Best Tankless Gas Water Heaters in 2021: Buying Guide & Reviews

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Best Tankless Gas Water Heaters 2021

If powerful performance is something that you seek in a water heater, the best tankless gas water heaters can meet your demand. In this article, we share a few tips that could prove useful to you in your search for a great water heater. On top of that, in the second half of the article we also have a detailed list of the best gas-running water heaters on the market for your perusal.

How Do Tankless Gas Water Heaters Work?

A traditional water heater (or “tanked water heater”) pre-heats water and puts it in a water storage tank. It is actually from this tank that your showerhead and taps draw that blissfully warm water.

A tankless water heater, on the other hand, does not store the water but heats it when needed. The water running through the unit gets heated as you use the appliances thanks to a number of electric heating elements. Without the need to constantly warm up the water in a tank, tankless heaters can save as much as $104 per year on your energy bill. This is the primary attraction of tankless heaters and it’s become so popular on the market that we got an entire article focusing solely on tankless heaters.

How to Pick the Best Tankless Gas Water Heater

Fuel Type

The heating element of a gas-running heater is a gas burner. Depending on the heater’s type, the burner can either use natural gas or propane as a fuel source. Both are readily accessible in virtually every part of the country at a relatively cheap price. 

This is one of the key advantages gas heaters have over electric heaters as, in many places, the cost of gas or propane can be significantly lower than electricity. Check your local fuel companies and compare those prices with electricity prices in your area to see if this is true for you. 

Because gas and propane release more energy, at a quicker rate than electricity, gas heaters can produce more hot water than the average electric heaters. If you are looking for maximum performance in a heater, a tankless gas water heater may be your best choice.

But deciding which one out of the two types of fuel can be quite challenging. After all, both of them sound as if they are the same thing. When you dig deeper, though, there are key differences that can give you clues as to which one would suit you better.

In terms of power, propane offers more energy per cubic foot in comparison to natural gas. One cubic foot of propane generates 2,516 BTUs in contrast to natural gas, which generates only 1,030 BTUs. As a result, propane is a better option overall if you have high hot water demand and need every bit of energy the burner can offer.

While propane does have greater energy potential than natural gas, when it comes to cost natural gas has the upper hand. Let’s assume that 1,000 cubic feet of natural gas costs about $15. Because natural gas can release 1,030 BTUs of energy per cubic foot burned, for $15 of natural gas you get about 1.3 million BTUs. 

This is roughly the same amount of energy you can get from 11.26 gallons of propane. Since propane is typically sold for about $2.50 per gallon, when the costs are all accounted for, natural gas is the more cost-effective option.

There’s also the question of environmental friendliness. Whenever “gas” is mentioned, all of us usually imagine plumes of toxic, lethal smoke liberally vented into the atmosphere through dark smokestacks and vents. However, it might come as a surprise that propane is actually a very eco-friendly fuel type. It burns much cleaner than natural gas, which is an actual greenhouse gas when released into the air or burned. 

Really, it boils down to which one out of the two has more desirable properties to you. For environmental friendliness, power, and efficiency, look to propane. However, if you value operating cost above all else, consider using a gas heater with a natural gas burner.


Sizing is where you take into account the performance and capacity of your heater and compare it with the hot water needs of your family. If this equation is not properly balanced, life with your brand new heater can get frustrating fast. 

For example, if your heater is not powerful enough to fulfill your family’s hot water demand, you will eventually find yourself rationing hot water so that there is enough to go around. On the other hand, if your heater is too powerful and consistently exceeds your ordinary demand significantly, the heater can unduly inflate your utility bill. 

In order to properly size your heater, there are two things that you need to consider: the flow rate of your heater and the temperature rise.

A heater’s flow rate refers to the amount of hot water it can produce at any given time. An average tankless gas water heater can produce about five gallons per minute under optimal temperature conditions. Five GPM can provide an ample supply to two standard showerheads and an extra faucet.

To find out your household’s flow rate, take a walk through the entire place and write down all of the appliances that will require hot water. When everything is accounted for, add them all up using the table below and you will have an idea of what your heater’s flow rate should be to cover everything.

However, keep in mind that unless you expect all of the appliances in your house to be used simultaneously, it is possible to have the heater’s flow rate be lower than the household’s maximum demand. Just make sure that it’s not too low.

ApplianceAverage Flow Rate
Tub4.0 GPM
Shower1.5 – 2.0 GPM (depending on efficiency)
Washing Machine2.0 GPM
Dishwasher1.5 GPM
Sink1.5 GPM

Temperature rise, or ΔT, refers to the difference between the inlet ground water’s temperature and the temperature that you want it to be raised to. The average groundwater temperature in the U.S is about 57°F. Let’s say that you want your tap water to run at a warm 110°F and set your heater accordingly, the ΔT in this case is 53°F.

While 57°F is the average, the groundwater temperature in your area can differ. It will be warmer in the southern states with hotter climates. In northern states, the groundwater can even be close to freezing. 

ΔT is important because it can tell you the heater’s efficiency. The greater the divide between the groundwater’s temperature and your desired temperature, the harder your heater will have to work. If groundwater is too cold, your heater will be able to supply less hot water to the appliances around the house.

Energy Factor (EF)

Energy Factor (EF) is the standard that is used to indicate how energy efficient a heater model is. Basically, EF can tell you just how much hot water the heater can produce after burning a certain amount of fuel. The higher the EF score, the more efficient the heater is.

But you don’t have to bother too much with mental math while out and about shopping. A fine trick to knowing the EF is to look for the Energy Star label. To be qualified for this label, a tankless heater must have an EF equal to 0.67 or above. For condensing heaters, the EF requirement for an Energy Star is equal to or above 0.8.

Nonetheless, be mindful that while EF will tell you the unit’s efficiency, it won’t tell you how much you will save using it. Energy efficiency doesn’t always equate to cost efficiency. As an example, the EF of electric heaters are always higher than even the best gas-fired heaters. But since electricity, in a lot of regions, is more expensive than gas, operating a tankless electric heater can turn out to be costlier than a lower-efficiency tankless gas heater.


When installing a new tankless gas heater, you will eventually have to discuss with your plumber about the venting arrangement. There are two major types of heater vent design: a non-condensing or condensing vent.

A non-condensing vent is the “traditional” way of venting the excess hot gases and steam. Everything is funnelled through a singular vent and out into the environment. While simple, this is fairly inefficient because a large portion of the energy released by the fuel is wasted.

A condensing vent makes use of this energy that’s normally wasted. By rerouting the hot steam back down to the bottom of the unit via a coil, cold inflow water can soak up the heat, effectively reusing the energy. With this arrangement, a condensing heater can have 96% thermal efficiency.

Condensing heaters can also cut installation costs pretty dramatically. Because the final gas that would be vented is significantly cooler (with the majority of its heat drawn back into the unit), the vent system can be made out of PVC instead of the stainless steel used in a non-condensing vent design.


On a traditional heater, the pressure relief valve that regulates the pressure inside water storage tanks can sometimes (though this is extremely rare) malfunction, causing pressure build-up that eventually makes storage tanks explode. 

This is not a problem in tankless heaters. Apart from the fact that there’s no tank, liquid water expands significantly slower than water vapor and if there happens to be any pressure build up, it can simply be vented out the instant you turn on your tap or showerhead.

This means many tankless water heaters don’t feature a pressure relief valve. Regardless, some models do if you use PVC pipings. While pressure build-up won’t make the pipes explode, it can potentially damage the unit and make the pipes burst and leak. The valve will automatically shut down the unit if it detects pressure beyond the heater’s working parameter.


It’s especially important to read up on the warranty information of your heater. It’s not just because a heater is an expensive appliance and knowing servicing information can save you a lot of troubles and money in the long run. The warranty information can also help your plumber approximate the lifespan of your heater as well as telling them how often maintenance should be conducted on the unit.

Depending on the model, the most common warranty periods are 3, 9, or 12 years.

Most manufacturers will also void their warranty if the heater runs into problems caused by incorrect installation or lack of maintenance.

Best Tankless Gas Water Heater Reviews

Here’s a quick list of all the products that we will be introducing in this section.

  1. Rinnai V65iN – Best to Buy in 2021
  2. Rinnai RU199iN – Best Value Tankless Gas Water Heater
  3. Noritz NRC66DVNG – Best Natural Gas Tankless Water Heater
  4. Takagi T-H3-DV-N – Best Whole House Gas Tankless Water Heater
  5. Camplux BW264 10L – Best Outdoor Tankless Gas Water Heater

Additionally, the comparison table below will tell you basic information about each.

NameFlow RateBTU RatingEnergy FactorWarranty
Rinnai V65iN6.5 GPM10,300 – 150,000 0.8210-year on heat exchanger, 5-year on spare parts, 1-year period on labor
Rinnai RU199iN11 GPM199,0000.91 – 0.9310-year on heat exchanger, 5-year on spare parts, 1-year period on labor
Noritz NRC66DVNG6.6 GPM120,0001.012-year on heat exchanger, 5-year on spare parts, 1-year period on labor
Takagi T-H3-DV-N10 GPM199,0000.9115-year on heat exchanger, 5-year on spare parts, 1-year period on labor
Camplux BW264 10L Outdoor1.6 GPM68,0000.881-year

1. Rinnai V65iN – Best to Buy in 2021

The Rinnai V65iN is considered for this title due to its well-rounded qualities: relative affordability, moderate performance, and reliability. While it definitely isn’t the most powerful heater on the list, the Rinnai V65iN can supply the demand of most families without a problem.

Rinnai V65IN Tankless Water Heater


  • Affordable
  • Great performance
  • Energy Star-qualified
  • Smart app available
  • Various safety mechanisms
  • Durable design
  • Comprehensive warranty


  • Long lag time

Under optimal climate conditions, the Rinnai V65iN can output a flow rate of 6.5 GPM. This is the equivalent of three standard showerheads and a kitchen sink. 

In order to get this kind of performance, the heater’s natural gas burner produces from 10,300 to as much as 150,000 BTUs worth of energy depending on the demand and temperature rise. For the heater to get the amount of gas that it needs, either a ½-inch or ¾-inch gas line is required.

The Rinnai V65iN is certified with an Energy Star label having a great Energy Factor of 0.82. In spite of the respectable performance, the heater’s innate energy efficiency will be able to save you more than a few pennies on the utility bill.

One problem that you may face while using the heater is that the lag time can be fairly long. Lag time is the amount of time that it takes for the heater to start up and heat the water stream to the set level. Most of the consumer reviews suggest that the Rinnai V65iN has a lag time of 40 seconds.

In terms of safety, there is a leak detection valve that will automatically shut off the unit if it was to detect a drop in pressure (signifying a leak) in the pipeline. The heater is also equipped with a scale detection sensor that will notify you when limescale has built up to a harmful level so that you can flush the unit. 

A temperature lock is in place to prevent major fluctuations in water temperature, effectively solving the age-old problem of suddenly experiencing a spike in water temperature that scalds you while you’re showering or washing. For people living in colder regions, there is an anti-frost feature to prevent the unit from freezing up due to the cold inlet water temperature.

Most components inside of the heater are made of metal, which promises great durability. Although there are select parts that the company still cast from thermoplastic, consumers have attested to the longevity of the unit.

The warranty policies are extensive. You will get a 10-year warranty on the heat exchanger, 5-year on spare parts, and a 1-year period on labor.

One interesting optional feature that makes this heater slightly more unique than its rivals is the Control-R Wi-Fi module that you can also purchase separately. This module allows the heater to be connected to the Internet and paired up with your smartphone through a special app. You can set the temperature and adjust the heater’s settings straight from your phone. Thus, this heater is also worth getting if you fancy yourself a smart home.

The Rinnai V65iN is a non-condensing heater, so a stainless steel vent is in order. But other than this pain point, installation will be fairly simple.

2. Rinnai RU199iN – Best Value Tankless Gas Water Heater

If you want the best performance and capacity for the price, the Rinnai RU199iN might be what you’re looking for. The performance is among the highest on the market and can fulfill the hot water demand of the largest households. Additionally, its great reliability, durability, and ease of use make this heater even nicer to purchase.

Rinnai RU199iN Tankless Water Heater


  • Powerful performance
  • Energy Star-qualified
  • Smart app available
  • Various safety mechanisms
  • Durable design
  • Comprehensive warranty


  • While a value buy, can be excessive for some households

With a good groundwater temperature (around 57°F,) the RU199iN’s natural gas burner is capable of producing hot water at a cool rate of 11 GPM. To get this kind of flow rate, the burner releases 199,000 BTUs at its peak and will require a ¾-inch gas line.

This flow rate is enough to run five showerheads simultaneously and still have excess to spare for about two kitchen sinks. Considering the fact that this is a feat that only the largest and heaviest of traditional heaters can do, which may cost you north of the $2,000 mark (installation included), the RU199iN pricing is just right to be called a valuable investment.

The energy factor of the RU199iN has been measured to be between 0.91 to 0.93, thus making it eligible for an Energy Star label that ups its value even higher. Integrated into the heater unit is a patented tech called Circ-Logic, which can recirculate pre-heated hot water back to you, essentially eliminating lag time. According to Rinnai, since you won’t have to wait for the heater to slowly kick in once you turn on your appliances, it can save you about 3 gallons of water that would normally be wasted when you take a shower.

In terms of safety, the heater unit is equipped with all of the usual mechanisms often featured in Rinnai’s heaters. It includes a leak detection valve, scale detection sensor, temperature lock, and furthermore, the heater has been approved by the South Coast Air Quality Management District. This means that the heater has low emissions and won’t reduce air quality in the surrounding area with harmful gases like NOx. NOx negatively affects the environment and is harmful to human’s health so this is a welcomed addition.

Warranty-wise, it has a 10-year warranty on the heat exchanger, 1-year on labor, and 5-year on parts.

The RU199iN is also compatible with the company’s optional Control-R Wi-Fi module for smart control. The module will allow you to remotely adjust temperature, settings, and even do voice commands to control your heater by using your smartphone.

3. Noritz NRC66DVNG – Best Natural Gas Tankless Water Heater

The Noritz NRC66DVNG is our selection for this section not just for its high-efficiency natural gas burner but also for its modest performance and affordability. Not every household will need a heavy duty water heater and the NRC66DVNG has enough capacity to provide just enough for the majority’s demand.

Noritz NRC66DVNG Tankless Hot Water Heater


  • Energy efficient
  • Condensing design
  • Great performance
  • Electronically controlled
  • Various safety mechanisms
  • Durable design
  • Comprehensive warranty


  • Long lag time

With a mid-sized natural gas burner capable of producing around 120,000 BTUs, the NRC66DVNG has a maximum flow rate of 6.6 GPM under optimal water temperature. This is the equivalent of roughly three standard showerheads and a sink running in tandem.

Unfortunately, it’s got a fairly long lag time from when you first turn on the tap or your shower until the water gets up to set temperature.

There are two reasons why this heater is considered as having “high efficiency”. The first reason is that the heater is Energy Star certified and has an impressive Energy Factor of 1.0. 

Secondly, it has a condensing design. As previously explained, instead of wasting the hot steam that would normally be vented back into the environment, the NRC66DVNG reroutes the steam back through the unit to allow the inflow water to soak up the excess heat. This won’t just give the heater a much-needed boost in heating time and efficiency, it also makes the final gas that’s to be vented considerably cooler. Instead of investing in an expensive stainless steel vent, you can vent the unit using a 3” Schedule 40/80 PVC, PP, or CPVC, which are all cheaper.

The heater is electronically controlled by an on-board computer and the unit ships with a remote thermostat for you to control temperature and settings.

In terms of safety, the heater has low NOx emission and won’t negatively affect the health of people in the family and the surrounding atmosphere.  The array of safety valves to control temperature is also on-board.

As for warranty, you have a 12-year period on any problems regarding the heat exchanger, a 5-year on parts, and a 1-year on labor.

4. Takagi T-H3-DV-N – Best Whole House Gas Tankless Water Heater

The Takagi T-H3-DV-N is a natural gas-fired heater with enough capacity to power a sizable household while still remaining relatively affordable. It comes with all of the bells and whistles you would normally expect from higher-end heaters.

Takagi T-H3-DV-N Tankless Water Heater


  • Powerful performance
  • Energy Star-qualified
  • SCAQMD-qualified
  • Computerized safety mechanisms
  • Durable design
  • Comprehensive warranty


  • Long lag time

The burner has a maximum gas input of 199,000 BTUs and is capable of producing an ample flow of 10 GPM at optimal temperature. With this kind of performance, you can power four bathrooms if you have warm groundwater in your area and approximately two to three bathrooms in colder regions. As a condensing heater it is also highly energy efficient, and has an Energy Star label to attest to its efficiency with an EF of 0.91.

A notable feature in the Takagi T-H3-DV-N’s is the computerized safety mechanisms. This means that you won’t need to do manual checks regularly or have to bother with pilot lights. Instead, the computers will regulate the heater’s operation from top to bottom and immediately notify you if it detects a problem. It can also automatically shut the heater unit down when severe malfunctions occur, protecting the unit along with the house.

The heater qualifies for the 2012 SCAQMD rule 1146.2, which means that it has low NOx emission.

Warranty is 15-year on the heat exchanger, 5-year on parts, and 1-year on labor.

5. Camplux BW264 10L Outdoor – Best Outdoor Tankless Gas Water Heater

For such a compact and versatile tankless gas heater, the great heating performance that you can extract from this unit makes it an excellent purchase for outdoor use. Additionally, all of these values also make the BW264 a brilliant heater for sailors and RV-ers.

Camplux 5L 1.32 GPM Portable Tankless Water Heater


  • Portable
  • Highly adaptable and versatile
  • Adequate performance for outdoor use
  • Packed with safety features


  • Limited warranty policy
  • Ineffective at high altitude

This propane-fired heater has a maximum output of 1.6 GPM, which is enough to fulfill the demand of one water-saving showerhead or a small tap. Unlike your indoor heaters that require complex electrical wirings and a full power grid, the BW264 can be kick-started by two D-cell batteries. The burner releases about 68,000 BTUs per hour and draws fuel from portable propane canisters (make sure that the propane canisters are properly pressurized at 0.4 PSI.)

Aside from the ability to ignite independently from a full power source, it also has a low start-up pressure at about 3.0 PSI. Since you can’t really trust the campground or trailer park’s water supply to be reliable all the time, this is a major advantage. Additionally, if you find the water supply to be too under-pressurized, the heater can operate in tandem with a 12-volt water pump to improve pressure.

It’s integrated with plenty of safety mechanisms, including a flame failure device that can cut off the gas supply and prevent volatile gas from reaching the burner when you push the off button. There’s also an overheating protection sensor to keep the burner from being damaged by excess heat. For use in frigid regions, it’s also equipped with an anti-freezing feature.

Keep in mind, however, that the heater shouldn’t be used to produce drinking water. It doesn’t have a filter and the heat treatment will not be enough to make the water safe to drink. In case you plan to go mountaineering with the heater, it shouldn’t be used at an altitude higher than 2,000 feet. If you live in high regions, it is best that you check your altitude before you invest into this heater.

Camplux only has a one-year warranty on the BW264, unlike other manufacturers with decade-long warranties on their units. But you shouldn’t worry too much about this: most consumers attest that they got as much as four years out of the heater before needing a replacement.

Gas Water Heater Safety Tips

Even though the reliability, inherent safety, and safety features of tankless gas heaters are as close to excellence as could be nowadays, it’s still good to develop a habit of safety. 

Make Sure there is Proper Ventilation

Improperly installed or placed vents, vents that are plagued with drafts, or defunct vents can cause serious harm. The noxious fumes could be drawn back and spread indoors rather than being ventilated into the open air outside.

Although installing and maintaining the vent is strictly technical and will require a professional’s touch, you can still check the vent’s air flow to see whether it is going up or down. You can also see for yourself whether the vent has been fitted properly by seeing if its diameter is the same as the heater’s diverter.

Regularly Check the Safety Valves

The temperature and pressure relief valves are there to protect the unit from being damaged in case the water temperature or pressure goes beyond the heater’s limit. Like previously mentioned, some tankless heaters have pressure valves, some don’t. Check the user’s manual to know for sure whether your heater model has one as well as where it is located in the unit.

At least once a year, you should turn the handle of the valve to see whether it is working properly. Water should drip freely from out of the unit (careful while you’re doing this, as the water may be scalding hot) and stop when you close up the handle. If it doesn’t behave this way then immediately contact your plumber as your valve may have malfunctioned. 

Consider Having a Carbon Monoxide Detector

Because both natural gas and propane release carbon monoxide when burned, which is toxic to the human’s respiratory system, it is important to have a carbon monoxide detector in the house. The gas is odourless and colorless and doesn’t react biologically with your body, meaning you won’t realize that it is leaking in your house until it is far too late.

According to the Consumer Product Safety Commission, carbon monoxide poisoning kills about 200 people each year and hospitalizes 10,000 others. Therefore, you should be very careful while dealing with this problem. A detector is a good place to start.


That concludes our buying guide and review article for this week on the best tankless gas water heaters! We hope that you have found the information to be useful and has helped assist you in shopping for that new, perfect heater for your home. As ever, if you have any questions or comments, please feel free to share those in the space below. Until next time!

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