4 (Real Ways) On How to Size a Tankless Water Heater

size a tankless water heater

Tankless water heaters are designed to provide a limitless supply of hot water. This new technology allows the homeowner to use hot water anytime they want conveniently.

If we are planning to invest in a tankless water heater, it is vital that we know how to size it up. In this article, we will learn how to size a tankless water heater like a pro and ensure that we get the right size for our household.

Types of Tankless Water Heater

There are two types of tankless water heaters in the market, namely:

  • Gas-fired tankless water heater- this type of water heater can heat water twice as fast as an electric water heater. It is efficient, but it requires more maintenance. A gas-fired water heater is ideal for busy professionals who don’t have much time to sit around and wait for the hot water to come back when we run out of supply. Gas-fired water heaters are ideal in accommodating a big household.
  • Electric tankless water heater- this type of tankless water heater is easy to install. However, it is not suitable for large household as it cannot heat large amounts of water. The initial cost for both plumbing and installation of an electric tankless water heater is lower. This is an ideal choice if we are living alone in an apartment.

Tips in Sizing a Tankless Water Heater

a small water heater

Here are the following tips to keep in mind when we are sizing a tankless water heater.

  1. Determine the area of the mounting space.

Compared to a storage tank, a tankless water heater is smaller and compact. Hence, we can mount this heater on our wall. Electric water heaters can take up to a 10-inch high and 7-inch wide range. Gas-fired units can take up to a 30-inch high and 20-inch wide range.

As observed, electric tankless water heater consumes small spaces compared to a gas-fired unit. Depending on what we’re planning to buy, make sure that we have adequate space to accommodate this tankless water heater into the wall. Also, consider the space that allows the heater to vent the fumes and prevent combustion. If we are planning to buy a gas-fired tankless water heater, it should have adequate space that allows the vent pipes to be installed.

  1. Measure the cold water temperature.

Another thing that can help in sizing our tankless water heater is to measure the cold water temperature using a thermometer. Consider it as our vantage point. Our tankless water heater will heat the water and maintain the desired temperature without compromising the maximum flow rate.

For instance, the desired temperature for kitchen sink hot water is at 110 F. If the measurement of the cold water temperature says 50 F, then our tankless water heater must be able to heat the incoming water temperature higher than 60 degrees. In that way, we can maintain the kitchen water flowing at 110 F.

  1. GPM Heating

We need to understand the basics of GPM heating to size our tankless water heater properly. For instance, a gas-fired tankless water heater can increase the temperature of up to 70 F or higher when 5 GPM water is being used.

For the electric tankless water heater, it can also increase the temperature of up to 70 degrees, but it will only occur when only 2 GPM of water is being used. As a general rule of thumb, the less water is used, the higher the water temperature of the water will be.

  1. Determine the size of our tankless water heater.

Measure the incoming water temperature. Like, it reads at 50 F. List down the maximum flow rate of gallons while the water is being used. For instance, one of our family members is taking a shower, and the GPM is 1.5, while the desired temperature is 105 F. Another person is doing the dishes with a 1.5 GPM, and 110 F desired temperature. At the same time, the washer is running at 2 GPM with the desired temperature of 120 F. The summation of the GPM in that given time is 5 GPM.

Now, to maintain the desired temperature while these functions all at the same time, we need to make sure that our tankless water heater can raise the temperature to 70 F at 5 GPM. A gas-fired tankless water heater is highly capable of it. Some high-end electric water heaters can also maintain this desired temperature if we want to save some space.

Installing the Tankless Water Heater

Now that we know how to size our tankless water heater, we can proceed to the installation. It can be a bit complicated because the house wiring, size of the gas supply, and the plumbing configuration are all involved.

Switching to a tankless water heater can be expensive. But it’s more expensive if our old unit is eating up all our energy bills and causing problems from time to time. If we are building a new home, it is vital to opt for a tankless water heater from the start. A tankless water heating system is cheaper to install compared to a gas-fired unit. This is due to the lack of required retrofitting in the installation.

Aside from that, it is vital that we hire a licensed contractor or plumber to install the tankless water heating system for us. Some manufacturers will void the warranty if they found out that the tankless water heater installation was not done by a professional.

But if we are a DIYer and want to install it ourselves, make sure to observe the required building codes and the necessary permissions before we install the tankless water heater.


This is for homeowners who opt for a gas-fired water heater instead of an electric unit. Make sure that we consider the venting before we install our tankless water heater.

Venting refers to the process that will rid the exhaust fumes that the gas-fired unit generates. Tankless gas water heaters produce a lot of fumes, and it also needs a larger venting. This indicates that a gas-fired unit requires more room than an electric gas unit.

If our household is tank-based and we want to switch to tankless, do not reuse our old venting. The exhaust temperature is higher with our tankless water heater. Also, the metal piping is required for gas-fired unit installation.


Going tankless will save us energy and money in the long run. While the initial cost for the installation and the unit may be a bit costly, it’ll save we from the endless repair of an old, conventional water heater.

Also, rest assured that we will never run out of hot water again while in the middle of a shower. A tankless water heating system is convenient, efficient, and cost-effective.

Follow our tips in sizing our tankless water heater and have an easy and hassle-free installation afterward. Knowing how to size our tankless water heater properly will save us time and money. It also helps we to choose the right tankless water heater for our household. Now we can benefit from the convenience and efficiency only a tankless water heater can offer.

Related Topic: 5 (Easy Steps) to Know How Long It Takes to Drain a Water Heater

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