about other appliances such as the washing machine and dishwasher, which may each consume up to 2 liters of hot water every minute. This amount of demand necessitates the use of something really strong. This is especially true when you require all three baths or showers at the same time. The peak demand is likely to be between 7 and 9 GPM at any one time. The Rinnai RL94iN – a natural gas tankless water heater with a maximum flow rate of around 9.4 GPM and a reasonably affordable price makes it the ideal choice for a home with three bathrooms and extra household appliances.

Rinnai RL Series HE+ Tankless Hot Water Heater

The Rinnai RL Series HE+ Tankless Hot Water Heater has the following features:

  • Natural gas or/and liquid propane as fuel
  • UEF (Efficiency Rating):0.81
  • Product Dimensions: 14 x 10 x 23 inches
  • Maximum Flow Rate:9.4 GPM
  • Product Dimensions: 14 x 10 x 23 inches Energy Star certified (rebate opportunities may be available)

Flow rate:9.4 GPM; Fuel Type:Natural Gas or/and Liquid Propane; UEF (Efficiency Rating):0.81; Product Dimensions: 14 x 10 x 23 inches; UEF (Efficiency Rating):0.81; Maximum Flow Rate:9.4 GPM Certificated by the Energy Star program (rebate opportunities may exist);

Best Tankless Water Heater for 4 Bathrooms

Tankless water heaters for large homes are not always the most straightforward to select. Particularly if you live in the northern hemisphere of the United States. The climate is chilly, and the temperature of the groundwater is low. The ideal pick must have a high performance rating, otherwise it will not be able to handle the tremendous load of four bathrooms well. Rinnai RUR199iN is, in my view, your best bet in this situation. It is a highly efficient tankless water heater that is also one of the most powerful models available on the market.

Please double- and even triple-check to ensure that it fulfills your criteria.

You should choose the Rinnai 199iN if you reside in the south or if you don’t intend on utilizing all four of your bathrooms at the same time (but still want to be able to use them anytime you need).

Rinnai RUR199iN Tankless Water Heater for 4 Bathroom House

The following are the features and specifications of the Rinnai RUR199iN:

  • Natural gas is the fuel of choice
  • The UEF (Efficiency Rating) is 0.93
  • The product dimensions are 22.1 x 14.8 x 33.9 inches
  • The maximum flow rate is 11 GPM
  • Pump for recirculation built within the unit

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