Can Tankless Water Heaters be Used in Areas with Hard Water?

I was recently asked about using a tankless water heater in a hard water area. In this Tankless Guide post, we take a look at hard water and your tankless water heater. Below is a simple process for determining whether or not you have a hard water situation and what you can do to reduce its effects.

What is hard Water?

No its not ice cubes. The technical description of hard water is based on the number of grains of hardness per gallon of water. OK so what is a grain? Well a grain of hardness is the amount of magnesium and calcium equal to the weight of a kernel of wheat. What is all translates to is

  • Less than 1.0 = Soft
  • 1.0 – 3.5 = Slight Hardness
  • 3.5 – 7.0 = Moderately Hard
  • 7.0 – 10 = Hard
  • 10.0 and Over = Very Hard Water

Hard water is very common

Nearly 85% of homes in the United States have hard water, according to the U.S. Geological Survey, and the figure is comparable for Canada.  Hard water can be a serious problem for appliances that use water, as calcium scale and lime accumulates in water intakes and pumps, making it hard to get the water it needs or eventually causing mechanical failure.

Very hard water can be a problem for tankless water heaters too.  A buildup of sediment can reduce the GPM you’re getting  and eventually damage the unit.  Read more on the importance of flow rate . Tankless water heater manufacturers like Navien, Titan, Rheem recommend that homes with a hardness of 11 grains per gallon (GPG) use a water softener to treat water before it enters the unit.  The 11 GPG amounts to roughly 210 parts per million (PPM) total dissolved solids, and we mention that because some tests give results in GPG while others give them in PPM.

The problem with hard water is that calcium will precipitate out in hot water, collecting in aerators, washing machine filters, appliance pre-filters and within the water system of appliances.  This can be a serious problem for appliances that use water, as sediment accumulates in water intakes and pumps, making it hard to get the water it needs or eventually causing mechanical failure.

A Quick Home DIY Hard Water Test

  • Take a empty plastic pop or water bottle with cap. 16 ounce or bigger. (make sure it is rinsed clean)
  • Add 8 to 10 ounces of luke warm tap water
  • add about 10 drops of your home dishwasher liquid
  • Put cap back on the bottle and shake the bottle for about 15  seconds
  • If the soapy liquid foams up quickly and leaves a nice foam in the top of the bottle you have good not hard water
  • If instead the water turns cloudy with a soapy film on the sides of the bottle rather then foam you most likely have hard water.

If you find from your your home test you most likely have hard water then I suggest you keep reading and check out some followup tests to confirm your results.

Hard water test kits

There are several ways to test for hard water. Water hardness test strips are available that will give you an accurate reading.  A second option is to use a test kit that can be purchased at local hardware and home improvement stores.  They will test for water hardness and also measure chlorine, nitrate/nitrite, water acidity, iron and more.  Most of these can be done at home.  Very precise water testing is available through your local health department.  They’ll supply you with jars to fill with water and then send off the samples to a lab for precise testing which also looks for bacteria in the water supply.

Test strips or DIY kits are sufficient in most homes to test for water hardness.  If you find that your water qualifies as hard water, especially if it surpasses 11 GPG which would be called very hard water, you should consider investing in a water softener.  Hard water and your tankless water heater, your dishwasher, your sinks, and your plumbing in general is a bad combination.  Hard water will shorten the life of your tankless water heater by as much as 25% to 33%.  The good news is that hard water can be alleviated with a softener, and that scale build-up in tankless water heaters is easier to get rid of than build-up in standard tank-style water heaters.


Dont wait until you have problems with your tankless heater, other appliances or your plumbing.  Test for hard water now and you’ll know whether or not a water softener will be useful in giving your appliances and fixtures the longevity you expect.  To further increase the durability of your tankless water heater, be sure to give it an annual cleaning — a task you can accomplish yourself with a little preparation



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