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How to Protect Your Tankless Water Heater From Hard Water


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Many experts believe that hard water is the number one enemy of an on-demand water heating system. Hard water can cause havoc on your home's appliances, including tankless water heaters. But there are preventative steps you can take to minimize the effects of hard water.

A tankless water heater will develop a build-up called limescale that will negatively impact the unit's efficiency by creating a layer of "insulation" between the heat exchanger and the water it is trying to heat. Eventually, it could cause the tankless unit to throw error codes or shut down all together. But we're getting ahead of ourselves. First, let's look at what hard water is and what causes it in the first place.

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What is Hard Water?

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.

The rain water is absorbed into the ground and travels to rivers and other bodies of water. Along the journey the soft rain water passes through soil and rock and begins to change to hard water as the minerals from the ground are dissolved into the water.

Hard water contains a high concentration of natural minerals particulary calcium carbonate and magnesium. These minerals are not a health concern, but they do cause problems for many home appliances.

Hard Water and 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 overheat 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 water heater operating again.

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Protecting Your Tankless Water Heater from Hard Water

Hard water will take years of service life from your faucets, dishwasher, washing machine and even your toilet! The effects of hard water can be problematic for all water heaters, not just tankless models. We highly recommend purchasing an inexpensive hard water test kit from Amazon so you can test your water yourself and know the hardness of your water. These types of test kits deliver results in only a few seconds.

There are many different ways to protect your tankless water heater from hard water. Below you'll find some of the more common methods.

Scale Inhibitor Systems

Flushing both traditional tank-style and tankless water heaters will help remove the limescale build-up after it has already developed. However, installing a Scale Inhibitor System can help reduce the limescale build-up from forming in the first place. 

A Scale Inhibitor System is a popular and relatively inexpensive method of protecting traditional tank-style 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 the ease of filter replacement. 

The incoming water travels through the Scale Inhibitor System where it's treated with polyphosphates which help inhibit scale build-up within your water heater. 

3M Aqua-Pure Whole House Scale Inhibition Inline Water System AP430SS, Helps Prevent Scale Build Up On Hot Water Heaters and Boilers

3M Aqua-Pure Scale Inhibitor

We really like the Aqua-Pure Scale inhibitor by 3M. It'll not only help prevent limescale build-up within your water heater, but it'll also protect your faucets, shower heads and other hot water-using appliances.

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 that enters your home.

There are 2 main types of water softeners:

  1. Salt Based
  2. Salt Free

Each works differently, and each has advantages and disadvantages. Let's take a closer look at each:

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Salt Based Water Softeners

Salt based water softeners 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. In other words, as hard water enters your water softener it runs through 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. 

Fleck 5600 SXT Water Softener Ships Loaded With Resin In Tank For Easy Installation (48,000 Grains, Blue)

Fleck 5600 SXT Water Softener

Fleck makes an excellent water softener that will protect your entire house from hard water. It includes the resin, brine tank, control head and tank. Add salt once you've installed it and you're set to go.

This video shows how a salt based water softener works.

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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. 

Nuvo H2O Complete Manor & Taste Water Softener & Filter System (Complete System)

Nuvo H2O Salt-Free Water Softener

The Nuvo H20 Salt Free Water Softener is an example of a catalytic system. It's capable of filtering roughly 50,000 gallons of water, which is the average annual water usage for 2 people. The hard water ions are neutralized and allowed to pass thru your plumbing without limescale build-up.

This video shows how a catalytic water softener system works.

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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, 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 with both metal and plastic pipes.

Eddy Electronic Water Descaler - Water Softener Alternative

Eddy Water Descaler

The Eddy Water Descaler is an example of an electromagnetic salt-free water softener. It's easy to install and can be used on both metal and plastic pipes.

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This video shows how an electromagnetic water softener system works.

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Final Thoughts

All tankless water heaters, even traditional tank-style water heaters, should have a sediment filter installed to filter the incoming water. It's important to remember that whatever method you choose to reduce your hard water issues, you'll still need to perform regular maintenance and flushing on your hot water heating system.

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.

Our favorite hard water system is the scale inhibitor, however you should take a close look at each and make the best decision for your individual situation, especially if hard water is a concern in your area.

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. 

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