What Size of Tankless Water Heater Do I Need - Ultimate Guide

Christine Herrington

Christine is our Chief Editor and a contributing writer to the site.

Last Updated on February 3, 2022

Tankless water heaters offer unlimited hot water on demand. In recent years, tankless water heaters have become popular in homes and commercial buildings, thanks to the numerous benefits they offer, from energy efficiency to long-term financial savings.

However, while they can provide hot water on demand, tankless water heaters are greatly limited by the amount of water they can heat simultaneously based on their tank size. Choosing the right tankless water heater capacity can be challenging without the proper knowledge. But by taking just a few factors such as household water needs and flow rate, you can find the perfect unit for your home.

Sizing Your Tankless Water Heater

It is essential to choose the correct size when purchasing a tankless water heater for your household to avoid inefficiency and inflated utility bills. If you purchase a small system, it won’t have the capacity to offer sufficient hot water to meet your household needs. If you buy an oversized tankless water heater, you will have a system that requires greater amounts of electricity. Therefore, proper sizing is crucial before making a purchase.

What is a Tankless Water Heater?

A tankless water heater heats the water directly on demand. Unlike other water heaters, it doesn’t hold water in a storage tank. The cold water is heated to the desired temperature and maintains temperature indefinitely.

There are two broad types of tankless water heaters: electric or gas. Gas-powered heaters are generally more efficient and heat water faster compared to electrically powered units. However, they require ventilation and have a higher initial cost of plumbing and installation.

Electric tankless heaters are much easier to install but are incapable of heating large amounts of water, making them unsuitable for larger households. Nonetheless, both the tankless water heater and the tankless electric water heater function in the same way; only the power source varies.

What Does the Size of a Tankless Water Heater Mean?

If you’re asking yourself, what size tankless water heater do I need? It is best to understand sizing before you make your purchase. Considering that tankless water doesn’t store water, the term “size” can be confusing.

Size in regards to tankless water heaters refers to the flow rate, which is how much water can be heated at once. The water flow rate is measured in gallons per minute (GPM). Size is, therefore, not the physical dimensions of the water heater but rather the heating capacity.

How to Size a Tankless Water Heater

To size a tankless water heater to determine whether it fits your needs, you need to determine the temperature rise and the flow rate depending on whether you will use it in the bathroom alone or the entire house. Use the following steps to determine the sizing of a suitable unit for your needs.

Step 1: Determine the Flow Rate

To determine a tankless water heater’s suitable size, start by figuring out your flow rate, usually indicated in gallons per minute (GPM). Determine how many devices you want to run simultaneously and find out their total flow rate. Then sum up their total flow rates in GPM.

For instance, you might expect to run a hot water faucet with a flow rate of 0.80 gallons per minute and a shower head with a flow rate of 2.0 gallons per minute at the same time. The flow rate through the tankless water heater would need to be at least 2.80 gallons.

Step 2: Determine the Required Temperature Rise

The next step is to determine your required temperature rise, calculating the output temperature minus the temperature incoming from the municipal water source. You can use a thermometer or use the US average groundwater temperature map if you reside in the US.

For the desired output temperature, most homes would prefer water heated to between 105°F–115°F. For instance, for people living in Michigan, where the feedwater temperature averages 40°F and your desired output temperature is 110°F. The required temperature rise would be 110°F – 40°F = 70°F.

Step 3: Choosing a Tankless Water Heater

After establishing your hot water demand at peak hours and the required temperature rise, the next step is to look for a tankless water heater that meets these requirements.

Most manufacturers provide sizing charts with their products specifying maximum flow rates at a given temperature rise or maximum output temperature rise at a given flow rate. Some tankless water heaters are also flow rated for different input/output water temps. Choose a unit size that either meets or slightly exceeds your peak hot water demand.

A Look at Common Scenarios

What Size Tankless Water Heater Can Replace a 60 Gallon Water Heater?

Since tankless heaters are extremely convenient and offer enormous financial benefits, many people are looking to replace their heaters with tankless water heaters.

If you’re wondering what size water heater do I need to replace a 50-gallon water heater, you can choose these two options depending on where you live in the US. For homes in the north of the USA, you’ll need a 27kW electric tankless water heater or a 10GPM gas tankless heater. An 8GPM gas tankless heater or an 18kW electric tankless heater is suitable for households in the USA’s southern region.

How Large Should a Water Heater Be For a Family of Four?

When choosing a tankless water heater for a family with four members, you have to consider that four people can simultaneously run multiple faucets, do laundry or use a dishwasher. So when considering, what size tankless water heater do I need to accommodate four people, you would need an 18-27kW electric tankless heater or a 7-10GPM gas tankless heater to suit their water consumption needs.

Enjoy Endless Hot Water With a Tankless Water Heater

Size is a critical factor when deciding the right tankless water heater for your home. You neither want a tankless water heater that is too small nor one that’s too big to needlessly consume energy translating to high operating costs. To find the correct size, you assess your home’s overall flow rate and the required temperature rise. The best tankless water heater size should exactly meet your household hot water needs for optimal operation efficiency.

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