How To Select A Tankless Water Heater

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.


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

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

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

Other Sizing Notes

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

Recommended Reading:

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

About Our Team

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

Tankless Water Heater Buying Guide

Tankless water heaters create more hot water while using less energy than the most efficient tank heaters, allowing you to save on energy, space, and money all while saving money and time.

Tankless Water Heaters

A tankless water heater, also known as an on-demand water heater, warms water only when it is required. It is possible to put these heaters in a closet, on an outside wall, or in any other suitable location within your home. They may be used to heat your entire home or just a single point of usage such as a shower, sink, or appliance, among other things. A tankless water heater, whether natural gas, propane, or electric, provides you with an infinite supply of hot water while saving you money on energy expenditures by eliminating the need to store, heat, and reheat the same tank of water.

How Does a Tankless Water Heater Work?

  • When you turn on a hot water faucet or turn on the dishwasher, the tankless water heater identifies the requirement for hot water and begins the heating process immediately. In order to provide water at the correct temperature, the temperature of the entering water is utilized to determine the amount of heat that must be produced by the burners. When there is no longer a demand for hot water, the unit shuts down and stops consuming energy, waiting until a new demand for hot water arises. The ultimate result is hot water that is not restricted to the amount of hot water stored in your tank.

Benefits of a Tankless Water Heater

You have the ability to convert any water source into hot water on demand. There is no limit to the quantity of heat that may be stored in a storage tank heater. Whether you’re cleaning the dishes, washing your clothes, taking a hot bath, or even doing them all at the same time, tankless water heater technology will provide you with the hot water your lifestyle requires immediately and efficiently.

Lower Energy Bills

When you use a tankless water heater, you may save up to 40 percent on your energy bills! Heat-only units are meant to be extremely energy efficient, heating water only when it is required.


Tankless water heaters have a lifespan of up to 20 years, which is often twice as long as a traditional tank-style device.

Space Savings

Traditional water heaters may consume up to 16 square feet of important floor space, which is a significant amount of room. Many tankless water heaters are roughly the size of a carry-on suitcase and may be mounted on nearly any wall inside or outside your home, depending on your preferences.

Clean, Fresh Water

Traditionally installed water heaters are susceptible to rust and scale buildup within the tank where the hot water used for bathing and drinking is kept.

Water heaters that do not require a tank provide you with continuous access to fresh, clean water that is heated on demand as it goes through the unit.

Choosing the Right Tankless Water Heater

It is critical to determine the size of the tankless water heater you will require. A heater that is too tiny will not be able to appropriately fulfill the hot-water demands of your house, while a heater that is too huge will be an unnecessary expense. How much water will you use at peak demand – for example, when the dishwasher is running, you are having a shower, and a load of laundry is being washed? What proportion of the incoming water will need to be heated in order to meet the demand? Before purchasing a tankless water heater, you should think about two things: the flow rate, which is the amount of water that flows through a fixture or appliance in gallons per minute (GPM), and the temperature rise, which is the difference between the groundwater temperature and the desired hot-water output temperature.

For example, you may be using the kitchen sink, washing laundry, and bathing all at the same time.

To find out the flow rate of a fixture or appliance (shower head, dishwasher, washing machine, and so on), consult the manufacturer’s handbook or look for the GPM stamped directly on the fixture or appliance.

Average GPM Usage for Common Appliances and Fixtures

It can be seen from the graph above that when the shower, kitchen sink, and high-efficiency washing machine are all in use at the same time, the total flow rate is 6.0 GPM. Depending on where you live, groundwater temperatures can range anywhere from the mid-30s to the upper 70s degrees Fahrenheit, measured in Fahrenheit. Groundwater temperatures in North America are divided into three climatic zones, each with its own average groundwater temperature:

  • Northern Zone temperatures range from 37 degrees to 51 degrees
  • Central Zone temperatures range from 52 degrees to 61 degrees
  • And Southern Zone temperatures range from 62 degrees to 77 degrees.

The average interior water temperature in a residence is between 110 and 120 degrees Fahrenheit. For the temperature rise, subtract the entering water temperature from the required interior water temperature to find the temperature increase. According to this formula, if your shower water temperature is 57 degrees Fahrenheit and your ideal shower temperature is 120 degrees Fahrenheit, the temperature rise is 63 degrees. Additionally, while purchasing a tankless water heater, in addition to being aware of the flow rate and temperature rise, you should consult size tables to assist you in selecting the most appropriate heater for your requirements.

Point of Use Water Heaters

An region of your home may require an additional burst of hot water from time to time. When this occurs, the usage of a small point of use (POU) water heater might be beneficial. In the case of a hot tub that operates on electric heat, a POU heater can be added nearby to provide additional heat.

Installed close to a room or appliance that requires more hot water, these compact tankless heaters provide instant hot water. Moreover, these appliances are useful for house renovations and extensions since they can easily be put in a cabinet or other compact location.

Hybrid Water Heaters

If you appreciate the convenience of a tank water heater while also seeking to reduce your energy expenses, a hybrid water heater may be the best option for you. These appliances, which combine the functions of a tank-style and a tankless water heater, are often more than twice as efficient as a tank-style water heater and may be installed utilizing your existing infrastructure. Hybrid water heaters are equipped with a heat pump that draws in and heats the surrounding air before releasing the heated air back into the area where the heater is located.

These hybrid water heaters, which heat water on demand, are extremely energy efficient.

Check with the manufacturer for installation suggestions.

With those figures in hand, a Lowe’s associate can assist you in selecting the most appropriate tankless water heater for your needs.

9 Tips on How to Choose a Tankless Water Heater

So, you’re considering of purchasing a tankless water heater for your house – but you’re not sure where to begin your research. Well, don’t be concerned since you’ve arrived to the correct web page! Tankless water heaters may help you save both money and space – This article will serve as your go-to resource for selecting the best tankless water heater for your needs at home. Let’s go through the top nine tips you should be aware of!

1. The Benefits of Tankless Water Heaters

Here are a few of the advantages of installing a tankless water heater in your home: 1. It is more energy efficient. These are the considerations you should bear in mind while you look for your ideal water heart.

Taking Up Less Space

When you use a tankless water heater, you may have immediate hot water for any use in your home. The water is heated as it passes through the gadget, rather than in a large unwieldy tank that you have to carry about all of the time.

Unlimited Hot Water

There is no limit to the amount of water that may be stored with this sort of water heater. You are free to leave the hot water running for as long as you desire. You’ll never have to worry about the person in the shower in front of you using up all of the hot water again!


When compared to a traditional water heater, tankless water heaters have a lifespan that is twice as long. The most basic of them can last you up to around 20 years.

Energy Bill

Installing a high-quality tankless water heater in your house has the potential to reduce your energy bills by half! These units are extremely energy efficient and may save you a significant amount of money.

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2. Find Out What Size Will You Need

One of the first things you should do is determine the precise amount of tankless water heater you will require. Every single day, the average family in the United States consumes 64 gallons of water, according to the United States Energy Information Administration. Undoubtedly, it is a substantial amount of water! So, how do you determine the size of the tankless water heater you’ll need for your home? Tankless water heaters are available in a wide range of styles and sizes to suit your needs.

Tankless water heaters also utilize the British Thermal Unit as an input to establish their efficiency ratings, which are displayed alongside their GPM. As a result, you will need to consider the GPM, BTUs, and temperature rise of your household. Here’s how you can find out:

Looking At BTU’s

Approximately the same amount of energy is necessary to raise the temperature of a pound of water by one degree Fahrenheit as contained inside a single BTU. As a result, the higher the BTUs of your water heater, the greater the amount of water that will flow. In most cases, comparing BTUs and GPMs reveals that 31k BTUs can create 1.2 GPM and 190k BTUs can provide 5.7 GPM, respectively.

Calculating GPM

Take a look around your home and count the number of gadgets that will require hot water from your tankless water heating system. Then figure out how many of them you’ll be able to employ at any one moment. Then you only need to add up the GPM of all of those devices. Flow rates for tankless water heaters, for example, will range from 1.2 to 6 GPM on average. In general, a basic sink would be rated at 1.2 GPMs, whereas a shower would be rated at 2.6 GPMs. As a result, a tankless water heater with a flow rate of 4 GPM can manage both a shower and a sink at the same time.

Calculating Temperature

Calculate the temperature of the water that enters your home and subtract it from the temperature you want your hot water to be when you want it. You’ll know whether or not a prospective new tankless water heater will be able to meet the hot water demand in your home once you’ve completed the calculations and determined the flow rate for the potential new water heater.

3. Take a Look at Warranties

Make sure to go through any warranties that may be included with a new tankless water heater before purchasing one. Most high-quality heaters will come with an extended warranty that will cover you in the event that it fails. You should also take into consideration the sort of water that enters your home. If you have hard water, this may result in your new equipment wearing out more quickly. Choosing a tankless water heater with a longer warranty will be important if your water is hard to begin with.

4. Water Heater Efficiency

Tankless water heaters can save you money because of their high efficiency – they will always surpass traditional water heaters in terms of performance. When evaluating the efficiency of a tankless water heater, the energy rating (EF – Energy Factor) is used to determine how efficient the device is.

EF Rating

The gadget will perform better and be more efficient if the EF rating is high enough. The quantity of energy that comes out your water heater is multiplied by the amount of energy that goes into the device to calculate the efficiency (EF).

5. Gas-Fueled Heaters

An excellent choice for those who have access to natural gas or propane is to install a gas tankless water heater. These units have the quickest reaction time as well as the highest heat output. Despite the fact that you get rapid heat with a short reaction time, utilizing a gas unit will normally consume more gas than using a regular water heater will. So keep this in mind if you decide to go with one of these options: you may have to bring in additional gas to your home.

Gas units will have an ignition system as well, which will play a role in a number of the distinctions between these units and their counterparts. When choosing one of these gadgets, it is important to consider three main sorts.

Power Ignition

There are no batteries or a connection to your electric system required for them. Once the water begins to flow through your new gadget and via a turbine, they are active.

Direct Ignition

Direct ignition will need the connection of the device to your electrical system or the use of batteries. As soon as water begins to flow through the gadget, this kind is easily lit by pressing a button.


In the same way that typical water heaters do, a pilot light will continually burn — this does not need that the water heater be connected to electricity or that any water flow be detected.

6. Electric Water Heaters

Purchasing an electric tankless water heater might be significantly less expensive than purchasing a gas unit. Electric heaters are often 300 to 500 dollars less expensive than gas heaters, depending on the model. They are also more energy efficient and ecologically friendly than older models. However, one disadvantage is that electric water heaters tend to consume more energy than gas water heaters. The electric ones, on the other hand, have a more straightforward design. In this case, working with electric rather than gas might be more convenient and less expensive when it comes to repair and maintenance.

Because they do not emit any exhaust, they do not necessitate the use of ventilation.

7. Your Climate

An other factor to consider when selecting a tankless water heater for your house is the environment in which you reside. This might assist you in determining the most appropriate unit for your household needs and requirements. The environment in which you live might have a significant impact on your GPM rating. As previously indicated, while computing the GPM rate, you take into consideration the temperature of the water entering your home. The colder the weather outside, the more difficult it will be to heat your water, which will result in a reduction in your GPM.

Furthermore, if you live in a warmer area for the majority of the year, having a higher GPM rating may not be your number one concern.

8. Venting a Tankless Water Heater

When shopping for a new tankless water heater, it’s important to think about the venting options available. Will you be required to vent your unit at any point?

Indoor Units

Not all tankless water heaters will be required to be vented — in fact, only indoor gas-fueled tankless water heaters will be required to be vented. You won’t have to worry about this because electric units don’t create any exhaust emissions, unlike gas units. Venting a gas unit is not the same as venting a conventional water heater, for example. It is usually a good idea to consider hiring a skilled expert to assist you with the water heater installation process.

Just keep in mind that if your unit will be installed outside, it will not be necessary to have venting. Ventilation is only necessary while working in a tight space or within a facility where people are present.

Outdoor Units

These sorts of devices are built to withstand the environment and are intended to be used outside. You no longer have to spend time attempting to find out how to vent an indoor space to the outside environment. Despite the fact that they are designed to survive the environment, they have some limits. If you live in a location where extreme cold temperatures are common, it could be a better idea to look for an alternative choice. One significant advantage of this sort of device is that it is far less expensive than indoor devices and is considerably simpler to install.

9. Tankless Water Heater Maintenance

It depends on the model of tankless water heater that you choose how much maintenance is required. Let’s take a look at each of them and the distinctions between them.

Maintenance For Electric Units

These are typically low-maintenance and require little upkeep. You can only clean the water inlet on these machines, which is all that can be done for them. In addition to that, electric tankless water heaters do not require any further maintenance to function properly.

Maintenance for Gas Fueled Units

Maintenance on gas units is a little more involved than on electric devices. As opposed to a simple cleaning, these machines will require frequent inspection and maintenance. This should also be done by a respectable organization with personnel who have undergone extensive training. Finally, when considering tankless water heaters, consider if you are willing to deal with the hassle of having to do maintenance on the unit or whether you would like to pay a professional to perform preventative maintenance on the unit.

Wrapping Up Tankless Water Heaters

When seeking to purchase a tankless water heater, there are a number of considerations and considerations to bear in mind. If you’re making a significant investment, take your time and thoroughly investigate the unit before making your selection.

  • Consider why you’re buying a new one in the first place. Determine the precise size that you will require
  • Examine the unit’s warranty to ensure that it is valid. See how energy efficient the item is by looking at its efficiency rating. Determine if an electric or gas heating system is more appropriate for you. Is it necessary to vent the unit
  • If so, how? You should take into consideration the climate in which you live. What type of maintenance will be required
  • Can you undertake the maintenance yourself or will you need to engage a skilled expert in your area

After you’ve taken all of this into consideration, you’ll be able to identify the specific unit that will work best in your house. All that is left is for you to do is purchase and install! If you have any concerns regarding selecting the best tankless water heater for your house, or if you require the services of a skilled expert to complete an installation, please visit ourcontact page and send us a note!

Read This Before You Buy a Tankless Water Heater

Consider the following: The method used by the majority of houses in this nation to heat water is ridiculously inefficient. Every year, we fill up large storage tanks of 40- to 50-gallon capacity with water and then continuously pump energy into them to ensure that we have hot water available anytime we want it. But, unfortunately, this is not always the case. The wait for the tank to reheat might be lengthy if a teenager is taking a long shower or the spouse is enjoying a long soak in the tub.

Then there are the niggling concerns such as: Is it clogged with silt that consumes energy? Is there a chance of a leak? Both of these worries are fair given the fact that tanks often fail between 8 and 12 years.

Tankless Water Heater Installation: Is It Worth It?

Investing in a tankless water heater has a number of benefits, as detailed above. It creates hot water just when you use it and for as long as you require it, resulting in a reduction of 27 to 50% in fuel expenses when compared to tank-type heaters. (A typical gas-fired tank wastes 40 to 50% of the fuel it burns, according to the manufacturer.) As a result, there is virtually little danger of a catastrophic leak occurring because there is no tank to collapse. Furthermore, since their introduction in the United States in the 1990s, tankless heaters have become increasingly sophisticated, with features such as built-in recirculating pumps (which provide “instant” hot water) and wireless connectivity, which alerts you via smartphone when a unit requires servicing.

Our tankless water heater guide will explain how they function, what you should know before purchasing one (and before the installation comes), and the idiosyncrasies of how they operate so that you won’t be caught off guard if you decide to go tankless.

How Does a Tankless Water Heater Work?

Doug Adams created the illustration.

  1. It all starts with the first turn of the hot-water faucet (1). A flow sensor (2) detects the presence of water entering the heater and sends a signal to the control panel, causing the heater to begin generating hot water. During operation of a natural-gas-fueled unit, thecontrol panel (3) activates thefan (4), which pulls in outside air, opens the gas valve (5), which allows the gas to flow into the unit, and ignites the burner (6). In order to transmit heat from the flames to water passing through the exchanger’s tubing, a heat exchanger (number 7) is used. The mixing valve (8) regulates the temperature of the superheated water that exits the exchanger. Whenever the temperature sensor (9) detects water temperatures that are too high or too low for the intended setting, the panel will modify the gas valve, the mixing valve, and the flow-regulating water valve (10) in accordance with the results. Ventilation is provided by a sealedvent (11) (or a couple of vents) via a roof or exterior wall, which removes exhaust gases and supplies combustion air to the burner.

Several people were thanked for their contributions: Phillip Maxwell, Residential Product Manager, Rheem; Eric Manzano, Product Training Supervisor, Noritz; Joe Holliday, Senior Vice President, Product and Business Development, Rinnai; and Fred Molina, Water Heater Products Manager, Bosch Thermotechnology

What to Know About Tankless Water Heaters

Thanks to Noritz for the use of his photo.

How Much Does a Tankless Water Heater Cost?

Prices range from approximately $170 for modest gas-fired units to more than $2,000 for high-output heaters that can serve two showers at the same time; $1,000 is a reasonable starting point for most buyers. Electric heaters without a tank range in price from $90 to $900. The expenses of a first-time installation are higher than the price of a simple tank replacement. Electric tankless water heater installation (see item below headed “Installing an Electric Tankless Water Heater”).

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How to Install a Tankless Water Heater

This is a work that should be left to the professionals, since it entails creating leak-free water, vent, and gas connections in the case of gas or propane units, or upgrading the wiring and circuit-breaker panel in the case of electric units, and it is best left to the professionals.

Tankless Water Heater Maintenance

Sign up to have a professional do an annual service that includes cleaning or replacing water and air filters, as well as inspecting the burner’s operation. The use of a vinegar flush every 500 hours in places with hard water prevents mineral accumulation, known as scale, from blocking the heat exchanger. That 20-minute task may be completed by a professional or by a homeowner.

How Long Do Tankless Water Heaters Last?

It is expected that gas-burning tankless water heaters would last 20 years or longer, which is two to three times longer than tank-type heaters. Tankless electric units have shorter life lifetimes, ranging from 7 to 10 years, compared to conventional units.

Where Can I Buy One?

They may be found at plumbing supply stores, big-box stores, and internet sellers, among other places.

Alternatively, you may order one via your plumber.

Pros and Cons of Tankless Water Heaters

Thanks to Noritz for the use of his photo.

PRO: They’re Compact

As a result of new federal requirements requiring stronger insulation to decrease standby heat loss, the size of newer tank-type water heaters has increased. Consequently, they may not be able to fit into locations where an older heater with the same capacity might. Tankless gas heaters are approximately the size of a suitcase and are mounted on the wall.

PRO: They’re Safer

A tank-type heater, on the other hand, may leak and spill gallons of water if it springs a leak, but it will not house Legionella germs or topple over in an earthquake. The air supply and exhaust vents are also closed to prevent backdrafting, which would otherwise allow carbon monoxide to enter the house.

PRO: They’re Easy to Winterize

Owners of vacation homes are well aware of how long it takes to drain a water-heating tank prior to closing up a house for the season. An electric compressor may drain a tankless heater in a matter of seconds, after which it can simply be unplugged.

CON: They’re Sensitive to Slow Flow

These devices automatically shut off if there is too much scale accumulation in the pipes, or if the aerators in the faucets and showerheads get blocked, or if a turned-down faucet limits water flow to around 0.3 gpm.

CON: The Payback Takes Awhile

An annual savings of only around $100 for a household using a $1,000 tankless gas heater vs a $400 tank-type heater is possible, depending on how efficient the heater is and how much hot water is utilized. The savings, however, begin to accrue after six years, when many tanks are reaching the end of their useful lives due to the extended lifespan of tankless gas systems.

New Tankless Water Heater Technology

Thanks to Noritz for the use of his photo. The advancement of tankless technology is ongoing. Here are a few of the most recent enhancements:

Higher Efficiency

Condensing gas heaters can extract up to 96 percent of the heat from a fuel, which is a 17 percent improvement over first-generation tankless devices. This is possible because of a second heat exchanger, which collects a large portion of the exhaust heat before it exits the vent. In addition to being around 25% more expensive than noncondensing heaters, condensing heaters produce acidic condensate that must be neutralized. If a heater doesn’t come with a built-in neutralizing cartridge, the installation will have to install one after the fact.

Instant Hot Water

Despite the fact that tankless water heaters heat water in around 15 seconds, you must still wait for the hot water to reach your shower head or faucet, just as you would with a tank-type heater. The recirculation pump should be used when the distance between the heater and the fixture is greater than 50 feet. This will conserve water and minimize the amount of time spent waiting. It is this pump that pushes the cold water in the pipes back through the heater. The pump can be activated by a timer, a push button, a motion sensor, a smart speaker, or a smartphone (see illustration above).

Wi-Fi Compatible

Tankless systems with digital connectivity let you to control the temperature as well as monitor gas and hot-water use from your mobile device. Furthermore, the device is capable of identifying the cause of a problem.

Please communicate this information to your plumber so that he or she may arrive on the job site knowing exactly what has to be done. This function also eliminates the need for any guessing when it comes to determining when it is time to descale.

Tankless Water Heater Rebates: A Great Way to Save

Carl Tremblay captured this image.

What Size Tankless Water Heater Do I Need?

Here’s how the specialists ensure that your water heater produces adequate hot water: 1. A large burst of BTUs is required for a tankless heater to convert cold water into hot water in a matter of seconds. However, if a heater’s Btu output is insufficient to meet demand, it will reduce the flow rate or, in the worst scenario, offer tepid water. A plumber considers three aspects when determining whether or not a heater will be able to satisfy the demands of a household:

  • The temperature of the water that enters the heater
  • The maximum demand for hot water expressed in gallons per minute (gpm)
  • The efficiency of the heater, as shown by its Uniform Energy Factor, which may be found in the product specifications
  1. The first step is as follows: A professional determines how many Btus per gallon of water heater is required to increase the incoming water temperature to 120 degrees (see the map on the next slide)
  2. Flow rates for all of the appliances and fixtures that may be consuming hot water at the same time are added together to form peak demand, which is calculated as follows: (These rates are detailed in the next slide.) As a result of not bathing or washing in 120-degree water, we save around 20% on our overall use. Water-saving fixtures and appliances, as well as delaying laundry while the shower is in use, can help you minimize peak consumption. In the calculation, the total Btu production is computed by inserting the Btus-per-gallon and peak-demand amounts in at different points along the way. If the difference in output is between two models, go with the one with the greater Btu rating to save money. You’ll also need two smaller units that function in tandem if your output is greater than 198,000 Btus, which is the limit for domestic gas heaters.

Btus Output Estimate

Not interested in completing the calculations? Make a rough estimate of how much heater output you’ll want using these statistics.

  • The following figures are for one bathroom for one to two people: 140,000 Btus
  • Two bathrooms for two to three people: 190,000 Btus
  • Three bathrooms for three to five people: 380,000 Btus

Btus Per Gallon by Region

  • Kitchen or bath faucets should flow at 1.5–2.2 gpm
  • Tub filler faucets should flow at 4 gpm
  • Dishwasher: 1–2.5 gpm
  • Washing machine: 1.5–3 gpm
  • Showerhead should flow at 1.25–2.5 gpm

How to Determine gpm?

To get the real gpm of a fixture, time how many seconds it takes to fill a bucket to the 1-quart mark and multiply that time by the number of gpm. gpm is calculated by dividing 15 by the number of seconds in a minute.

Electric Tankless Water Heater Facts

Thanks to Stiebel and Eltron for their assistance. In addition to gas lines and propane tanks, tankless water heaters operated by electricity can provide the benefits of on-demand hot water to homes that do not have them. Compared to gas or propane tankless heaters, these systems, which heat water using thick copper rods, are significantly quieter and roughly a third smaller in size. And because they do not require vents, they can be fitted practically anyplace, even beneath sinks and in small closets, without compromising performance.

In locations with warm groundwater, that amount of hot water may be sufficient to feed a whole house; but, in colder climates, they are better suited to point-of-use service, where the demand for hot water does not become excessive.

Furthermore, electric heaters have a lifespan that is approximately half that of gas heaters: Warranty periods typically range from three to five years.

Tankless Water Heater Installation

Doug Adams created the illustration. What you and your plumber should look for before the installation day is as follows:

Gas Line

If you want your tankless heater to work effectively, you must connect it to a gas supply line that supplies enough volume at a high enough pressure to run the burner. In many circumstances, this will need increasing the diameter of the supply pipe to 3-4 inches in diameter. Furthermore, if the pressure is insufficient, the gas provider will be required to change the regulator on the meter. For your information, some tankless systems, like as ones manufactured by Rheem, are capable of working with a regular 12-inch gas line as long as it is not more than 24 feet in length.


Tankless gas heaters that do not condense employ stainless-steel vents that can resist high exhaust temperatures. Condensing systems feature a cooler exhaust and use PVC pipes, which are less costly than other types of exhaust. Installing a concentric vent, which has an exhaust pipe inside a larger air-intake pipe, is easier than installing a traditional vent since only one hole in the wall needs to be made.

As a point of reference, vent runs have traditionally been limited to a maximum of 10 feet. However, more powerful fans, such as those found in Rinnai’s Sensei series, now enable vents to be extended up to 150 feet.

Water Hardness

Heat transmission is slowed and water flow is restricted when scale deposits accumulate in a heat exchanger (or on electric heating components) over time. If you currently have whole-house water softening, scale will not be an issue for you. However, if your water is not being softened and its hardness surpasses 120 milligrams per liter, it is worthwhile to invest in a treatment system to remove the hardness. For your information, a specific, point-of-use cartridge, such as the TAC-ler water conditioner (Stiebel Eltron), can be used to change the hardness of water without the use of salt or other chemicals.

Outdoor Tankless Water Heater

Matt Risinger captured this image. If your environment and local rules allow it, think about the advantages of hanging a heater outside in the winter.

  • Saves space: You won’t have to create place for another appliance in your home as a result of this. Installation is straightforward: Because of the built-in exhaust vent, there is no need to drill a large hole (or two) through the side of the building. Service is simple: A plumber may come to your home at any time, whether or not you are there. However, take in mind the following: Regulations governing construction: If you want to install it outside, you may require approval from your local building department. Weather conditions that are cold: Even at temperatures as low as 22 degrees Fahrenheit, internal heaters keep components warm, but exposed water lines must be insulated and covered with heat tape that activates automatically in freezing conditions. Southern states are less concerned about frozen pipes than those located north of the Mason-Dixon Line.

Tankless Water Heater Venting

Carl Tremblay captured this image. Are you in need of assistance with repairs around your home? A house warranty may be of assistance. The This Old House Reviews team has put up some in-depth guidelines that you can read here:

  • Home warranty providers that are the best
  • Reviews of American Home Shield, AFC Home Club, Select Home Warranty, and Choice Home Warranty are all available.

How to choose your whole house tankless water heater

* The smallest suggested model is based on average winter ground water temperatures in the specified location, as well as a flow rate of 3 gallons per minute in the model. Greater flow rates will be achieved by using a bigger tankless water heater than what is provided. The temperature of incoming water varies based on the source of the water, the location of the water, the season, and the environment. Please measure your needed flow rate as well as the temperature of the incoming water to ensure that you are selecting the proper model for you.

Application Flow Rate in GPM at 60 PSI
Typical Shower 1.5 to 2.0 GPM
Typical Bath Tub Faucet 2.0 to 3.0 GPM
Bathroom Vanity Sink Faucet 0.5 to 1.5 GPM
Kitchen Sink Faucet 1.0 to 2.2 GPM
Clothes Washer 1.5 to 3.0 GPM

The selection of an ideal electric tankless water heater for your house or company is based on four major considerations:

  • Incoming water temperature, intended maximum flow rate, desired features, and electrical service in your house are all important considerations.

Incoming Water Temperature

It is intended to provide you with a general model recommendation based on the usual winter arriving water temperatures in various locations of North America, as seen on the right. When a maximum flow rate of around 3 gallons per minute (or less) is needed, the model advised on the chart for your location is considered the lowest recommended model for an all-purpose home water heater; however, purchasing a more powerful model will result in greater flow rates. We also presume that your home has enough electrical service to handle the model that we propose – see the section below for more information.

See also:  How To Turn On Electric Hot Water Heater

Maximum Flow Rate Desired

The greatest amount of hot water that you require should be considered before making a final selection on which tankless water heater type to purchase. The majority of people tend to overestimate their need for hot water at several times. See the chart on the right for some general flow rate recommendations for various hot water uses throughout the house. When it comes to hot water usage, most homes will only require one large use at a time, or a maximum of two flow-restricted showers, for example.

  • It is recommended that you test the flow rates at your showerhead and at numerous outlets around your home in order to obtain a more accurate approximation of your exact flow rate requirements.
  • In northern latitudes, on the other hand, your flow rate expectations should be much lower (up to 3.5 GPM), depending on the temperature of the incoming water you use.
  • The average shower temperature is between 98 and 104 degrees Fahrenheit.
  • Unless your house or company is powered by 240 volts (208 volts), your maximum flow rate will be reduced.

It’s as simple as looking up your incoming water temperature and desired flow rate to figure out which model to buy. Keep in mind that the temperature of entering water might fluctuate throughout the year depending on the weather conditions.

Incoming Water Temperature
Models 40F 45F 50F 55F 60F 65F 70F 75F 80F
Tempra 36/Tempra 36 Plus 3.8 GPM 4.1 GPM 4.5 GPM 4.9 GPM 5.4 GPM 6.1 GPM 7.0 GPM 8.2 GPM 9.8 GPM
Tempra 29/ Tempra 29 Plus 3.0 GPM 3.3 GPM 3.6 GPM 3.9 GPM 4.4 GPM 4.9 GPM 5.6 GPM 6.5 GPM 7.8 GPM
Tempra 24/Tempra 24 Plus 2.5 GPM 2.7 GPM 3.0 GPM 3.3 GPM 3.6 GPM 4.1 GPM 4.7 GPM 5.4 GPM 6.5 GPM
Tempra 20/Tempra 20 Plus 2.0 GPM 2.2 GPM 2.4 GPM 2.6 GPM 2.9 GPM 3.3 GPM 3.7 GPM 4.4 GPM 5.2 GPM
Tempra 15/Tempra 15 Plus 1.5 GPM 1.6 GPM 1.8 GPM 2.0 GPM 2.2 GPM 2.4 GPM 2.8 GPM 3.3 GPM 3.9 GPM
Tempra 12/Tempra 12 Plus 1.3 GPM 1.4 GPM 1.5 GPM 1.6 GPM 1.8 GPM 2.0 GPM 2.3 GPM 2.7 GPM 3.3 GPM
  • Generally speaking, the areas colored in light grey in the accompanying chart are only suited for tiny point-of-use applications (for example, a single sink). Homes with modest hot water demands and showerheads with low flow rates may find the areas colored in yellow to be acceptable for their needs. Before making a final model decision, it is advised that you test your real water flow rate and incoming water temperature thoroughly. Those areas outlined in red reflect the most suitable model options for the majority of whole-house water heating requirements. It is normally acceptable to operate two showers at the same time with regular showerheads in areas where the maximum flow rate is 4.0 gallons per minute or above

Due to the possibility of considerable differences in incoming water temperatures between your property and others in your near vicinity, evaluating your incoming water temperature is often a more accurate method of identifying which model is best suited to your home or company.

Desired Features

Stiebel Eltron now provides two Tempra Series Models, the standardTempra and theTempra Plus, which are both available for purchase. In terms of the fundamental distinction between the two versions, the Tempra Plus models are distinguished by the inclusion of Stiebel Eltron’s unique Advanced Flow Control technology. When the unit’s sensors identify that the flow demand is greater than the heater’s capacity, this technology automatically changes the flow of water to meet the demand. Especially helpful in cold areas and other situations where the electrical service size does not permit the installation of a bigger model, yet it may be necessary to manage high flow needs on occasion.

All versions in the Tempra Plus Series are also equipped with a complete digital temperature setting display.

Either model series is suitable for ordinary residential applications and may be used interchangeably.

Electrical Service

The last factor to consider when choosing an electric tankless water heater is the electrical service available to your house or place of business. Single phase electricity is required for all Stiebel Eltron tankless water heaters. The voltage range is 208-240 volts. Due to the fact that a tankless water heater operates on demand, it must transmit a significant quantity of heating power to the water in a short period of time. As a result, tankless water heaters have a high peak electrical demand despite the fact that they will significantly cut your total power use.

Your electrical panel’s main breaker (the one that allows you to cut off electricity to the entire house) will reveal how many amps of electrical service your home is capable of providing.

Model RecommendedBreaker Maximum Amperage@ 240 V Maximum Amperage@ 208 V Recommended Household Electrical Service Recommended MinimumWire Gauge
Tempra 36 / 36 Plus 3 x 50 AMPS * 150 AMPS 132 AMPS 300 AMPS 3 x8 AWG*
Tempra 29 / 29 Plus 3 x 40 AMPS * 120 AMPS 105 AMPS 200 AMPS 3 x8 AWG*
Tempra 24 / 24 Plus 2 x 50 AMPS * 100 AMPS 88 AMPS 150 AMPS 2 x8 AWG*
Tempra 20 / 20 Plus 2 x 40 AMPS * 80 AMPS 70 AMPS 125 AMPS 2 x8 AWG*
Tempra 15 / 15 Plus 2 x 30 AMPS * 60 AMPS 52 AMPS 100 AMPS 2 x10 AWG*
Tempra 12 / 12 Plus 1 x 50 AMPS * 50 AMPS 44 AMPS 100 AMPS 1 x8 AWG*

In order to be used in most installations, the breaker size and wire gauge must comply with all relevant local, state, provincial, and national electrical codes in your region. These are merely guidelines that will apply to the majority of installations. Some codes necessitate the use of an electrical sub-panel for the installation of a heater, particularly when the heater is not located within direct line of sight of the primary electrical panel. A voltage loss of less than 3 percent under load should be maintained by properly sizing the wires.

The specs shown above are only suggestions.

It is recommended that you consult an electrician before placing an order.

Prior to making a purchase, it is suggested that you obtain an estimate from a professional electrician.

Remember that uninstalled items returned because of insufficient electrical service may be subject to a restocking fee if the unit was not installed properly. For additional information, please check our POLICIES page.

How to choose a Tankless Water Heater for your home, your needs, and your budget.

Congratulations on making the decision to purchase a tankless water heater for your house! Now, how do you pick the best one for your house and your budget? What are some tips? Petro is here to assist you in sorting through the options available to you in order to select the most appropriate tankless hot water heater for your needs. First and foremost, selecting a tankless water heater over other options for heating water for your house is possibly the most cost-effective approach to save money on your utility costs over the course of a calendar year.

It’s also crucial to note that tankless water heater installations are often more expensive than their tank-bound counterparts; however, the energy savings realized over time will more than offset the higher initial expenditures.

This document gives an outline of such concerns, and as usual, your Petro team of experts is available to assist you with any questions you may have.

Determine the maximum number of devices.

Choosing the right number of tankless water heaters for your house is one of the most critical factors to make when shopping for these appliances. This is calculated based on your typical hot water use, which is a function of the number of people that use hot water in your household. For example, if you have 10 people living in your house, you may require more than one unit. However, if you live alone or with only one other person, one hot water heater unit may be sufficient for your need. Petro’s highly skilled experts have years of expertise in the water heater sector under their belts.

Determine the required temperature rise.

Following the efficiency with which a tankless hot water heater warms water to the desired temperature, the next most essential consideration is the cost of the unit. This is referred to as a “temperature increase” in the business. It is necessary to know a little bit about your property in order to determine the appropriate temperature increase capabilities for your tankless water heater. The average temperature of the water that enters your home is maybe the most significant issue to consider.

Choosing the appropriate temperature rise factor for your tankless water heater is a calculation that is dependent on the temperature of the water that is being heated.

It is possible to determine your typical water temperature using a variety of online heat map tools; Petro can also easily aid you in finding your average water temperature.

Tankless water heaters that are less powerful will be required in warmer climes.

If you live in a colder region, you will need a unit with a larger temperature increase factor. Petro, one of our tankless water heater specialists, can assist you in determining which type of tankless water heater will work best for your home because they have vast knowledge in their service areas.

Determine the right size.

It might be difficult to choose the proper size of a tankless water heater for your needs. You must figure out how much hot water you are consuming, as well as the temperature rise necessary by your household appliances, before proceeding. This is due to the fact that it takes time to heat water. The unit’s size is proportional to the amount of temperature rise required. Also, if there are several persons utilizing the hot water, or if you want to have various appliances operating at the same time, a bigger unit is required.

Flow rates.

One of the most significant factors in deciding the appropriate size of a tankless water heater is understanding how much water and at what temperature particular appliances require hot water. Here’s a helpful chart that shows the average temperature and flow rates for the day (measured in gallons-per-minute, or GPM).

  • When the water is heated to 102 degrees, the typical flow rate of a bathtub is four gallons per minute. Shower Flow Rate -At 104 degrees, showers have an average flow rate of 2.5 to three gallons per minute
  • However, at lower temperatures, the flow rate is lower. Flow Rate of the Washing Machine – At an average temperature of 120 degrees, the flow rate of the washing machine is two gallons per minute on average. In general, dishwashers have a flow rate of 1.5 gallons per minute, and they operate at an average temperature of 110 degrees. In the kitchen, the average flow rate is 1.5 gallons per minute, with an average hot water temperature of 110 degrees
  • In the bathroom, the average flow rate is 1.5 gallons per minute

Select the power source – Natural gas, propane or electric.

Tankless water heaters may be powered by three distinct types of energy sources: natural gas, propane, and electricity, to name a few. If you are considering the use of any electric-powered item, you must first assess how it will affect the total electricity use of your home. Some older homes may have electrical systems that are less reliable. In this case, an electric tankless water heater for the entire house isn’t necessarily the greatest solution because it might overburden an older system.

As a result, they are most frequently employed as a whole-house solution for hot water requirements.

According to the United States Department of Energy, electric units save you around $40 per year in energy expenditures, but natural gas and liquid propane units save you approximately $100 per year.


When considering whether or not to install a tankless water heater in your house, maintenance is an important concern, just as it is with any high-usage device. Petro Home Services recommended that you get your tankless water heater serviced once a year to keep it in good working order. This assists you in achieving the higher life expectancy of these units as compared to tank-bound counterparts. Technicians with extensive experience will replace the air filter and check that the burner is in proper operating condition.

Once every 500 hours, you should use vinegar to remove mineral buildup, sometimes known as “scale,” from your unit.

The best way to maintain any water heater, whether it is a tankless one or not, is to have it serviced by a qualified professional.

When it comes to hot water heater troubleshooting advice or determining when it might be time to upgrade or replace your water heater, the professionals at Petro Home Services have the answers you need.

We’re here to help.

Petro House Services understands how to keep your home functioning smoothly all year long, and we can help you with that. Petro has a wealth of knowledge and expertise dealing exclusively with tankless water heaters, so you can be confident that your unit will operate efficiently for the duration of its expected life span. In addition, you will enjoy consistent hot water and energy savings for many years to come. Call Petro Home Services at 800.645.4328 now to learn how to select the best tankless water heater for your needs, keep your hot water running, and save money on your home’s energy costs.

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