what size tankless water heater do i need

what size tankless water heater do I need to Know Tankless water heater pros and cons

Are you thinking about buying a tankless water heater? But confused about which size of water heater you should buy? 

Well,you are in the right place then. Because here, we are going to talk about the things you should consider when buying a tankless water heater and how to find the perfect size heater for you. So, let’s get started-

Before we get started with the consideration and size of the water heater, let us tell you first what a tankless water heater is?

As you can see the tankless name itself tells that the water heater is without any tank. Means tankless water heater does not store the water in a tank because it has not tanked.It heats the water as it flows through it.

So, as it does not store the water and heat as flows it works by the provided limitation of how much they will heat at the given time.

With tank water heater can run as many fixtures as you want and after a time the stored hot water in the tank runs out. But with a tankless water heater, the hot water never runs out, but if you use some fixtures, then it cannot keep up.

If you buy a too-small size one you won't be able to cover up all your household needs because of its small capacity system. You will run out of hot water soon. But If you buy a too big one, then it will be a wastage of your money. To prevent either one from happening, you need to find out the proper size for you before you purchase one.

How to know the perfect water heater size for you?

To know which tankless water heater is perfect for your house, you need to follow a few steps to find it out-

Step 1

To choose the right tankless water heater for you, first you need to figure out how many fixtures you are going to run at the same time.

When you figure out the number of fixtures you will need to run at the same time; then you have to calculate the flow rate. The flow rate is how many fixtures you will run and the amount of water will run from them.

Example: If you run one shower at a time then your flow rate will be-

One shower = 1.6 GPM (gallon per minute).

But if you want to use two showers and one kitchen sink at the same time, then your flow rate will be

Two shower = 3.2 GPM (1.6*2)

One kitchen sink = 1.3 GPM

A total of 4.5 GPM will be your flow rate.

This is the desired flow rate you want to run through your water heater and your water heater will provide you unlimited hot water depending on this flow rate.

If you want to reduce your flow rate, then you need to install water fixtures with low flow.

Step 2

Now that you know how much water you need for your household, you have to find out how much water you need to heat and how warm you want it to be.

To do that, you have to determine the temperature of the groundwater of your place, where the heater will be installed. This water will go into the water heater and turn it into your desired temperature.

This factor is a very important one to consider while choosing your tankless water heater. Temperature rise is the difference between the temperature of your groundwater and the water you want from your heater.

You need to subtract your ground water temperature from theheated water temperature, to get your desired water temperature.

Example: If your groundwater temperature is 50°F and you want water from your heater around 105°F or 110°F heated,

Then you need your water temperature to be-

105°F- 50°F = 55°F hot water you want from your heater.

You need to buy a water heater that provides 55°F hot water.So you will need a water heater that can heat water at 55° degrees at the flow of 4.5 gallons per minute.

Now check the sizing chart to see the model you are considering to buy can supply this flow rate of water at this temperature with the groundwater of where you are.

Step 3

After calculating the amount of water, you will need to heat and the required temperature, the next thing needs to consider before buying a tankless water heater is that if it has the capacity to run your desired flow rate at the same time. 

If you are running more one shower or maybe some other fixtures of the house at the same time, then you need to make sure that your heater has the capacity to run this much of flow altogether.

Example, the smallest models of a tankless water heater provides hot water for only handwashing. This means it will only run into one sink fixture machine with a smaller amount of flow.

A medium model water heater will provide endless hot water for a single fixture only.

But a larger model tankless water heater will cover your entire house with endless hot water for as many fixtures as you want.

The more fixtures you will need to run and the colder temperature you live in the more powerful tankless water heater you will need.

Tankless water heater or Tank water heater?

There’s always been a chaos of which water heater is best to use, Tank water heater or tankless water heaters. If you are also confusing between these two, then, let's check it out yourself which water heater has the best features and is best to use.

Tank water heater

Tank water heaters are the traditional water heater that stores the hot water in a tank. Basically these water heaters come with a huge tank which holds 50 to 60 gallon of water. So, you heat the water you need and store it in the tank and use it simultaneously.

Tank water heaters have less energy efficiency then tankless water heaters. But they are less expensive and you can fit it in your basement. Tank water heaters stores heated water but water might lose the heat for the environment around it. Tank water heaters are easy to maintain, its low-cost results easy to replace and repair.

It lasts 10 to 15 years but that's enough as compared with the price. But it costs a higher utility bill. They take a lot of space as they come with a huge tank. If you have a small house, then it will be difficult for you to place it. It cannot be placed outside of the house as tankless water heaters can.

You can turn on as many fixtures as you want and hot waters will flow until the tank runs out. Using multiple fixtures at the same time with tank water heaters may increase utility bills. You will always have hot water storage available but you won't be happy with the bills you will get because of it. And you might need to repair it often which will cost extra you might not want to spend.

Tankless water heater

A tankless water heater often called an on-demand water heater. It heats the water as it flows through it. It does not have any water storing system.

Using multiple fixtures at the same time may result in the heater running out of hot water. But if you choose the right water heater thinking about the many fixtures you want run at the same time then the hot water from a tankless water heater will be endless.

A tankless water heater is not huge in size, so it takes very little space to place it. You can place it outside of your house. It doesn't need vending or doesn't need a gas line coming into the house.

As the water heats as it flows, so there's no possibility of cooling off the water and no loss of energy. It saves energy from 10% to 30%. The electric tankless water heaters are 99 – 100% efficient.

These water heaters are a lot costly. It will take 2,000 to 4,500 from buying to installing a tankless water heater. But these water heaters will last 20 to 25 years and won't take a lot of repairs.

You don't have to wait for your water to become hot to use with the tankless water heater. You will have hot water whenever you need it. As it only heats water a few gallons per minute, it doesn't take a lot of time to do it as tank water heaters take because it has to heat a whole tank of water to make it warm enough use.

Tankless water heater pros and cons

Now that we know a lot of things about tankless water heaters let's check out the pros and cons to make sure if you really want to buy it or not-


  • Tankless water heaters save more energy than losing it.
  • It has a lot of efficiencies.
  • It lasts more than 20 years.
  • Small in size so doesn't take a lot of space in your house. You can set it anywhere. You can even place it outside of your house if space challenging.
  • Doesn’t need vending or gas lines all over the house.
  • You can use as much hot water as you want without even waiting for it to become hot.
  • Saves paying money for overtime use.
  • Saves a lot utility bill.


  • It costs a lot more than a tank water heater.
  • Installation and replacement can be really costly.
  • If you use more water than the required flow rate, then it may have a problem keeping up with you, and you might experience some running cold water shower.


Water heaters are the mandatory product for regular water use in cold areas. And it is also a hard decision to choose the right water heater for your needs. But we hope you get some information about it from this article to be confident at buying the right water heater for you.

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