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Tankless Water Heater Buying Guide

How to Pick the Perfect Tankless Water Heater

By  |  Product Expert

In the average home, heating water accounts for 30% of the energy budget.

Think about it, you have to have a hot water tank large enough for cooking, cleaning, bathing, and other tasks. We've all experienced that occasional "cold shower" that's a result of too many tasks being done at the same time. You also know that If you have a large family, it's best to be the first in to take a shower.

What happens when you aren't doing all of these tasks or not even home? You still have around 50 gallons of water that needs to be continuously heated, regardless if you use it or not. That's a lot of wasted energy.

Many of us only consider our water heater when it fails, usually replacing it with an identical unit to restore service as soon as possible.

This also means that many of us are used to living with a bulky, inefficient, unreliable storage tank water heater taking up space in a basement or closet.

You don't have to suffer any longer. Thankfully, we now have tankless hot water heaters that are compact, efficient, and supply hot water on demand, lowering your energy and maintenance costs.

Water Heater Efficiency
Water heaters are rated according to their Energy Factor (EF). The energy factor is the ratio of energy used by the water heater that is actually turned into hot water for use in the home. Typical big storage tank type water heaters have an energy factor between 0.6 and 0.7, while tankless water heaters can have energy factors as high as 0.98.

Storage tank water heaters waste energy in two main ways. They are inefficient in turning the fuel they use into hot water, and they then lose the heat to the environment by storing the hot water in a large tank. Tankless water heaters avoid both of these problems. They use sophisticated heat exchangers that can capture nearly 100% of the heat produced from burning the fuel, and they produce hot water as it is needed, so no heat is wasted waiting for a use.

How Do Tankless Water Heaters Work?

Tankless water heaters monitor the flow of water through the water heater to determine when hot water is needed. Once flow is detected, the burner is turned on and water is heated as it passes through the water heater. The unit further ensures that an appropriate temperature is delivered by tempering the output with cold water and by adjusting heat output.

Waiting for a demand before producing hot water does have a couple of minor drawbacks. These include the potential for hot water being unavailable at very low flow rates, and the occasional occurrence of the “cold water sandwich”. If a hot water tap is turned off and then quickly turned back on, it is possible that a pocket of water will pass through the water heater without being heated. This resulting pocket of cold water is sandwiched between the hot water that was produced before the tap was turned off and the hot water produced after the flow was restored. Manufacturers and installers have come up with a variety of measures to reduce the severity or eliminate the cold water sandwich phenomenon.

How Do I Select the Correct Tankless Heater for My Home?

There are two factors that need to be considered when sizing a tankless water heater. First, you need to find out how much hot water will be needed at any given time. This can be accomplished by figuring out which fixtures you will want to operate simultaneously and how much hot water each will use. This is called the flow rate and is measured in gallons per minute (GPM). You can determine the total rate at which the water heater will need to produce hot water by adding the flow rates for each fixture together.

The second factor that you must consider is the temperature differences between the water entering the heater, the ground water temperature, and the temperature desired at the fixtures. Typical fixture temperatures are 110° F – 120° F, while ground water temperature can range from approximately 70° F in southern parts of Florida, to near freezing in the northern parts of the US.

The greater the difference between the ground water and fixture water, the fewer fixtures a water heater will be able to supply. The tables and figures below will help you to determine your load and approximate your temperature difference. Using the calculated flow rate and temperature difference, you can select your tankless water heater from the comparison chart provided. Flow rates at temperature differences other than the two included on the comparison chart can be found in manufacturer documentation.

What Are My Home's Specific Flow Rate Requirements?

Use the two tables below to calculate the total flow rate your home needs. Be sure to only include the maximum number of fixtures you will want to operate simultaneously in the calculation table.


Fixture Type Flow Rate (GPM) Quantity Total for Fixture Type







Total Flow Rate


Click the print button to print the form above.    


Where Do You Live?

Use the map to locate your ground water temperature and enter it into the table below to determine the temperature rise for hot water at 110°.

Use this tool to calculate the Required Temperature Rise (F) based on Ground Water Temperature (F) and Required Temperature Rise (F)


Desired Outlet Temperature (F)
Ground Water Temperature (F)
Required Temperature Rise (F)  
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