What Size Tankless Water Heater Do I Need

What Size Tankless Water Heater Do I Need? [5 Factors Explained] ?

what size tankless water heater do I need

Technology is evolving every day and right now, we are almost set to leave out the general water heater systems behind and move on to the new tankless water heaters. They are ready to serve you for your on-demand water supply. Not only they are improved in technology and efficiency, but they will also start saving your money on both the electricity and water bills.

But getting the best Tankless Water Heater is the ideal thing to consider before installing them. Thinking of this, the first question that comes to our mind is what size tankless water heater do I need? Selecting the right Tankless Water Heater size for your home is very important if you are going to buy it for the first time.

Technically, the Tankless Water Heaters are quite different from the storage tank water heaters because they can store water to use it later. This is the reason why most of the people get confused in choosing the ideal Tankless Water Heater according to the size and the storage space. Most of the people think about choosing the total gallon capacity which may not be ideal for every home.

In fact, the Tankless Water Heater is most specific with their rate of flow per minute which is the reason why you need to concentrate highly on the size to avoid misuse of electricity as well as water. This guide on How to Size a Tankless Water Heater will surely help you to fulfill your requirements.

1. What Size Tankless Water Heater Do I Need for My family and their Needs?

While choosing any tankless heater, the first thing that you must consider is about how many people are actually staying in your home or even if you are using for a commercial bathroom, how many people are going to use this product. Apart from this, the next thing that you need to consider is the mode of operation.

Is the heater going to serve just for the bathroom or through the entire house?

Most of the modern Tankless Water Heaters are capable of delivering heating water to the entire household if the configuration is done perfectly. However, to get this advantage, you need to make sure that the Tankless Water Heater has a big size because the smaller sized ones will not have much potential to deliver warm water to the entire house.

residential water heater sizes

In the case of storage tank heaters, the mechanism of the heating system is easier. This is because it has a storage space and it is easier for daily usage. In fact, because of this automatic storage system, you can consider buying according to the water required.

But they do not store hot water. In fact, the storage water heaters heat the stored water. Because of this mechanism, they are incapable of delivering instant hot water to the household on demand. Thus, it makes sure that the same storage capacity will not be ideal to calculate the size of the tankless water heater.

2. Keep the Gallons per Minute in Mind

The next most important thing that you need to consider is the flow rates per minute. The capacity of a Water Heater is measured by the rate of flow of water and makes it ideal for your use. So, if you want to get the best results, the ideal way to do this will be to take the net flow rates of all the hot water faucets if you are planning on using it every time.

This will differ from home to home and from the different uses. So if you are planning to provide water to the entire house, you need to always consider a situation where two showers are running.

gpm guide

Considering the situation, the one thing that you need to do is, to sum up, the essentials that you use at the same time. So consider two showers and a hot water tap which will be running at your house all the time. Any regular shower has a flow rate of 1.5 gallons per minute. So this means that you need to have at least 5 gallons per minute flow rate in such a situation.

However, if you are just using the Tankless Water Heater for your bathroom, you can buy any water heater will a smaller capacity. Apart from this, let us come to considering the dishwasher while cleaning the vegetables. This will make the dishwasher to be very helpful with the continuous flow of water. And at the same time, it provides sufficient hot water instantly.

3. Appliance Flow Rates

A lot of things depend on the flow rates of any appliance in your home that one thing that matters the most. The typical flow rates mean that the Tankless Water Heater must have the capacity to make sure that all the flow rates are covered up in this. There are different flow rates for the different appliances that you need to consider.

Apart from this, you need to consider some situations where you may need to open two showers or two taps with hot water flowing through at the same time. So the Tankless Water Heater must have the capacity to make sure that it can deliver proper flow rates to the appliances. Here is a complete list that you can easily follow.

flow rate guide
  1. Bathroom Faucet: The flow rate is relatively high from 0.5 to 1.5 Gallons per minute because of the multiple appliances present.
  2. Kitchen Faucet: The flow of water is kitchen faucet is much higher. It is around 3 to 7 gallons per minute.
  3. Shower Faucet: The shower faucet comes with shower caps which reduces the rate of water wastage which makes sure that you can get the ideal water flow. The flow rate is around 1 to 2 Gallons per minute
  4. Dishwasher Flow Rate: The dishwasher flow rate is just average and it consumes around 2 to 2.5 Gallons per minute because it has storage technology, it will be ideal to choose a tankless water heater with such capacity.
  5. Clothes Washer: The clothes washer flow rate depends on the number of clothes that you are going to wash. However, it requires a minimum of 1.5 to 3 gallons per minute for a better result.

4. Temperature Rise

Apart from finding out the amount of water that you need, it is also very important to understand how much water you need to heat. Is it just because of the bathroom or a quick shower? Or are you planning to use the shower for a longer time? The temperature rise depends on a number of factors. And it is also an important factor that you need to consider.

The temperature rise means the difference between the temperatures of water the tankless heater adapts and the temperature of water it produces. This relativity depends on how warm the heater needs to be and what will be the ideal temperature for your use. To figure out the temperature you can give this article a read.

To consider the factors, let us get an example. Suppose the heater has a temperature of incoming water of 50 F and the temperature of the shower is around 105 F. this means that the temperature rise is around 55 F. So the water heater can always heat the water at a flow rate of 1.5 gallons per minute and each gallon by 55 degrees.

However, if you consider the Tankless Water Heaters, there are two different types present in the market today. Both of them are the electric Tankless Water Heater and the gas Tankless Water Heater.  Different places in the world have a different range of temperatures.

Thus, it becomes very difficult to pick the right size from available Tankless Water Heater sizes which also complements the right type of the Water Heater.

5. Using all at once

The last and probably the most important thing that you need to consider is the peak time usage. Any Tankless Water Heater is capable of individual works from the shower to the kitchen faucet.

But it is the peak hour performance that matters the most. You may have situations in your home when two people are having showers in two different bathrooms. This means that you must get a Tankless Water Heater that has the capacity to deliver a decent flow of rate while you are using multiple appliances at the same time. Suddenly, if your shower stops giving you hot water you can find out the reason from here.

By planning the maximum possible appliances running at the same time, you need to make sure that you do not get the best results.

Gas Versus Electric Tankless Heaters Capacity

Both the Gas and electric tankless water heaters come with different mechanisms which helps them to get the best results. In different zones, you may require different tankless water heaters. It means that you can get an ideal tankless water heater that will get you the best results.

Coming to the specifications, the gas tankless heater can handle more than 5 gallons per minute. And the electric heaters can handle around 2 gallons per minute. Though it depends on the amount of water that you need, the gas-based models have a better capacity.

Example Case

Let us just consider the peak hour running when everyone is busy either for a wedding or an occasion taking place at home. So we can consider a situation where many people are using the hot water heater at the same time and it is continuously running for water supply throughout the entire household.

So, here are the appliances currently running with it. We have shared a tankless water heater size chart to make it easy for you to understand.

Appliance Number Flow of Water
Showers 2 3 GPM
Bathroom Sink 2 1 – 3 GPM
Kitchen Sink 1 3 – 7 GPM

So, if we take this data, this means that nearly 13 Gallons per minute of water is already used in the home. So you may need to consider any large sizing tankless water heater for the best results. You can also go through this detailed guide on hot water heater sizing.

Frequently Asked Questions

Q1. What Size Tankless Water Heater Do I Need To Replace A 50 Gallon Water Heater?

A1. Finding the right size of your Tankless water heater must be the first thing that you need to consider. If you have a previous 50 Gallon Water heater, you need to consider buying something big. You can follow the steps mentioned below to calculate the minimum capacity for the water heater that you need. 

1. Calculate the flow rate at first. Start with the peak hours when you may need more than one hot water faucets in the kitchen and the bathroom together. So the first thing that you need to do is, to sum up, the water flow. Considering a Kitchen faucet and shower, it is just around 4.7 gallons per minute. 
2. Now you need to know the flow rate. It is around 60/seconds to fill the bucket. Once it is done, know the temperature rise that you require.
So if you are buying a Tankless water heater with 4.7 gallons per minute flow rate, it will take about 12 minutes to complete 50 gallons. So ideally you need to purchase a Tankless water heater which has a high flow rate.

Q2. Do Tankless Water Heaters work at High Altitude?

A2. Most of the gas models may not work as much effective as you need in case of a very high altitude. Even if they do, it requires a lot of time to get the water heated up. Coming to the specifications, any altitude over 4000 feet high and the heater starts losing 4% of the efficiency for every 1000 feet.

Q3. Do Tankless Water Heaters run out of hot water?

A3. In some of the cases, there are some disadvantages of owning a small tankless heater if you are using it for regular reasons. If the flow capacity is a bit short, you may face issues of losing out of hot water after the use. This happens mostly in cases if the flow rate is not sufficient and you are using multiple appliances with the product.

Q4. How long do Tankless Water Heaters last?

A4. Most of the tankless water heaters have a high long life of nearly 20 years. Since there is no storage feature, you will not have to worry at all about the iron forming and also the filtration. The best part is that tankless water heaters come with replaceable parts that increase life expectancy.

Q5. What Size Tankless Water Heater Do I Need For A Family Of 5?

A5. For a family of 5, the average water consumption would be around 5.2 gallons of water per minute. So keeping the peak hours at mind, one mind the first thing that you need to consider is the flow rate and the temperature rise.
After getting an accurate temperature rise, you need to consider the flow rate along with the number of faucets that you have. If your house is connected with more hot water faucets, you need a tankless water heater with more size.

Q6. Do Tankless Water Heaters need to be Serviced?

A6. Every good thing in your home requires a service or maintenance to keep. Though it is true that the tankless water heaters do not require much maintenance because of the mechanism, you may have to keep maintaining both the heating and the cooling systems. In case of a gas water heater, check for the gas components regularly.

Q7. How to do AO smith water heater sizing?

A7. To do the perfect sizing, you need to ensure how many people live in your home. The sizing should be based on a few factors like the size of your house, the faucets that you will use the members, the rate of water flow, and the type of ignition.
According to this, you can choose the right size. You can collect all the data and then keep the purpose of your use. Based on all these factors, choosing the right size of the AO smith model will be perfect.

Q8. Which water heater size for a family of 4?

A8. Any family of 3-4 people can be considered as a medium family. But choosing the right size all depends on where you are going to use the heater. If you just consider using the heater for one shower faucet and one for washing, you can consider 25 gallons for every couple.
So if there are around 4 people, consider buying a 50 gallons heater if it is electrically-based. If it is gas-based, consider buying a bigger model to avoid water loss.


Tankless Water Heaters have come up with the most ideal advantages for a person to enjoy instant hot water. The best part is that it does not take much time for the people to enjoy the hot water because of the fast mechanism. But the mechanism is different from the regular storage tank water heaters.

Thus, selecting the ideal size is very important. So try to choose an ideal Tankless Water Heater for your requirements according to the factors mentioned above. We also hope that we answered your question what size tankless water heater do I need at it’s best. Still, if you have any doubts and queries mention it in comments. Happy Shopping!

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