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Tankless Water Heater Vs. Tank Water Heater

Be sure to check out the end of this blog to find out what we consider to be the best tankless water heaters.


Tankless water heaters have many names. They can be referred to as; instantaneous, continuous flow, inline, flash, on-demand, point of use, or instant-on water heaters. All of these names are water heaters that instantly heat water as it flows through the device. These devices do not retain any water internally except for what is in the heat exchanger coil. Copper heat exchangers are preferred in these units because of their high thermal conductivity and ease of fabrication.


Tankless heaters may be installed throughout a household at more than one point-of-use (POU). A tankless water heater may be used with a central water heater, or you may use several tankless water heaters to replace the centralized water heater to provide all the hot water requirements for an entire house. The main advantages of a tankless water heater is a plentiful continuous flow of hot water (as compared to the limited volume of the conventional tank water heater), and potential energy savings under some conditions. The main disadvantage of these systems is their high initial costs (equipment and installation) and low gallons per minute (GPM) of hot water production.


Most newer homes are where you will see tankless water heaters installed. The reason being is that cost to upgrade older homes electrical systems outweigh the benefits provided by tankless systems. Most older homes have a 200-amp service total. You will need about 150 AMP for most whole-home tankless water heater systems. The cost to upgrade your home's electrical service can range from $1500.00 to $5000.00 depending on where you live and the level of upgrade you need.


The most important thing to consider when purchasing tankless water heaters is the flow rates. If you buy a system that has a flow rate of 4 GPM (gallons per minute) then you will not be able to use hot water anywhere else besides your shower simultaneously. This could be problematic for people who have large families. Below is a list of common GPM used by various fixtures.


Shower: 2.5 – 3.5 GPM

Kitchen Sink: 0.75 – 2.5 GPM

Dishwasher: 1 – 3 GPM

Clothes Washer: 2 – 4 GPM

Bathtub: 2 – 4 GPM

Bathroom Sink: 0.75 – 2.5 GPM


Tankless Water Heaters and Scale Accumulation

Tankless water heaters are not immune to hard water problems. As water moves through the heat exchanger, the minerals will precipitate out of the water. Unfortunately, because the system is designed to heat water quickly, the water passes through smaller tubes in the heat exchanger to ensure the water heats quickly. This causes calcium carbonate to precipitate out of the water and the build-up as scale.


As the scale builds up it will cause the heat exchanger to overheat. Every tankless water heater on the market has a safety shut-off when a heat exchanger overheats. This prevents damage to the unit and is a safety precaution. If the unit is reset without the problem being corrected, it will only function for a very short time before the unit shuts itself down. It is also not fail-proof and should not be relied on as a signal to perform maintenance of your system. This can cause permanent damage to the system and will not be covered by most warranties.




Tankless Water Heater Maintenance

Most tankless water heaters require yearly maintenance. The primary purpose of this maintenance is to deal with any scale build-up. If you manually clean your water heater, the period between cleanings depends on how hard your water is and how much the unit is used. The general guideline is to perform maintenance at least once per year. This, however, will not be adequate in hard water areas.


If you don’t have access valves installed, then cleaning your unit of scale will be nearly impossible unless a plumber comes and installs access valves to the tankless water heater. Using a HydroFLOW water treatment device will help you avoid this expensive installation of access ports and provide your tankless system with hard water protection.


Some manufacturers such as Rinnai, have built-in scale detection, which will alert the homeowner on when the unit needs to be flushed. The scale detection for Rinnai is only available on their ultra and luxury series. These warning systems shouldn’t be used as a signal to maintain your tankless water heater. This can cause permanent damage to your very expensive tankless water heater system.


Tankless Water Heater Troubleshooting

The most common cause of your tankless water heater not heating the water properly is due to scale buildup. The best way to fix scale problems in your tankless water heater is to install a HydroFLOW Pearl on the incoming cold-water line going into your tankless water heater. This will ensure that you can avoid this problem and protect you from the damage it can cause to your expensive tankless heater.


Both electric and gas tankless water heaters are less likely to experience issues than tank water heaters, but problems can still occur. Before troubleshooting, turn off the heater to initially assess what is wrong. It is also recommended to check your owner's manual for any troubleshooting steps not covered in this article. If you are unsure what you’re doing, please consult a professional plumber.


Other Common Issues Tankless Water Heaters can Experience

A Lack of Hot Water

Electric tankless heaters may stop providing hot water because of a worn-out heating element. To replace the element, drain the heater, remove the cover, and disconnect the wires.

For gas tankless heaters, the problem is usually a switched-off gas valve or an obstructed vent.

Finally, just like tank heaters, tankless heaters can only provide a certain amount of hot water at once. This is more often a problem for electric heaters, which are best suited as point-of-use units and for small homes. The homeowner will need to upgrade to a larger heater or purchase a second unit.


Water That is Too Hot

In addition to a thermostat set too high or a broken thermostat, tankless heaters can produce water that is too hot due to a faulty temperature sensor. Use a multimeter to figure out if the sensor is broken or needs repositioning.


Heater Shuts Down

When demand for water becomes too high, a heater can shut down. Just reset it.


Buildup of Minerals

To avoid mineral buildup, tankless water heaters need cleaning at least once a year (twice in areas with hard water), or as mentioned above, the installation of a HydroFLOW Pearl will prevent this from ever being an issue.


Cold Water Sandwich

Unfortunately, a cold-water sandwich is unfixable. Some models suffer from this problem, others do not. This is when water comes out cold, then gives a short burst of hot water before returning to cold again.


Ignition or Flame Failure

Tankless gas heaters often show an error message if they have an ignition problem. Confirm the valves are open and, if there is one, check the fuel source.

Flame failures are usually caused by blocked venting.


Blockage in the Air Supply or Exhaust

Other problems suffered by gas tankless heaters relate to air supply and exhaust. Remove any obvious blockages and check the heater meets clearance requirements., even when you don’t need it.

Whereas the majority of problems you’ll see when troubleshooting water heaters will be the above, sometimes issues are more complex. Yet other times, the water heater has simply reached the end of its life and you need to upgrade.


How Does a Tankless Water Heater Compare to a Tank?

You are probably familiar with the standard tank heater. This is a large cylinder that sits in your utility area, and it heats and maintains your water at a constant, preset temperature. These heaters come in various sizes and may be fueled by natural gas, oil, propane, or electricity. They work by heating the cold water that enters your home to the desired temperature and storing it until needed. They work 24/7, so you always have hot water waiting.


Tankless heaters are much smaller because they do not hold water in a reservoir. Instead, the water is supplied to a heat exchanger inside the tankless heater, which heats the water instantly as it passes through the heat source. They typically run on gas or propane, but some models use electricity. When you are not running water through them, they are essentially in standby mode, not using any fuel or heating water.



Because a tank-style heater works at maintaining a reservoir of water hot, it will be using energy as the water cools, even when you do not require hot water. If you go out of town for a day or two and use no hot water, your tank still heats the water and uses energy.

Tankless heaters only turn on when water passes through them, so they typically use less energy overall. They do not use a constant supply of energy, but they use more energy minute for minute when they are operating.


The tipping point for energy-efficiency comes when you use large amounts of water every day. For a household that uses 41 gallons or less of hot water daily, you will use 24% to 34% less energy with a tankless heater. This savings decreases as you increase the number of gallons used per day. If you fill a large soaking tub each night with hot water and use more water for washing machines, dishwashers, and showers throughout the day, a tank-style heater comes out ahead. But for homes that use less water, tankless styles save more energy.



Of the two models, tank-style water heaters are easier to install. It takes only minutes to position the heater in place and connect the water and fuel supply. Because the tank is usually in a basement, utility closet, or another easily reached area, installation is not considered invasive or difficult. Tankless heaters, however, in a retrofit situation the installation can be more complex. Typically, the plumbing will require modification, which may include opening walls and associated repairs 



Tank-style water heaters are priced mainly by their size. For a home with three or four people, you need a 50-gallon tank, which costs around $500 for a gas-fueled model. Installation costs around $45 to $65 an hour, which adds $90 to $200 to the total cost. This makes the final price between $590 and $700 for the average water heater.


Tankless heaters typically cost more to install but have similar purchase costs. A single-point unit, fueled by gas, costs around $500 and has similar costs for the plumbing installation of $90 to $200. Added to that, however, are the other installation costs, such as the new plumbing which runs between $250 and $500, the wall that has been opened for $200, and an access door to reach the heater for repairs for an additional $100. This makes the costs of a tankless heater between $1,140 and $1,500. Something also to consider is that if you select an electric tankless water heater you will need to include the cost of an electrical service upgrade to accommodate the energy needs of a tankless hot water heater. This is a very expensive cost that can be nearly $5000.00 depending on the service upgrade and your electric company.



A standard, tank-style heater lasts on average between 10 and 12 years. The reason why it does not last longer is usually due to sediment and poor water. Over time, corrosion from the constant water and mineral build-up wear away valves and the bottom of the tank. Regular draining and cleaning of the sediment at the bottom of the tank can prolong their lifespan.

Tankless heaters can last up to 20 years with proper maintenance. Because they are not constantly subjected to standing water, they do not corrode or experience the same type of mineral buildup. They can, however, develop mineral problems in areas with hard water, which makes proper maintenance important to their longevity.


Hot Water Runs Out

One of the biggest problems with a standard storage-style tank is that it heats and maintains the water inside, and once you use that water, you must wait for it to reheat the new water before you have more hot water available. So, if you have a 50-gallon heater and your family takes several back-to-back showers, you need to wait for it to heat up before running a sink full of hot water. The only way to solve this issue is to purchase a tank that is larger than what you typically use in a day.


Tankless heaters do not run out of hot water because they heat the water as it enters the system. So, if you have freshwater entering the heater, then you have a constant supply of hot water as well.



Storage-style tank heaters have very little maintenance. It is recommended that every year, you empty the water to remove some sediment, and then refill. Otherwise, they do not require any regular care.


Tankless heaters also require regular, yearly maintenance. Many internal parts need to be kept in good repair to ensure that the heaters run the way they should. Most suppliers have a maintenance plan that they recommend for the specific models they sell.



Tank-style water heaters are simple to repair and replace. They are easy-to-reach and have few parts that may need replacing. This makes repairs fairly inexpensive if something goes wrong.


Tankless heaters, however, are more complex, which makes repairs or replacement more challenging. The most common component failure is the heat exchanger which may require a certified technician to repair or replace.


Storage Space​

Tank-style heaters have a footprint of anywhere from 16 to 36 inches in diameter. This means that you need to have at least this much space somewhere in your home to place it.

Tankless heaters are installed inside or on your wall, so they do not take up valuable space within the home itself. They can be installed on any “water wall” with a 6-inch thickness or more, including outside walls to help save space.


Inconsistent Temperatures

One advantage of a storage tank-style heater is the fact that you get consistently hot water until the hot water is used up. This is usually sufficient for a shower, bath, or using the washing machine. It is also possible to use one tank to operate multiple water appliances simultaneously or on different floors or areas of the home.


Tankless heaters, however, are generally installed at a single point on the line. In most cases, that is close to the room where it will be used in. So, near a bathroom or kitchen, for example. For homes with multiple bathrooms, you may need multiple tankless heaters so that each water source has a dedicated heater. Finally, some models of tankless heaters may give you what is known as a “cold water sandwich.” This means that occasionally the amount of water coming through the heater is too much for it to handle, so in the middle of your hot shower, you get a blast of cold water before it reverts back to hot. Most newer models are designed to avoid this problem, but it can still occur.


Based on the information above here is the breakdown of the Pros and Cons of a tankless water heater vs a tank heater.


Pros and Cons of a Tankless vs Tank Water Heater 

Tankless Pros

  • Energy-efficient.
  • Takes up no space in a home.
  • Costs less to run.
  • Lasts twice as long.
  • Endless supply of hot water.

Tank Pros

  • Consistent water supply.
  • Fast installation.
  • Non-invasive.
  • Easy to repair.
  • Low maintenance.
  • Heats water for an entire house, no additional equipment required.

Tankless Cons

  • Expensive and difficult to install.
  • Needs regular maintenance.
  • May require a water softener or water conditioner to avoid failure in areas with hard water.
  • Inconsistent hot water supply.
  • Lack of adequate GPM, you may need multiple heaters.

Tank Cons

  • Not energy-efficient.
  • Takes up space.
  • Not as long-lasting.


What Tankless Water Heater Should you Purchase?

We have taken all the top tankless water heater manufacturers and evaluated them for several different criteria. Based on the criteria listed below, here are the best tankless water heaters to purchase.


  • Customer Satisfaction – This is determined by the total of reviews and stars received from various review sites.
  • Price – Just because something is expensive does not mean it’s worth buying. Price is a major determining factor for most consumers when making a purchase. So, we evaluated a product on its price compared to other similar products on the market. Based on price, we rated each with a “Great Value”, “Average”, “Expensive.”
  • Energy Factor – Energy Factor (EF) is a measurement that is used to describe the energy efficiency of a water heater. To calculate a water heater's energy factor, you take the amount of energy the water heater puts to use and divide it by the total amount of energy that goes into powering the unit.
  • GPM – Gallons Per Minute is one of the most important factors when considering a tankless water heater. If the GPM is too low, then you will be unable to run appliances and showers simultaneously. It is important to understand this when selecting a tankless hot water heater because it won’t be able to keep up with your household needs.
  • Product Features – Some tankless water heaters have added features that add value to their products. Some examples of these are built-in maintenance alerts, product warranties longer than the industry standard, quality of materials used to manufacture their product and other safety features to name a few.


The best point of use tankless water heater


ECO90 Self-Modulating Instant Hot Water Point Of Use Tankless Water Heater

  • Customer Satisfaction – 4.4
  • Price – Great Value
  • Energy Factor – 0.998
  • GPM – 1.5 
  • Features – The ECO90B electric water heater promises no "cold sandwiching" with a unique heating element that avoids corrosion. It is also self-modulating for stable temperature control.





The best electric tankless water heater


Stiebel Eltron Tankless Water Heater – Tempra 29 Plus

  • Customer Satisfaction – 4.5
  • Price – Average
  • Energy Factor – 0.99
  • GPM – 6 GPM
  • Features – Advanced flow control that controls the water temperature keeping it constant and avoiding "cold sandwiching", 7 Year Warranty.




The best gas tankless water heater


Reliance TS-540-GIH 199K Tankless Natural Gas Water Heater

  • Customer Satisfaction – 4.8
  • Price – Average
  • Energy Factor – 0.95
  • GPM – 8.4 GPM (the manufacturer's website says 10 GPM)
  • Features – Design that prevents corrosion, 12-year warranty, built-in freeze protection.




The best propane tankless water heater


20HI Indoor 6.0 GPM, Liquid Propane

  • Customer Satisfaction – 4.9
  • Price – Great Value
  • Energy Factor – 0.83
  • GPM – 6 GPM 
  • Features – EZ-Flush kit included, 10-year warranty, smart adjust technology that keeps the temperature stable.



The best commercial tankless water heater


Rinnai CU Series Commercial Tankless Gas

  • Customer Satisfaction – 5
  • Price – Expensive
  • Energy Factor – 0.97
  • GPM – 11 GPM 
  • Features – Maintenance alerts, multiple venting options, built-in freeze control, 8-year warranty.










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