Best Tankless Gas Water Heater Buyers Guide 2019

    Natural Gas Flame

    The popularity of on-demand water heating systems has increased over the last few years, and gas tankless water heaters are often the favorite choice. Gas tankless systems have many advantages over electric units, but they also have their share of short comings.

    The Cost of Gas Tankless Water Heaters

    As a general rule, you can expect to pay more for a gas tankless water heater, and it should be noted that an annual professional inspection is recommended to keep it running in tip-top shape.

    From an operating cost standpoint, gas units tend to out perform electric tankless systems. However, this can be misleading since fueling a tankless with gas is less expensive than electricity.

    Gas systems actually have a lower EF rating (efficiency) than electric units. But we'll dive deeper into all of the details later. 

    Take a look at the best tankless water heaters on the market today.

    Pen and paper

    How to Buy a Gas Tankless Water Heater

    There's a lot to consider when it comes to buying a tankless water heater, and it's easy to get overwhelmed. Whether your building a new home or replacing an old unit, it's important to purchase a unit that can meet your household's hot water demands.

    This article will help you find the right tankless gas water heater for your home. You'll have the knowledge you need to research and compare appliances, so there won't be any suprises down the road. 

    If you're still on the fence about which energy source is right for you, check out our article Gas vs Electric to see a full comparison between the two.

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    How Does a Gas Tankless Water Heater Work

    Tankless units are often called on-demand water heaters because they only operate when there's a need for hot water. When a hot water tap is opened, cold water flows into the tankless water heater and triggers the gas burner to jump into action.

    The cold water is warmed as it travels through the heat exchanger, so that when the water leaves the appliance and enters your plumbing it is hot.

    No Storage Tank Limits

    Where a tank-style water heater can only deliver as much hot water as the size of it's tank, an on-demand tankless system is capable of delivering a seemingly endless stream of hot water. 

    Unlike a storage tank, where hot water is held in an enclosed container full of sediment and rust, a tankless water heater always delivers fresh hot water.

    Its Important to Choose the Correct Size Tankless

    Although tankless water heaters aren't restricted by tank size, they are limited by flow rate. This means that there's less room for error when you select the size of your tankless water heater.

    A tank-style water heater uses a storage tank to hold a reserve of hot water, but an on-demand system doesn't have this buffer. If the household demand exceeds the appliance's ability to heat water, the unit will be forced to deliver lukewarm water.

    Correctly sizing your tankless water heater will ensure that you purchase a heater that's capable of meeting your household hot water needs.

    Man with paper and pencil behind three sizes of houses

    Selecting the Right Size Gas Tankless Water Heater

    Tankless water heater systems are sized by the amount of water the unit is capable of heating at any given time. This is called flow rate, and it's a critical factor when shopping for a tankless unit.

    If you purchase a tankless water heater system that has too low of a flow rate to meet your household's hot water needs, the unit won't be capable of delivering enough hot water. Nobody likes a cold shower, so it's critical that you select a tankless unit that's large enough to deliver hot water during your family's peak usage.

    The 2 Things You Need to Know to Size a Tankless: 

    • Temperature Rise: The difference between the incoming ground water temperature and the heated output temperature.
    • Flow Rate: The amount of water a tankless water heater is capable of heating at any given time.

    All manufacturers size their tankless models using both these two factors (BTU is also a factor, but we'll cover that later). If you know these two items, you'll be able to speak the same language as the manufacturers. Although, each manufacturer's approach may differ slightly, you'll still be able to compare apples-to-apples.

    Temperature Rise

    To determine the temperature rise you'll need to know the ground water temperature for where you live. Using this chart, you can simply locate where you live and jot down the ground temperature for your area.

    The incoming water temperature varies throughout the year. It will be colder during the winter, so it's best to use the lower temperature to ensure the unit is sized correctly.

    Most people set their water heater between 110-to-120 degrees Fahrenheit, but if you like your hot water slightly warmer or cooler you should use that number instead. Keep in mind, that setting your hot water above 120 degrees could cause scalding in young children or older adults.

    Now, simply subtract the two numbers. As an example; if your ground temperature is 47F and your desired hot water temperature is 120F, your temperature rise will be 73 degrees.

    This means that your tankless will need to heat the cold incoming water 73 degrees in order to reach the desired 120 degrees.

    Flow Rate

    Finding the correct flow rate for your hot water needs is easier than you  might think. First, determine when your household uses the most hot water. This is called peak demand and in most cases, a family's peak demand is in the morning when people are showering and getting ready for school or work.

    Next, count the number of open faucets and appliances used during a typical peak demand period. Do you frequently run the clothes washer during this time? The dishwasher? How many people are showering? Are any sinks used?

    Flow rate is measured in gallons per minute (GPM), so for each fixture or appliance used during your peak demand period you'll need to know it's GPM. Here's a handy chart to help:




    1.5 - 2.2 GPM


    1.25 - 2.5 GPM

    Bath Tub

    4.0 GPM

    Washing Machine

    1.5 - 3.0 GPM


    1 - 2.5 GPM

    If you're not sure of a faucet's GPM you can go old school and grab a bucket and mark off the 1-quart line. Then open the faucet and set the timer. Divide how many seconds it took to hit the 1-quart line into 15. You now know the GPM for that faucet! 

    If you typically run 1 shower (1.5 gpm) and a washing machine (2.0 gpm), your peak hour gpm would be 3.5. Using the example above, you would need to purchase a tankless water heater that could deliver 3.5-gallons of water per minute (gpm) with a temperature rise of 73-degrees.

    This video will show you how to size your tankless water heater:

    British Thermal Unit (Btu)

    Another factor that should be considered when purchasing a gas tankless water heater is Btu. The U.S Energy Information Administration defines Btu as "the quantity of heat required to raise the temperature of one pound of liquid water by 1 degree Fahrenheit at the temperature that water has its greatest density (approximately 39 degrees Fahrenheit)."

    When shopping for a gas tankless water heater always purchase the one that can deliver the highest Btus for your required flow rate. As an example, if you need a tankless that can deliver a flow rate of 3.5 gpm at a temperature rise of 70-degrees, and you've found two models you like, select the one with the higher Btu rating. It'll be able to deliver more power.

    Gold hot water faucet handle

    Gas Fueled Tankless Water Heaters

    Gas tankless water heater units are able to deliver a higher flow rate than electric systems. In addition, they offer a quick response time and high heat output, and they're an excellent choice if you have access to natural gas or propane in your area.

    Uniform Energy Factor

    Before June 2017 gas tankless water heater's efficiency was measured by an Energy Factor (EF) rating. Today, the Uniform Energy Factor (UEF) is now the standard for measuring energy efficiency.

    The UEF is thought to be a better representation of how the consumer uses hot water. There are three criteria used during the testing to determine the UEF rating: First hour rating/first hour delivery, capacity, and estimated energy costs.

    Appliances with the highest UEF rating will offer the most energy savings since they will operate the most energy efficiently.

    Fuel Supply Line

    The fuel supply line is the gas line that leads into your house and connects to the tankless unit. This line must be able to provide enough fuel for the large burners to deliver instantaneous hot water.

    A traditional water heater will have an output in the range of 75,000 Btu/h, but a tankless unit is frequently more than double that amount commonly weighing in at up to 200,000 Btu/h.

    If your home isn't prepared to deliver enough fuel, you'll need to increase the size of your gas fuel line. Which can be a costly addition to the installation.


    Gas tankless water heaters use an ignition system to ignite the burner that heats the water. There are 3 main types available. As a general rule, the more complex the ignition system, the more expensive the tankless water heater. 

    Standing Pilot Light

    • A standing pilot light is always on and ready to heat the water.
    • Generally used in lower-end units.
    • There's less energy savings with this type of pilot light because fuel is being burned whether the unit is heating water or not.
    • Less energy savings equals higher operating costs.

    ​Direct Ignition

    • A direct ignition system delivers a spark to the main burner when it detects water flow.
    • These systems offer significantly improved energy efficiency over the standing pilot light units.
    • Depending on the design, the unit either requires an electrical outlet nearby or cell batteries to operate.

    Hydro-Power Ignition

    • The burner is ignited when water flows into the unit and activates a small turbine.
    • Does not require an electrical connection or batteries to operate.
    • Manufactured by Bosch.


    Non-condensing gas tankless water heaters need air to carry out combustion. Once combustion occurs the exhaust needs to be directed outside through a vent. The venting system is more complex than what is used for traditional water heaters, but it also offers more flexibility, as the vents can run through the roof or horizontally thru a side wall.

    The venting material from a tank-style water heater will need to be replaced with a corrosive-resistant Category III vent made from stainless steel. As you might imagine, its also more expensive.

    Due to the combustion efficiency, condensation frequently develops within the vents. The condensation is highly acidic and standard venting material would be quickly "eaten" away, which is why a heavy-duty vent is required. ​

    Venting is not necessary for outdoor or indoor condensing models.

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    Types of Gas Tankless Water Heaters

    There are 3 basic types of gas tankless water heaters:

    • Non-condensing tankless water heaters.
    • Condensing tankless water heaters.
    • Outdoor tankless water heaters.

    Non-Condensing Tankless Water Heaters

    Non-condensing tankless water heaters are designed to be installed indoors. They are generally less expensive to purchase and more expensive to install since they require Category III venting. These types of systems have two different vent configurations.

    Direct Vent​

    • Air is drawn from outside the house into the tankless water heater for combustion.
    • There are two vents: One for air intake and the other for exhaust.
    • A direct vent system allows the tankless unit to be installed in smaller areas.

    Power Vent​

    • The air inside the house is drawn into the unit for combustion.
    • The exhaust is vented outside.
    • Installation must be in an area that provides adequate air flow for combustion.

    Condensing Tankless Water Heaters

    Condensing tankless water heaters offer a more sophisticated venting option. They are more expensive to purchase, but less expensive to install since they eliminate the need for venting altogether.

    • A condensing unit extracts the heat from the exhaust and eliminates the need for expensive venting.
    • These units achieve a higher EF (efficiency) rating. Generally in the mid-to-high 90's.
    • Although, condensing units are more expensive than non-condensing units, the fuel costs are lower to operate the unit, offsetting the increased upfront cost.
    This video shows how a condensing tankless water heater works:

    Outdoor Tankless Water Heaters

    An outdoor tankless water heater is designed to endure the weather. They are installed on the exterior of the house and utilizes the free air flow outside to vent the exhaust. In other words, no additional venting is needed.

    If you live in an area where an outdoor unit makes sense, this can be a great choice especially if your house is already built and you are transitioning to a tankless unit.

    Since they are located outside, they are typically easier and less expensive to install because venting isn't necessary and fewer modifications will need to be done to your home. 

    Higher-end tankless units are designed with self-warming components that help prevent the unit from freezing, and allows them to operate in low temperatures. Although, it should be noted, that outdoor units may not be the best option if you live in a climate that experiences freezing temperatures on a regular basis. 

    Wrenches in case

    Tankless Water Heater Maintenance

    Always check your tankless water heater owner's manual for specific maintenance requirements and recommendations. Some manufacturers recommend performing maintenance every 6 to 24 months.

    Higher-end units will frequently be designed to resist lime scale build-up, and some units will even alert you when maintenance is necessary.

    But if you live in an area where the water supply is especially hard, you should flush your unit more frequently and explore other proactive preventative options. 

    There are many maintenance tasks such as flushing the unit, that the homeowner can complete. The use of a sediment filter is also recommended. 

    Have Your Tankless Unit Inspected by a Professional

    Even with performing regular maintenance, a gas tankless water heater should be inspected annually by a trained professional. He will be able to verify that combustion is occurring safely and the unit is performing as expected.

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