If you’re a plumber working in Arizona, Nevada, or Texas, you already know the drill. You get a call from a homeowner who’s ready to upgrade to endless hot water. They’ve heard the hype about tankless water heaters—the efficiency, the space savings, the endless supply. But then you look at their zip code, or maybe you just know the local water quality report by heart, and you think: Hard water.
It’s the invisible enemy of plumbing fixtures everywhere.
A common question we hear at the supply counter is: How well do tankless water heaters work with hard water?
The short answer? They work great—until they don’t. But with the right know-how, installation prep, and maintenance plan, you can help your customers enjoy the benefits of tankless systems without the headache of premature failure. Let’s break down exactly what hard water does to these units and how you can be the hero who keeps the hot water flowing.
Understanding the Technology: How Tankless Works
Before we get into the nitty-gritty of mineral deposits, let’s do a quick refresh on the mechanics. Unlike traditional tank-style heaters that keep 40 to 50 gallons of water hot 24/7 (essentially paying to heat water no one is using), tankless units operate on demand.
When a tap is turned on, cold water travels through a pipe into the unit. A gas burner or an electric element heats the water instantly as it passes through a heat exchanger.
The keyword here is exchanger. This component usually involves narrow passageways designed to maximize surface area for rapid heat transfer. While this is brilliant engineering for efficiency, it also happens to be the Achilles’ heel when you introduce mineral-heavy water.
What Exactly is “Hard Water”?
We throw the term around a lot, but what is it chemically? Hard water is simply water that has high mineral content. It’s formed when water percolates through deposits of limestone, chalk, or gypsum, which are largely made up of calcium and magnesium carbonates.
In the regions we serve—especially Arizona, Nevada, and Texas—hard water isn’t the exception; it’s the rule.
Why does this matter to a plumber? Because when hard water is heated, the calcium and magnesium separate from the water and form solid deposits. This is what we call limescale or scale. You’ve seen it crusting over showerheads and leaving spots on glass doors. Now, imagine that same crust forming inside the delicate, high-performance heat exchanger of a tankless unit.
The Potential Problems: When Minerals Attack
So, how well do tankless water heaters work with hard water if left untreated? Not very well for very long. Here is the specific damage hard water inflicts on these systems:
1. The Insulator Effect (Reduced Efficiency)
Scale is a terrible conductor of heat. As limescale builds up on the inside of the heat exchanger, it creates an insulating layer between the heat source (the burner) and the water.
This forces the unit to work harder and burn more fuel to achieve the set temperature. The promised energy efficiency—often the main selling point for the customer—goes right out the window.
2. Overheating and System Stress
Because the burner has to run hotter to penetrate that layer of scale, the metal of the heat exchanger can eventually overheat. This thermal stress causes the metal to become brittle, leading to cracks and leaks.
Most modern tankless units have sensors that detect this rise in temperature. To protect themselves, they will throw an error code and shut down. If your customer calls saying their heater keeps turning off, scale buildup is a prime suspect.
3. Flow Restriction
Remember those narrow passageways we mentioned? As scale accumulates, it physically narrows the path for the water. This restricts flow, leading to a drop in water pressure on the hot side. Eventually, the flow sensor might not even register enough movement to ignite the burner.
Preventative Measures: The Best Defense is a Good Offense
Does this mean you should talk your customers out of tankless units in hard water areas? Absolutely not. It just means the installation package needs to include a strategy for handling the minerals. Here are the best preventative measures to recommend.
Water Softeners
The gold standard solution is a whole-home water softener. By using ion exchange to swap calcium and magnesium ions for sodium ions, a softener eliminates the problem at the source. If the water entering the tankless unit is soft, scale cannot form.
For customers investing in a high-end plumbing system, a softener is the best insurance policy for their appliances.
Scale Inhibitor Systems
If a full softener isn’t in the budget or the customer doesn’t want the maintenance of salt bags, consider a scale inhibitor system. These are cartridge-based filters installed on the cold water line feeding the heater.
They don’t remove the minerals, but they alter the structure of the water or the minerals themselves so they can’t adhere to the heat exchanger walls. It’s a cost-effective middle ground that works well for many households.
Isolation Valves (The Service Kit)
This is non-negotiable. Every tankless installation in a hard water zone must include isolation valves (service valves). These valves allow you to cut off the water supply to the unit and hook up a pump to flush the system without disconnecting the plumbing from the wall. Without these, maintenance becomes a nightmare.
Benefits of Tankless Systems (Even with Hard Water)
Despite the challenges, tankless water heaters remain a fantastic option for our region. Why?
- Longevity: Even with the added maintenance, a well-cared-for tankless unit can last 20+ years, compared to the 10-12 years of a traditional tank.
- Space Savings: In many modern homes or retrofits, reclaiming the square footage of a giant tank is a huge win.
- Endless Supply: For large families, never running out of hot water is a luxury they are willing to maintain.
The key is setting expectations. Let your customers know that how well do tankless water heaters work with hard water depends entirely on how well they treat the equipment.
Maintenance Tips: The Flushing Protocol
This is where you, the professional, come in. Offering an annual maintenance plan is a great way to secure recurring revenue and keep your customers happy.
Here is the standard procedure for flushing a tankless unit to remove scale:
- Safety First: Turn off the gas supply (or breaker for electric units) and shut off the main water inlet and outlet valves on the isolation kit.
- Hook Up: Connect a submersible pump to the cold water service valve using a washing machine hose. Connect another hose to the hot water service valve and run it into a 5-gallon bucket.
- The Solution: Fill the bucket with food-grade white vinegar (undiluted). You can also use specific descaling solutions sold at our supply counters, which often work faster.
- Circulate: Place the pump in the bucket and turn it on. Open the service valves. Let the vinegar circulate through the heater for 45 to 60 minutes. The acidity breaks down the calcium deposits.
- Flush: Turn off the pump, close the service valves, and disconnect the hoses. Crucial step: Open the cold water main valve and the hot water service valve (drain) to flush fresh water through the system for 5 minutes. This clears out any remaining vinegar or loose debris.
- Clean the Filter: Don’t forget the inlet screen filter! Pull it out and rinse off any debris.
- Reset: Close the service valves, open the main hot outlet, turn on the gas/power, and test the system.
Educating the Customer
The biggest friction point often comes from customers who think “tankless” means “maintenance-free.”
When you’re writing up that quote, explain the local water conditions. Use analogies—compare it to changing the oil in their car. They wouldn’t drive 50,000 miles without an oil change and expect the engine to last. The same applies to their water heater.
If you position yourself as a partner in maintaining their home’s health, rather than just someone selling a box, you build trust. And trust leads to those referrals and seasonal maintenance contracts.
The Verdict
So, back to the big question: How well do tankless water heaters work with hard water?
They work excellently, provided you respect the chemistry. In Arizona, Nevada, and Texas, hard water is a fact of life, but it doesn’t have to be a death sentence for plumbing appliances.
By recommending softeners or inhibitors, installing isolation valves every time, and educating your clients on the necessity of annual flushing, you ensure they get the endless hot water they paid for.
Need to stock up on isolation valve kits, descaling solution, or check out the latest tankless models?
Van Marcke Plumbing Supply has you covered. With locations across the Southwest and over 6,000 brand-name products, we’re your local partner in getting the job done right. Check us out online at PlumbersOnly.com or swing by the counter—we’ve got the coffee on.