What Size Tankless Water Heater Do I Need


What Size Tankless Water Heater is Best_plumbing cypress texas


Before purchasing an energy-efficient on-demand water heater you might get puzzled and ask yourself ‘What size tankless water heater do I need’? If you take a very small tankless water heater, then you might have cold water in a shower when someone is using hot water. Because the small size tankless water heater is unable to supply water as you need.

More Reasons to Select Proper Sizing

This is one side of the coin. On the other hand, if you purchase a big tankless water heater, then you will have to spend a lot of extra money but you might not use that full capacity at all. It means you have to pay without using the tankless water heater.

At the initial stage, you have to pay an extra amount of money for its price. Not only that you also have to pay an extra dime for maintenance and other plumbing costs.

Hope, now you realize the importance of selecting the right size of tankless water heater for your home. A question might jump up in your mind about how you know about the right size of the tankless water heater you need. To get the answer just keep reading. The answer lies in the rest of the article.

To select a tankless water heater, it’s required to consider temperature rise and flow rate for your whole house or a particular appliance like a bathroom. Here we will explain in 5 easy steps how you calculate the right size of tankless water heater. By doing this you can avoid spending extra money or having a cold shower.

Let’s dive in the deep:

Step-1: Determine Overall Flow Rate (GPM)

determine the flow rate plumbing cypress texas

On an average tankless water heater can produce 2-5 gallons of hot water per minute. According to energy.gov, tankless water heaters can produce higher flow rates than tankless water run by electricity. For a traditional storage water heater, it’s easy to determine the number of family members. But this process is not possible to maintain for tankless water heaters as you know there is no tank at all.

But we need to calculate this. The question is “how”?

To determine this you need to know about some fixtures including the tankless water heaters demand. Here you require fixtures. No matter how much people use the showerhead, kitchen, or other fixtures.

Why need fixtures, not number of users?

The answer to the question is you have an unlimited source of hot water. However, if a number of people use showers or other fixtures at a time, then tankless water heaters may not be able to meet up with the demand. For maximum demand, you have to calculate how many gallons of hot water you require at a time.

You may think calculating this is too professional. No, it’ not as you assume. It’s really so used to calculate:

One Method:

First, What you need to do is prepare a list and select the highest demand fixtures you want to run simultaneously.

Secondly, you have to find out those fixtures’ flow rates. How much hot water they use per minute. Then to find the total use of hot water accumulates the numbers you already got.


If you don’t know the actual flow rate of your fixtures, then you can use this table to get the average flow rate for most of the fixtures

FixtureGallons Per Minute (GPM)
Lavatory faucet0.5-1 GPM
Kitchen faucet1.5-2.2 GPM
Showerhead2-2.5 GPM
Tub faucets4-7 GPM
Washing machine2 GPM
Dishwasher1.5 GPM

Another Method:

If you aren’t comfortable with the above method, then you may apply a scientific method by yourself. Don’t worry about this ‘scientific’ method. You have not been a scientist to apply the method.

It’s simple. Take a one-gallon bucket. Now turn on the fixtures (faucet, showerhead, or bathroom tab) to determine how much time required to fill with water. Then apply the formula stated below:

bucket plumbing cypress texas

Flow rate = 60/ how many seconds require to fill the bucket.

Note: If you don’t want to waste a few gallons of water, then you may fill one-fourth of the bucket. Now apply the formula below:

Flow rate = 15 / how many seconds require to fill the bucket.

Example: Now we are going to present an example. By seeing that you can determine the total required flow rate.

Let’s say, you have two bathrooms. And you want to use both of them simultaneously. Besides, you also want to use a bathroom sink and one washing machine also. If you take the above table as a reference, then we can complete this equation.

GPM rate plumbing cypress texas

(2 showerheads * 2.5 GPM) + 1.5 GPM for a kitchen sink + 2 GPM for a washing machine = 8.5 GPM for the whole house.

From the above equation, we are seeing that if you want to use all fixtures, then your tankless water heater should be able to supply more than 8 GPM hot water.


Remember you have to count those fixtures and appliances only which you want to run altogether. It’s not mean that you count them as only the reason you own them.

Over and Underestimate

To stay in a safe position, we took the probable maximum flow rate from the chart. But this rarely happens in real life as we believe overestimation is better than underestimation.

At this point, you may take a decision to purchase a whole house tankless water heater. Besides, you may purchase a smaller one or extra smaller one for the bathroom, shower head, and other smaller units.

The decision depends on one factor that is the length of the pipe. Your fixtures should be as close to the heater as possible. Otherwise, you have to wait a bit longer for a hot water supply. A small point of water use of a demand type heater may solve the problem of the temperature of the water.

You have to take measures to decrease the demand for hot water. If you use a low flow rate showerhead, then your demand for hot water will be decreased. In that case, you wouldn’t need to get a big and expensive tankless water heater.

Nowadays technology may help you to do that. The Latest model lower flow rate showerhead has the same water pressure and gives you the same high-pressure experience.

The downside of Low Showerhead

The lower flow rate of showerheads has a con. There is a link between the flow rate and the activation of tankless water heaters. If the flow rate would be too slow, then the tankless water heater may not be activated.

They have different activation points based on the flow rate. So, you should select one carefully.

Step 2: Determine The Temperature of Your Location

determine the temparature rate plumbing cypress texas

At this point, you have determined the total demand for hot water. Now you have to decide how much hot water you want. You also need to settle the rising temperature level. Then you can calculate the temperature you require rising. It’s very important to determine the actual flow rate of a specific tankless water heater because the flow rate depends on the rise of temperature.

Some state’s temperatures might rise as high as 75°F, on the other hand, some might have less than 35°F. This variation has an effect on getting hot water in specific GPM. In Boston You can’t get 11 GPM with a specific tankless water heater in winter. But, you must get that in Miami by using that. The flow rate creates a difference.

If you live in a warm climate then, your water temperatures might be higher. Most of the uses you want your water heater between 105-115 °F

Where to Find the Flow rate

You can find the highest flow rate from the label attached to the body of a tankless water heater. This will help you to make perfect decisions about a model.

The cited numbers are true for warmer climates with a lower increase in temperature. And we know the rise in temperature depends on climate. If you live in the Northern states of the USA you will get fewer gallons of water than usual.

USA temparature map plumbing cypress texas

Groundwater temperature varies from state to state in the USA. Some states have lower temperature groundwater, some have high on the other hand some have a medium temperature. You need to subtract the groundwater temperature from the desired hot temperature water outgoing.

For example, if you live somewhere in Kansas City. Your average incoming cold water temperature is almost 52 °F. Then if you want to get 110 F hot from your shower head then you have to heat water at 58 °F. It means for your area you have to increase the temperature by 58 °F.

At this point, you are ready to go to the next point because you have successfully calculated the temperature rise in your area.

Temperature Rise Needed = Desired hot temperature water – Incoming Groundwater Temperature

Step 3: Consider The Pick Time Load

consider the pick time load plumbing cyconsider the pick time load plumbing cypress texaspress texas

This step would be a part of step 1 as it talks about the flow rate. If you didn’t calculate the flow rate till now then please, go back to step 2 and calculate the flow rate.

A question may arise which point are we going to make here? The answer is when you will try to calculate the flow rate and select the tankless water heater, consider your pick load, not average demand. It’s very important. I think I can clarify this.

You mustn’t like to have a cold shower at a busy hour as your tankless water heater didn’t supply enough hot water. You should be prepared for the worst. To stay on the safe side you should count on a bit more than the average demand for hot water. This overestimation would not hurt you much.

Step-4: Accumulate All together

accumulate all together plumbing cypress texas

Now you have two crucial pieces of information. One is your demand for hot water and two is how much the temperature rises. Now it’s time to select a perfect tankless water heater as per your requirement. Let’s assume that you are from South Carolina. You want to have a tankless water heater that would be able to provide hot water for using a shower, a washing machine, and a kitchen sink.

How big a tankless water heater do I need?

The answer is very easy. You need tankless water which can increase the temperature of 50 °F for 6 gallons of water per minute.

Remember, it’s better to purchase an oversized tankless water heater than a smaller one which would be unable to supply hot water as per your demand. To find out the real flow rate, you can check the specification of a tankless water heater.

Besides this, feel free to leave your thoughts in our comment box. Our expert professional plumbers will help you to select a tankless water heater.

Step-5: Determine Your Power Source

determine your power source austin's best plumbing

Now, you have to consider the source of your power to run the Water heater. Gas or electric run tankless water heater Which one is best?

There is no easy answer to this question. It depends on your circumstance and choice. Tankless water heater electric is suitable for appliances and homes needed less amount of hot water. In places where the temperature of warmer incoming water, as well as GPM, is less, tankless water heaters would be a perfect choice.

On the other hand, if you live in a place where groundwater temperature is Low with high hot water demand, then tankless water heater gas will be a great choice.

First, you have to consider the availability of the energy source you want to use for tankless water heaters.

Do you have the access to gas or electricity?

This is an important criterion for selecting tankless water heaters. To select the power source for a tankless water heater, you have to keep in mind some other reasons. Most of the area for long term use gas is more cost-effective.

Let’s assume, availability and the price of gas and electricity are the same in your area then you have to consider the features of tankless water heaters. Tankless water heater gas can heat water quickly than an electric one. This model is less expensive for the price of gas also, though it may change as per the location you live.

On the other hand installation of tankless water heaters is more cost-effective and easy as well. Because they don’t need to power piping and venting as well. Easy installation cut the labor cost also so the primary cost would be less compatible.

Think twice if you select a tankless water heater electric though your city has the full electricity service. If you don’t have a backup battery attached to the tankless water heater, then you may lose hot water in case of disruption of electricity. Now, we are accustomed to the important terminology and the reasons behind selecting a tankless water are clear.

Sizing Guide For Tankless Water Heaters

Please don’t forget it’s an estimation only. Changing a tankless water heater is surely more than a table. To select the tankless water heater perfectly you have to find your hot water demand and rise of temperature.

Tankless water heater sizing calculator

If you have a simple calculator then, you can determine the flow rate and temperature rise. Now you can learn how to determine the GPM of a tankless water heater. Your task is to select the total number of fixtures or appliances you want to run at the same time. We will calculate the total GPM as per the average number.


What Size Tankless Water Heater to Replace a 50 Gallon Water Heater?

Suppose you want to replace a 50 gallons water heater. Then the question came forward: what size tankless water heater do I need a 50-gallon water heater? The answer is you need a good equivalent tankless water heater. Before purchasing a tankless water heater, you have to learn something:

You can’t determine the size of a tankless water heater considering your existing storage water heater. These two are totally different from each other. As a tankless water heater has no storage system, existing storage capacity doesn’t have any role to select a tankless water heater.

Tankless water heaters don’t work like storage water heaters. As tankless water heaters supply hot water continuously, so how big your previous storage heater doest matter. If you want to upgrade or replace your heater from 40 to 50 gallons, then you don’t need a big tankless water heater.

The right size of a tankless water heater is one that can heat water at a specific temperature to provide hot water. Here, the considerable things are flow rate and temperature rise issues. Don’t think it’s a complicated or confusing way.

Let’s try to explain one more time:

We know that the sizing of a tankless water heater totally depends on the flow rate and temperature rise.

Count all the fixtures you want those to supply hot water simultaneously. It’s easy to count all the appliances or fixtures to supply hot water. At this point, you have to remember that, how many bathrooms you own doesn’t matter. Crucial things are how many of them you want to provide hot water at a time. Accumulate the flow rates

So, what’s the flow rate?

Flow rate is the amount of water those flows throughout the fixtures per minute. If you don’t have any idea, then carry on an experiment. Take a one-gallon bucket. Measure how fast a faucet, bathroom sink, or a showerhead can fill one-fourth of that bucket.

Now it’s time to try the formula.

Flow rate = 15 / how many seconds require filling one-fourth of a bucket

Determine the temperature rise of your area

To determine temperature rise you have to subtract groundwater temperature from the temperature you want.

Select the right model which meets the requirements

We must agree that the task is not so easy. There are so many trustworthy as well as renowned brands available in the market. All these may make you confused.

At this point, we can help you to make the decision. You have to sort out the available brands and models as per your requirement. Then a huge number of brands or models come to a number. This time you can pick one which meets your criteria like a power source and budget of course.

Let’s have a sizing example

Say for example you are living in Cypress, Texas. You are planning to change your storage heater model to a tankless water heater. You want to run two showers and a washing machine at a time. So, to fulfill the above requirement we have to do simple math.

Let’s do some math.

As per the table presented earlier in this article, we know the showerhead flow rate is on an average of 2-2.5 GPM (gallon per minute), so your two shower heads have a total 4-5 GPM. And a washing machine consumes almost 2 GPM on average. That means your total demand for hot water is 6-7 GPM.

Your area’s groundwater temperature is between 60 and 67 °F. That means your upcoming tankless water heater will have to raise water temperature about 110 F – 60F = 50 °F.

Note: We assume the desired temperature of hot water is 110 F.

Now, you know your requirement to select the model. You can go through a tankless water heater list provided by manufacturers. As per requirements, you can select one which would be able to supply 6 – 7.5 gallons of water per minute with 50 °F rise in temperature.

What size tankless water heater do I need to replace an 80-gallon water heater?

If you have a big family. Then you may have a question like ‘What size tankless water heater do I need to replace an 80-gallon water heater?’ The process to select the right tankless water heater is the same as we said before. You have to determine two things like firstly GPM and secondly temperature rise.

As you have a big house that’s why you want to get a whole family tankless water heater. That type of tankless water heater is more powerful than others in the market. And those would be able to supply hot water into 3 big size bathrooms and a washing machine at a time.

Now it’s time to think about the budget. A whole family tankless water heater is a bit expensive. There are models available run by natural gas or propane at cheaper prices. Those are powerful like others. But the problem comes from the cool weather. They work worse in freezing weather.

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

We have already known that it doesn’t matter how many members you have in the family in case of a tankless water heater. Instead, the crucial part of a tankless model is how many people use hot water at the same time.

Let’s Know About This in Detail. We know the difference between a tank and a tankless water heater.  

First, you have to understand the difference between storage and tankless water heaters. In terms of a tank water heater, your shower and use of hot water are limited as per the availability of the tank’s hot water reserves.

Due to store limited gallons of hot water you may have experienced running out of hot water.

On the other hand, tankless water heaters supply unlimited hot water. As it doesn’t have any storage tank to reserve hot water, so there is no limit for supplying hot water.

When hot water reaches into your faucet or shower head you may be ensured that there would not be any shortage of hot water. For hot water, you and your family wouldn’t have to be worried.

Instant hot water models heat water at the time of flowing from the ground source to the user point. So, it can be said that until you cut off the power and groundwater supply, you need not be anxious about the hot water supply.

Limitations of Tankless Water Heater

Superb performer tankless water heaters also have limitations.

Then, what’s that?

Instant hot water heater models fully depend on the ability to heat water at the time of supplying. In other words, it depends on BTU (British Thermal Unit).

But, it is not true that two different models with the same BTU perform the same. One of them might be more skilled than the other.

In this situation, a user might be puzzled. What should he do? It’s convenient for a user to find out the flow rate at the temperature rise. It means how much hot water can supply a tankless water heater per minute.

What Does Temperature Rise Mean?

Temperature rise means the difference between the temperature of output water and groundwater temperature. Don’t think it’s a complicated issue. Let’s make the whole thing easy.

How Big of A Tankless Water Heater Do I Need?

We have already covered most of the important topics. You may read the whole article to know more. Here we will explain everything briefly. To select a tankless water heater for a family with 5 members you have to determine the pick flow rate.

Then you have to select a tankless water heater model that can supply and demand hot water with temperature rise. Now I will show this with an example:

Let’s say you live with your big family in Texas or California. You may want to run two or three showers simultaneously. Besides this, you want some extra capacity for certain situations. For that, you want to use a washing machine and kitchen sink.

Each showerhead has an average flow rate of 2 GPM and every washing machine or sinks also has the same rate of water flow.

So, Total flow rate is = (2-3 shower head *2 GPM) + 2 GPM for one washing machine or one sink = 6-8 GPM

The groundwater temperature of your area would be between 57 °F to 67 °F. So, it’s required to raise the temperature 110-57 / 67 = 53 °F to 43 °F.

Sizing of Tankless Water Heater


Now come to the focus point of sizing a tankless water heater. As per calculation, a family with 5 members would need A minimum of 6 GPM tankless water heaters.

What’s our suggestion?

Here we give some suggestions for the best tankless water heater. We think the best option will be:

Rinnai RUC80iN Ultra Series Natural Gas Tankless Water Heater:

Tankless Water Heater Electric

You have other options besides a tankless water heater run by gas. You may try tankless water heater electric. We recommend Ecosmart ECO 36 36kw 240V Electric Tankless Water Heater:

Think again

At the time of selecting a tankless water heater electric, you have to remember some points. First, this model would be more expensive.

tankless water heater gas save 10% energy austin's best plumbing

Secondly, A tankless water heater run by gas uses 10 percent less energy than an electric one. That means huge savings every year.

Thirdly, for installing a tankless water heater electric, you have to upgrade your home’s electric service line. Besides, this model is more costly than tankless water heater gas. If you have available fuel options of gas or propane, then you should go for a tankless water heater by gas.

What Size Tankless Water Heater Do I Need For A 2, 3, 4 Bathroom House?

If you want to understand the sizing of tankless water heaters clearly, then you should keep going.

Read the specifications

You have learned how to calculate the demand Which requires selecting the best tankless water heater model. And to select a perfect tankless water heater model you need to go through the specifications of tankless water heaters.

First, you should know the Which are the most important specifications of tankless water heaters. Not only that you should know which things need to be considered at the time of selecting a tankless water heater.

The two most important things are the temperature rise and flow rate.

The more tankless water heater has the flow rate, the more it can be able to supply hot water.

It’s time to find your preferred flow rate. To know that you have to determine the peak load of your house and family. After doing so, select a tankless as per that.

We have already covered the sizing of tankless water heaters. You may go through that.

You have to accumulate the flow rate of all the fixtures you want to run simultaneously. Now you will find pick demand by adding all fixtures flow rate.

Most probably you don’t have any idea how to do that. If so, you may use the calculator below. Write down the number of fixtures and then you will get the total flow rate. If you still have any confusion, feel free to ask me the question below in the comments section.

Now you know your demand. Let’s determine the temperature rise. Temperature rise means the temperature difference between outlets and the groundwater.

Just subtract the groundwater temperature from your preferred output water temperature. You will have the temperature rise.

Normally shower has 110-115° F temperature. You can use the map we present earlier to get your area’s temperature. Alternatively, you can examine the number stated below.

Nice, you know 2 most important things for selecting the best tankless water heater fie 2 (3 or 4) bathroom. Now follow our main rating and find a model which one would be the perfect choice for your demand. Remember the flow rate you find on the table is the highest load to operate a tankless water heater.

Say for example you have a home with a bathroom. You want to run a shower and another appliance simultaneously. For a one-bedroom home, you require a tankless water heater that has the ability to heat a minimum 3 gallons of water. We suggest Rennie or ecosmart tankless water heaters for you.


Best Tankless Water Heater For 2 Bathroom House

Now we assume you have a home with 2 bathrooms. Again your tankless water heater would be according to your requirement. In this case, you may not like our recommendation.

To give a starting point I will suggest to you our choice model. If you have 2 bathrooms or (2 bedroom houses) and you want to run a shower at a time. Under this requirement you have a tankless water heater that would be able to supply 5 gallons of hot water per minute. I recommend Rennie v 65 INK:

Rinnai V65iN Indoor Gas Tankless Water Heater

The price of Rinnai V65 IN model is astonishing! Its capacity is 6.5 GPM. It’s a perfect tankless water heater for a family with 2 bathrooms. This model is suitable for a moderate size home in an area with a warm or moderate climate.

But if you live in the Northern area, I will recommend a more strong model of the tankless water heater. If you don’t have gas as an alternative power source, then you need an electric model tankless water heater.

Note: I always like gas or propane than electricity.

Best Tankless Water Heater for 3 Bathrooms

Let’s assume you have a home with 3 bathrooms (3-bedroom house). There you want to use at least 2 showers and washing machines or dishwashers simultaneously. Remember a washing machine or dishwasher may require 2 gallons of hot water per minute.

So, for this level of demand you really need something powerful. Your demand would fit with a tankless water heater able to supply 7 to 9 GPM.


To fulfill your demand we recommend super performer Rennie RL 94in. This model’s price is competitively low with the highest flow rate about 9.4 GPM. A home tankless water heater powered by gas is the best choice for 3 showers and other home appliances.

Be careful

The Highest flow rate makes you confused. Don’t be fooled. Remember this is the highest score in a warm climate. In real life, it will provide you with a 7 to 9 GPM flow rate that you need according to temperature rise.

This is not the best model also. Then, why do we recommend this ?

Because, this model is probably the best model that can meet your demand in terms of price.

Best Tankless Water Heater for 4 Bathrooms

If you live in a big house, then it would be not easy to select a tankless water heater. Especially if you live in the Northern area of the United States. Over there the climate is adverse for producing hot water. For instance, the weather is cool and the groundwater temperature is low.

For these reasons, you have to get a high-performance tankless water heater which would be able to carry the load of 4 bathrooms.

We think, to handle such a situation RUR199iN would be the best choice. It’s so efficient as well as one of the powerful tankless water heaters. Though it’s not cheap.

Check a Few Times

If you find your tankless water heater which would be able to meet your demand, check at least 2 or 3 times. Otherwise, you would spend a few thousand dollars for Lukewarm water, Which must not be expected of you. So, be careful to get a tankless water heater unit to supply enough hot water you want.

If you are a resident of the South area of the USA and you don’t have any plan to use 4 bathrooms (still you have the option to use all 4 bathrooms at a time), then Rennie 199iN is a perfect choice for you.

Rinnai RUR199iN Tankless Water Heater for 4 Bathroom House

Hot Tip To Get Better Performance

Now I am giving you tips for yourself to improve things in your plumbing pipes.

Insulate your plumbing pipes. Insulated pipes keep hot water warm at the end of the output line. It would be very helpful when your faucet, shower, or sink situated a bit far from the tankless water heater because water has to travel a long way to reach the output. At the time of flowing that long way hot water loses temperature and becomes cool.

People Also Asked.

Is a 50 gallon hot water heater enough for a family of 4?

Answer: The answer to the question is yes and no as well. To understand this, Just notice at the below table.


MemberRequire water (gallons)
1-2. 30-40
2-3. 40-50
3-4. 50-60
5+. 60-80
So, as per above chart we can say if you limit the hot water use upto 50 gallons,then the answer is yes. On the other hand, if your family uses hot water more than 50 gallons, then you have got a 50+ gallon hot Water heater.

Can I replace my hot water heater with a tankless?

Answer: yes, why not? You can replace a tankless water heater with a conventional storage tank heater. This model is not only cost-efficient but also ensures a hot shower always. You can replace the tank heater by an expert plumber.

How much water is used in a 5-minute shower?

Answer: 10 to 25 gallons.

According to the U.S Environmental Protection Agency (EPA) a full-size bathtub requires about 70 gallons of water. As per that calculation, a shower uses 10 to 25 gallons of water.

How much water is used in a 20-minute shower?

Answer: It depends on the flow rate of the water heater. A low flow rate shower use 2 gallons of water per minute. So, in a 20-minute shower, it requires 40 gallons of water.

On the other hand, a standard showerhead uses 2.5 gallons of water. So, in 20 minutes it requires 50 gallons of water.

How many showers can you get out of a 50 gallon water heater?

Answer: A 50 gallons hot water heater is the most common water we install. This type of model is perfect for 3-5 people.

Your 5 people family can take shower by using 17.2 gallon hot water within an hour.

What uses more water, a bath, or a shower?

Answer: Normally a shower takes less water than a bath. A standard showerhead flows 2.5 gallons of water per minute. That means your shower uses 25 gallons of water in 10 minutes.

On the other hand, a full bath takes up to 70 gallons of water. That means the difference is 45 gallons! It’s huge, isn’t it?

Can a tankless water heater fill a tub?

Answer: Obviously, a tankless water heater can fill a tab. The question is how fast? It depends on the size of the tankless water heater.

A small tankless water heater may take a long time to fill the tub.


According to the energy department of the USA, 18 percent of home energy use is for heating water. And smaller than required tankless water heaters may not supply the hot water you need, on the other hand too big tankless water heaters would be a source of wasting money. So, properly selecting a proper size tankless water heater is a must. By following the steps and examples stated above, hopefully, you have gotten the answer of what size tankless water heater do I need.

If you want to know more about this, please call Austin’s best plumbing company Plumberson.

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