Yes, you can, but plan for scale. Hard water leaves mineral deposits inside a tankless heater's heat exchanger, so expect at least yearly descaling and consider a water softener or scale prevention system. Check the manufacturer's manual for hardness limits and warranty terms before you buy.
Why does hard water matter more for a tankless heater?
Hard water carries dissolved calcium and magnesium. Penn State Extension notes that these are highly soluble minerals and that they build up on contact surfaces, possibly plugging pipes and damaging water heaters. The University of Nebraska-Lincoln explains the heat part: in a hot water heater, heat removes some calcium carbonate and magnesium carbonate from the water, resulting in scale buildup, which can slow the heating process and increase energy use.
A tank heater collects a lot of that scale as sediment at the bottom of the tank. A tankless heater has no such tank. Water passes through a heat exchanger, and Navien describes what happens over time: the waterways of a tankless water heater build up mineral deposits much like the lime in a coffee maker. Navien says these excess deposits are important to remove because they can erode and clog the walls inside the heat exchanger, reducing efficiency, reducing flow, lowering overall performance and potentially causing premature failures.
That is the core risk. It is not that a tankless heater cannot work with hard water. It is that scale is the main enemy, and it works slowly and out of sight. Our how hard water damages plumbing page shows the same pattern in pipes and fixtures.
### How hard is the water where I live?
Phoenix water is hard by most measures. Our how hard is Phoenix water page gives the numbers, and the number matters for your decision. Penn State says that levels below 7.0 grains per gallon will probably not cause major scaling and soap film, so a home well above that line should expect scale to show up.
The best way to know your own number is to test it. Penn State and Minnesota both suggest a test kit or an independent laboratory. Write down the reading before you buy so you can compare it with the heater manufacturer's guidance. If a lab report lists hardness in milligrams per liter, Penn State explains that this is the same as parts per million, and that grains per gallon is the other common unit, so convert carefully before you compare numbers.
How often should I descale a tankless heater?
Navien recommends descaling a tankless water heater at least once a year. It also says the frequency can vary: the higher the mineral content of your water, the higher the set water temperature and the more hot water your home uses, the more often you will need to descale. With hard water, plan on the annual schedule as a minimum, and ask your installer whether your model and your water call for more.
Descaling, which Navien also calls flushing, involves circulating an approved descaling solution or household white vinegar through the unit. Navien's instructions call for a solution circulating for at least 45 minutes and a fresh water flush for a minimum of 10 minutes afterward. The equipment it lists includes a descaling solution, drain hoses, and a 5-gallon collection bucket.
Navien says descaling can be done by a professional or by a homeowner with the right equipment, materials and skills, but it recommends hiring a professional. Its reasons: it takes plumbing experience, and if descaling is done incorrectly and the water heater is damaged, you could void the warranty. Our how often to descale a tankless water heater in Phoenix page has more on scheduling.
What can I do to reduce scale?
You have three broad choices, and they can be combined.
Soften the water. A whole-house softener removes the calcium and magnesium before they reach the heater. The University of Nebraska-Lincoln says ion exchange can remove nearly all the calcium and magnesium from source water. Penn State adds that scale can build up on water heaters and decrease their useful life, and it counts protecting appliances among the advantages of softening. A softener adds its own upkeep, so read how to maintain a water softener. Minnesota's health department also raises points to weigh, such as extra sodium in drinking water for people on restricted diets, and environmental concerns about salt discharge.
Use a scale prevention system. Some manufacturers sell chemical-free scale prevention systems designed to keep scale from forming. Navien, for example, describes its own PeakFlow system as a chemical-free water treatment system that prevents scaling before it starts. Ask any maker to show its published results and the conditions it was tested under, and confirm that the product suits your model.
Descale on schedule. This is the minimum, and it works alongside the other two. A heater that is descaled once a year and protected by treatment will last longer and run more efficiently than one that is neglected.
What should I check before I buy?
- The manufacturer's water quality limits. Ask for the written hardness guidance for the exact model, and what the warranty says about scale damage. Navien's guidance treats regular descaling as part of routine maintenance, and you should expect other makers to have similar terms.
- A service access plan. Descaling needs isolation valves and service ports so that a technician or homeowner can connect hoses. Ask the installer to include them.
- Your usage. Navien notes that higher temperatures and heavier hot water use raise the descaling frequency. A large household on a high set point should plan more maintenance.
- A comparison with a tank. Tank heaters have their own hard water issue with sediment and anode rod wear. Our will hard water ruin a hot water heater page compares the two.
- Electric or gas. Arizona homes vary. Read do electric tankless water heaters work in Arizona before you choose a fuel.
### What happens if I never descale?
Navien warns that neglecting the routine can lead to serious problems over time. Its list: mineral buildup can damage the heat exchanger and may interfere with normal heating, and the heat exchanger could malfunction and overheat the water in your system, causing damage and premature failure of the unit. Less dramatic signs come first, such as slower heating, lower flow and error codes. Our common tankless water heater problems page lists them.
So, should I get one?
Yes, if you plan for it. A tankless heater gives you hot water on demand and frees up floor space, and hard water should not stop you. It should shape the plan. Budget for a yearly descale, decide up front whether you want a softener or scale prevention, and get the manufacturer's hardness guidance in writing. If you are unsure, ask a licensed plumber to test your water and explain the tradeoffs for your home.
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