How Untreated Well Water Affects a Tankless Water Heater

Your tankless water heater was supposed to be an upgrade, offering endless hot water and lower energy bills in a unit that takes up a fraction of the space. But a few months in, the hot water pressure feels weaker and the temperature swings between comfortable and scalding. The display may flash an error code you’ve never seen before.

Instead of the water pump unit failing, your well water may be corroding it from the inside.

Across Maryland, Virginia, and West Virginia, well water chemistry varies from one property to another. Two neighbors a quarter mile apart can have completely different hardness levels, iron concentrations, and sediment loads. When that water runs untreated through a tankless water heater, problems can start quietly and get expensive fast.

Why Tankless Units Are Sensitive to Well Water

In a traditional storage tank water heater, which in most homes hold 40 or 50 gallons, sediment settles to the bottom, and minerals have room to spread out. 

A tankless unit works differently. It lacks the large tank that serves as a settling basin, and instead pushes water through a narrow heat exchanger at high velocity every time you open a hot water tap. The minerals in your well water tend to concentrate on those exchanger walls instead of dispersing through a large volume of standing water.

Fine sand, silt, and particulate iron that would sink in a storage tank travel straight through the tankless system, hitting inlet screens, flow sensors, and the exchanger itself. The unit works harder than it should from day one, and the damage accumulates with every gallon.

What Hardness and Scale Do Inside the Unit

Calcium and magnesium cause hard water, and they are common in the aquifers across the region. The Maryland Geological Survey has documented that groundwater in the Frederick and Hagerstown Valleys consistently tests as hard to very hard because of the limestone and dolomite bedrock underneath. 

For reference, the USGS classifies anything above 180 mg/L of calcium carbonate as very hard. And many wells in these valleys exceed that threshold.

If your well water is untreated, then when the burner in your tankless water heater fires, those minerals can bake onto the heat exchanger surface. The scale that develops acts as an insulator, forcing the unit to run longer and burn hotter to reach the same setpoint. 

A 2009 study by Battelle Memorial Institute, commissioned by the Water Quality Research Foundation, found that tankless water heaters running on water at 26 grains per gallon of hardness had to be delimed after the equivalent of 1.6 years of use, and did not return to their original efficiency.

Manufacturers know this. Rinnai’s installation manuals cap recommended hardness at 200 mg/L (roughly 11.7 grains per gallon) and exclude scale damage from warranty coverage. Both Navien and Noritz recommend professional descaling at least once a year, with more frequent service in hard water areas. 

Iron, Manganese, and Sediment Problems

Orange and rust-colored staining on your fixtures can be caused by iron concentrations as low as 0.3 parts per million. As the Water Quality Association notes, iron bacteria flourish in iron-rich water and clog pipes and fixtures over time. This reduces flow rates through the plumbing feeding your tankless unit.

At just 0.05 ppm, manganese produces dark brown or black discoloration on fixtures and inside plumbing. But beyond staining, manganese forms black oxide deposits that coat the tankless unit’s temperature and flow sensors. 

The National Ground Water Association flags the risk of plugging at manganese concentrations as low as 0.02 ppm, which is well below the EPA’s 0.05 ppm secondary standard. Those deposits also don’t respond well to the standard vinegar flush that clears calcium scale, so a homeowner who descales on schedule can still have manganese fouling the unit.

Wells can also introduce fine sediment such as sand, silt, and particulate minerals, which can foul the inlet screen that protects the heat exchanger. Once that screen is partially blocked, the unit’s flow sensor reads a lower volume, and the burner may not fire at all or fire erratically. 

Signs Your Water Is Working Against the Heater

If your tankless heater seems to struggle, a few patterns point to water chemistry rather than equipment failure. Longer waits for hot water to reach the tap, especially if the delay has become gradually worse, often signal a scaled heat exchanger restricting flow. Temperature fluctuations during use, where the water shifts between hot and lukewarm, point in the same direction. 

Error codes that appear more frequently, especially those tied to ignition, flow detection, or overheating, often trace back to scale or sediment interference.

Outside the heater, look at your fixtures. White, chalky buildup on faucet aerators, spotting on glassware after a full dishwasher cycle, and laundry that comes out stiff or dingy all indicate elevated hardness. If those signs appear alongside heater performance issues, water is the common denominator.

Treating the Water Rather Than Replacing the Heater

When a water heater starts underperforming, the instinct is to call for a repair or price out a replacement. But if the water itself is the problem, a new unit will follow a similar path. Water treatment is the fix that actually lasts.

What treatment looks like depends on what your water contains. A water softener addresses hardness. Filtration handles iron, manganese, and sediment. Conditioning systems tackle multiple issues in sequence. 

The right approach starts with knowing exactly what the water contains. Tri-County Pump Service, Inc. offers a free in-home water analysis that tests for hardness, iron, and pH. For concerns beyond those three parameters, lab-based testing is available at an additional cost. With more than 30 years of experience working with well systems across Maryland, Virginia, and West Virginia, our technicians match treatment to what the test actually reveals for your property.

If your tankless water heater is underperforming and you are on well water, the answer might not just be a new heater. It might be treating the water running through it. Call Tri-County Pumps at (301) 882-2776 or schedule a water analysis online to find out what your well water is doing to your equipment. We’re here when you need us. Let’s Keep It Flowing®.