A tankless water heater makes hot water only when a tap or appliance calls for it, so it has no storage tank of heated water sitting ready. Instead, cold water passes through a heat exchanger, a set of narrow internal passages heated by a gas burner or electric elements. In homes with hard water, dissolved minerals, mostly calcium and magnesium, can come out of solution when they are heated and settle as scale on those surfaces. Over time that scale can narrow the passages and make the unit work harder.
This guide is for homeowners in St. Louis and nearby communities who want to understand what scale looks like in a tankless unit, how to check whether your water is a likely cause, and what routine care the manufacturer's instructions usually cover. It does not replace the manual that came with your unit.
How Scale Builds Up
Scale forms most where the water is hottest. The heat exchanger is the most common place, and the burner or element area can collect deposits too. Because the unit heats water only while it runs, scale can build up gradually across many short uses. Homes with frequent short draws may see buildup at a different pace than homes with long showers and large laundry loads, but the pace depends on the water supply and the unit, so no timeline applies to every house.
Signs to Watch For
Scale rarely causes a sudden failure. More often, a homeowner notices changes over months:
- Hot water that seems to arrive more slowly or at a lower temperature than before
- The unit running longer or cycling on and off more often during a shower
- Rumbling, popping, or banging sounds during heating
- Fault or error codes on the display, which the manual should explain
- Reduced flow at one or more fixtures while others seem normal
Any of these can have other causes, including a clogged inlet screen, a faulty sensor, or gas supply trouble. Treat them as a reason to check the unit, not as proof of scale.
Check Your Water First
Many homes in and around St. Louis have hard water, but whether yours does depends on your own supply. Two simple checks help. First, review your home's water quality report or ask your water provider for the most recent annual report, which may list hardness. Second, use a home test kit, which typically uses test strips or a drop-based method to give a general hardness reading. Compare the result with the scale guidance in your unit's manual or the manufacturer's literature. The test tells you whether scale is a likely factor before you decide on any next step.
Flushing a Unit Safely
Many tankless units have a service mode or maintenance connections that let you circulate a descaling solution through the heat exchanger. The manual names the approved solution and the steps. In general:
- Shut off power to an electric unit at its breaker, or set a gas unit to off using its controls, following the manual's shutdown steps.
- Close the isolation valves on the cold inlet and hot outlet if your unit has them.
- Connect the hoses and pump as the manual shows, keeping the solution contained in a bucket.
- Circulate the solution for the time the manual gives, then rinse thoroughly with clean water.
- Restore the valves and power, then test hot water at a nearby tap.
Wear eye protection and gloves, and keep children and pets away from the work area. Never mix descaling solutions with other cleaners, and do not run the unit dry.
When to Call a Technician
Stop and contact a licensed plumber or appliance technician if your unit has no isolation valves or service connections, if the manual calls for steps you are not comfortable performing, or if you see error codes that persist or signs of overheating. If you smell gas, leave the home and call your gas utility or the emergency number right away. A qualified professional can inspect the heat exchanger, check the water supply, and advise whether a different approach fits your home. Keep the manual and any service records together so the next visit goes faster.
Keeping Records
A simple log of the date you tested your water, any error codes, and any flushing work makes patterns easier to spot. Over a few seasons, you may find that scale symptoms follow changes in your water supply, seasonal temperatures, or household habits. That record is useful to you and to any technician you bring in later.