How to Protect Your Tankless Water Heater From Hard Water

Hard Water and Water Softeners

Many experts say that hard water is ​​the number one enemy of an on-demand water heater system, and that certainly could be true provided you are using a sediment filter. The level of "hardness" in water is determined by the amount of dissolved minerals within the water supply. Different geographical areas are prone to different levels of water hardness.

​​Rain water is called soft water because it's relatively pure and mineral free. As rain water is absorbed into the ground it passes through soil and rock on its way to rivers and other bodies of water.

Along this journey, the soft rain water changes to hard water as the minerals from the ground are dissolved into the water. Hard water contains a high concentration of natural minerals, particularly calcium carbonate and magnesium.

  • Hard water is not a health concern but it does cause problems for many home appliances. 

​​​When mineral laden hard water enters our homes, a scaly build-up begins to form as some of the minerals attach to the surfaces that come in contact with the water. Over time, the build-up of minerals becomes a hard and scaly layer called limescale.

Limescale build-up causes a tankless water heater's heat exchanger to work harder than it should in order to bring the cool water entering the unit to the desired hot temperature.

​Eventually, the heat exchanger will over heat due to the increased workload. In many cases, an error code is triggered and the unit will be automatically shut down. Service and/or repair may be required to get the tankless unit operating again.

Protecting Your Water Heater From Hard Water

The effects of hard water can be problematic for all water heaters, not just tankless models. Flushing both traditional and tankless units will help remove the limescale build-up once it has already developed. However, installing a Scale Inhibitor System can help reduce the limescale build-up from forming in the first place. This video explains how a Scale Inhibitor System works:

A Scale Inhibitor System is a popular, relatively inexpensive method of protecting traditional and tankless water heaters from hard water.

The filter is installed on the incoming water supply BEFORE it enters the water heater. A shut off valve should be added to the pipe on each side of the filter to allow for ease of filter replacement. 

The Aqua-Pure Scale Inhibitor will not only help prevent limescale build-up on a water heater, but it will also protect your faucets and other water-using appliances. 

Aqua-Pure AP430SS Hot Water System Protector
  • Binds calcium and magnesium hardness minerals, preventing them from precipitating out of solution and building up on metallic surfaces
  • Inlet/Outlet Size:3/4 inch NPT,Dimensions height 10 1/4 inch(26.0),Diameter 2 3/4 inch.Contains siliphos, a 100% Pure FOOD Grade scale inhibitor approved by usda
  • Stainless Steel head
  • Flow rate 6 gallons per minute
  • Max operating Temp. 100F

Hard water will not only impact your water heater, it will also take years of service life from your faucets, dishwasher, washing machine and even your toilet!

A good first step is to test your water for hardness. This nifty test kit from Amazon can test your water in only a few seconds.

Since installing a water softener system can be a major expense, and there are companies who may be less than honest with their sales tactics, it's a good idea to know the hardness of your water upfront.


Industrial Test Systems WaterWorks 481108 Total Hardness Test Strip, 3 Second Test Time, 0-1000ppm Range (Pack of 30)
  • Total hardness water quality test strips evaluate calcium levels in residential, industrial, or educational applications
  • Tests from 0 to 1,000 ppm with sensitivity indicators at 0, 40, 80, 120, 180, 250, 425, and 1,000 ppm (mg/L) or 0, 2, 5, 7, 11, 15, 25, and 58 gpg
  • Three-second test time for immediate results

Whole-House Water Softeners

For areas with extreme hard water, you may want to consider installing a whole-house water softener. These systems treat all of the water, both cold and hot.

​There are 2 main types of water softeners: Salt Based and Salt Free. Each works differently, and each has advantages and disadvantages.

Salt Based Water Softeners

Salt based water softener have been around for more than 100 years and are likely the most affordable way to treat household water.

These systems completely remove the calcium and magnesium from the water before it enters your plumbing system.

The hard water minerals are replaced with sodium ions,  by binding negatively charged potassium, sodium or hydrogen resin to positively charged metal ions. The process draws the hard minerals from the water.

Fleck 5600 SXT Water Softener Ships Loaded With Resin In Tank For Easy Installation (48,000 Grains, Blue)
  • Manufacture 5 year warranty Valve/ 10 year warranty Tanks
  • Comes standard with Uprgarded Paddle Wheel Meter
  • Ships with resin pre loaded in fiber glass mineral tank for easier installation
  • Free Upgraded Paddle Wheel Meter
  • Made In USA Structural Tank Ships Loaded With Resin In Mineral Tank for easier installation

In other words, as hard water enters your water softener it runs thru resin beads which attracts and traps the "hard mineral" molecules. The water that enters your home is left mineral free. 

​Over time, the resin beads become "full" of the hard water minerals and a process called regeneration needs to occur to keep your water softener working effectively. ​

Salt Free Water Softeners

Salt Free Water Softeners work very differently and are commonly called water conditioners or scale inhibitors because the hard water minerals are not removed from the water.

​Although, salt free water softeners don't reduce the hard water minerals, they will help prevent the build-up of lime scale. There are 2 common types of salt free systems: Catalytic and electromagnetic.

Catalytic Systems

These systems use a process ​called epitaxial crystallization. In theory, the calcium is changed from the type that deposits limescale (calcite) to a type that doesn't (aragonite). ​

The hard water mineral ions are bonded to a chelating agent that suspends them within the water. In other words, the minerals are not removed, but pass thru the water and are stabilized making them unable to cause limescale. 

The Nuvo H20 Salt Free Water Softener is an example of a catalytic system and it's available from Amazon.

This Nuvo is capable of filtering roughly 50,000 gallons of water, which is the average annual water usage for 2 people.

Hard water ions are neutralized and allowed to pass thru your plumbing without limescale build-up.

Nuvo H20 DPHB Home Salt Water Softener System
  • Water flow: 12-15 gpm with 1-Inch ports (no change to existing flow)
  • Capacity: about 50,000 gallons (average annual water usage of household of 2)
  • Uses 20-Inch Salt-Free Replacement Cartridges, Eliminates existing damaging scale-
  • other products may prevent scale buildup but don't remove existing scale, No harmful salt avoid the hassle of constant salt replenishment
  • and protect your health and our environment from the harmful effects of salt used in other softeners.

This video shows how a catalytic water softener system works.

​Electromagnetic Systems

With an electromagnetic system, the ions of the hard water minerals are altered as the water passes through a magnetic field. These systems are very easy to install and many people like that there are no chemicals used in the water treatment process. However, and there is some debate on their effectiveness. 

The Eddy Water Descaler is an example of an electromagnetic salt free water softener. 

This system is easy to install and can be used on both metal and plastic pipes.

Eddy Water Descaler Electronic Water Softener
  • Reduce lime scale in the home and business
  • Easy customer installation - Metal or Plastic pipes
  • No salt alternative to a water softener
  • Emulates softened water without altering TDS (keeps healthy minerals)
  • 12 month 100% Money-Back Guarantee + Lifetime Repair or Replace warranty

This video shows how an electromagnetic water softener system works.

Final Thoughts 

If you live in an area that's prone to hard water, you may want to consider installing a whole house water softener, as these systems will not only protect your tankless unit, but also your other appliances, faucets and plumbing.

All tankless water heaters, and even traditional water heaters, should have a sediment filter installed to filter the incoming water. We recommend installing a scale inhibitor to your water heater, especially if hard water is a concern in your area. 

Keep in mind that without a water softener regular maintenance and flushing can keep your tankless running efficiently. However, these tasks can become burdensome, especially in very hard water climates.

  • Even with a scale inhibitor, we recommend flushing your tankless water heater periodically. 

Be sure to hire a reputable company if you don't plan on doing the work yourself. Keep in mind that there are many different options available to treat hard water. You should be able to find a method that best meets your situation and budget. 

Last update on 2018-06-23 / Affiliate links / Images from Amazon Product Advertising API