What Is The Most Common Issue With Tankless Water Heaters?

Homeowners in Modesto love the space savings and endless hot water that a tankless unit brings. Yet even strong systems have weak points. The most common issue we diagnose with tankless water heaters is scale buildup that triggers error codes, reduces hot water flow, and shortens component life. Hard water is the driver, and our region has plenty of it. If you have to reset your unit, wait through cold-water bursts, or watch your unit “short-cycle,” scale is the first place to look.

This article explains what scale does inside a tankless heater, how it ties to hiccups like temperature fluctuation and ignition trouble, and what you can do to prevent it. You’ll also see when a quick flush fixes the problem and when a professional technician should step in. If you’re searching for tankless water heater services near me in Modesto, Riverbank, Salida, or Empire, this walk-through will help you decide the next best move.

Why hard water causes the number-one issue

Stanislaus County water often measures in the hard range, which means it carries calcium and magnesium. When water heats, minerals fall out of solution and form scale. In a tankless unit, that scale sticks to the heat exchanger and clogs inlet screens, mixing valves, and flow sensors. The heater has to fire harder to reach set temperature, then overshoots, then throttles. This creates uneven water temperatures, wider swings between hot and cold, higher gas or electric consumption, and stress on the heat exchanger.

Over months, that scale narrows the water path. Your unit senses low flow and throws a code. Many brands label it as a flow error or heat exchanger temperature rise error. Some models lock out to protect themselves. Homeowners often describe it as “the shower went lukewarm, then cold, then the heater shut off.”

The early signs that scale is building

Before a unit throws a code, it leaves clues. We see the same pattern across Modesto homes, from College Area bungalows to newer builds off Pelandale Avenue.

• Water temperature wavers during a shower, especially when someone turns on a faucet elsewhere in the house.
• The hot water comes on slower than it used to, even though the distance to the bathroom has not changed.
• The unit is louder on startup, with more frequent cycling on and off.
• You notice small bursts of cold water during long showers.
• Your gas or electric bill creeps up without any change in use.

These symptoms also show up with venting issues and undersized gas lines, but in our area, scale is the most common culprit. If your tankless system is more than a year old and has never been flushed, assume scale is at play.

Why scale leads to so many different problems

A tankless water heater is a small, controlled combustion or high-output electric system. The burners or elements heat water that passes through a compact heat exchanger. For the unit to work, it must measure water flow precisely, modulate flame or current smoothly, and vent safely. Scale interferes with each of those steps.

Flow and temperature sensors Knights Plumbing and Drain tankless water heater services near me lose accuracy when mineral deposits coat their surfaces. The heat exchanger transfers energy less effectively with scale blocking thermal contact. The unit compensates by raising flame intensity or power output, which can cause temperature overshoot and then a safety throttle. The result is the familiar “hot then cold” yo-yo. In severe cases, scale creates hot spots that stress the heat exchanger metal. That shortens the life of a very expensive part.

In short, scale is the root condition that shows up as poor temperature stability, ignition failure, error codes, and full shutdowns.

A Modesto-specific view: water chemistry and usage patterns

We maintain systems across Modesto, from La Loma to Village One, as well as Salida and Riverbank. In these neighborhoods, water hardness often sits high enough to create noticeable scale in as little as 6 to 12 months without maintenance. Households with teenagers or multi-generational families run even more hot water, which accelerates deposition.

We also see issues in homes with oversized showers and body jets. High flow demands push a tankless unit close to maximum output. Add scale to the mix and the unit cannot hold setpoint. If your showers in East Modesto run five to ten minutes and then cool off, this mix of high demand plus scale is likely why.

What a proper descaling looks like

A thorough flush involves more than hooking up hoses and calling it done. The process matters if you want a lasting fix.

We power down the unit and isolate it using the service valves. We connect a pump and circulate a food-grade descaling solution through the heat exchanger for 30 to 60 minutes, depending on buildup. We monitor pH and flow, then rinse with clean water until the discharge runs clear and neutral. We clean the cold-water inlet screen, check the domestic recirculation filter if the unit has one, and inspect the pressure relief valve.

After flushing, we test combustion or electrical draw, verify CO levels for gas units, check venting, and confirm outlet temperature stability with a digital thermometer. On many service calls in Modesto, the outlet temperature holds within 2 to 3 degrees after a proper flush, where before it would swing by 10 to 20 degrees.

Other common complaints that stem from scale

Temperature swings lead the list, but we frequently fix three related issues. First, ignition failure or flame loss on gas models. Scale causes slow heat transfer and creates higher exhaust temperatures, which can trip safeties. Second, flow sensor errors. If mineral crystals roughen the sensor path, the unit reads low flow and shuts down. Third, noisy operation. Whistling or rattling often comes from restricted paths and unstable flame due to poor modulation against a scaled exchanger.

When we pull the cover on Modesto units that show these problems, nine times out of ten we find white mineral deposits on fittings and in the exchanger. After a descaling and sensor cleaning, the unit runs quietly and reliably again.

When it is not scale: edge cases worth calling out

As common as scale is, we always rule out a few other causes.

• Gas supply undersized or pressure too low. We measure inlet pressure at rest and under load. If your home has a long run of half-inch line feeding a high-BTU unit, the burner starves under demand. We see this in older West Modesto homes that added a tankless unit without changing gas piping.
• Venting issues. Negative pressure in tight houses or blocked vent terminations can kill flame stability. Bird nests and debris in terminations are more common than many expect.
• Cold-water sandwich. This is a normal effect of hot water left in the line between cycles. A recirculation pump or mixing strategy can reduce it. Don’t confuse it with broad temperature swings.
• System sizing. A 120,000 BTU unit serving multiple bathrooms and a laundry room may never hold temperature during peak use. We check fixture counts and flow rates before we blame the heater.

We test these factors on every visit so you get the right fix, not just a flush and a hope.

How often should you flush a tankless unit in Modesto?

Most manufacturers recommend annual descaling. Given our water hardness, many homes benefit from a six to nine-month interval, especially if you:

• Use a lot of hot water daily, such as large families or in-law units on one system.
• Run a recirculation loop, which increases total heated water volume.
• See early signs like slow hot-water delivery or minor temperature drift.

If you add a whole-home softener, you can extend the interval. We still inspect yearly because even softened water can carry small amounts of minerals and because filters, vents, and condensate drains need attention.

Quick homeowner checks before you call

This short list can save you time. If these steps do not help, a pro service visit is your best move.

• Clean faucet aerators and showerheads to rule out fixture restrictions.
• Check your tankless unit’s inlet screen. Many models have a small mesh filter at the cold-water inlet. Shut water off, remove, rinse, and reinstall.
• Verify the set temperature on the controller. Many homeowners accidentally drop it to the 100 to 105 range.
• Look for visible error codes on the display. Note them for the technician.

Stop there. Avoid opening sealed combustion chambers or disassembling venting. Those tasks need special tools and safety checks.

Cost and time: what to expect from a professional flush

In the Modesto area, a descaling service with inspection commonly takes 60 to 90 minutes. Pricing varies by unit type and access, and whether we add parts like sensors or gaskets. As a general range, homeowners budget for a straightforward flush and tune in the low to mid hundreds. If the heat exchanger shows severe scaling or corrosion, additional work may be recommended. We provide upfront pricing on-site after a quick evaluation, so you know the number before we begin.

The value is simple: cleaner heat exchangers restore performance and reduce stress on the system, which protects your investment. A heat exchanger replacement can cost several times more than routine maintenance. Regular descaling keeps you off that path.

Preventive options that work in Modesto

You can reduce scale and extend the life of your tankless unit with a few practical steps.

Set realistic water temperatures. Many households are comfortable at 120 degrees. Higher setpoints accelerate scale formation. If you need hotter water for a specific fixture, we can discuss mixing solutions that keep the heater at a lower, gentler setting.

Consider a whole-home water softener or a scale-inhibition system. Softening removes calcium and magnesium ions, which cuts down deposits dramatically. If a traditional softener does not fit your goals, we can review scale media systems that reduce adhesion. These do not remove hardness but can lower buildup. We install both options routinely in neighborhoods from Village One to North Modesto and can explain the trade-offs clearly at your home.

Schedule maintenance based on your usage. A condo in Downtown Modesto with one bath and light laundry can often stay on an annual cycle. A four-bath home with a recirculation loop near Sylvan District might need twice per year. We set reminders so you do not have to track it.

Keep combustion air and vents clear. Trim shrubs near vent terminations and avoid storing items against the unit. Good airflow supports clean burning and steady temperatures.

What happens during a Knights Plumbing and Drain service visit

We start with a short conversation about your symptoms and usage patterns. Then we run the unit under real demand to reproduce the issue. We measure temperature rise, verify flow, and check for error codes. Next, we inspect the heat exchanger, sensors, inlet screen, and recirculation components if present.

If scale is evident, we perform a descaling flush, clean screens, and reset the unit. We test gas pressure or electrical draw, check venting, and confirm stable outlet temperature under a typical household load. If we find upstream concerns such as low gas pressure or a restrictive vent, we present options. You get clear notes about what we did and what to watch next.

For many Modesto homes, this visit is enough to restore consistent hot water the same day. If you searched for tankless water heater services near me, this is likely the service you need.

Real example from a Modesto home

A family near La Loma Park called about showers that turned lukewarm after five minutes. The tankless unit was five years old with no maintenance history. We found temperature swings of 15 degrees during a ten-minute test. The inlet screen was partially clogged, and the heat exchanger showed heavy scale.

We flushed the system for 50 minutes until the return line cleared. After cleaning the screen and confirming gas pressure, the temperature swings dropped to 2 degrees. We recommended a six-month flush cycle and discussed a softener. They chose to install a scale-reduction system instead. Six months later, the follow-up flush was quicker, and the exchanger stayed clean. Their showers stayed steady.

How to tell if your tankless heater is sized and configured correctly

Because scale often exposes sizing gaps, we check that your unit matches your home’s demands. We consider total fixture flow rate, groundwater temperature, and target output temperature. In Modesto, winter groundwater can dip enough to reduce the unit’s effective capacity. If you have added bathrooms since the original install, the heater may be undersized for peak use. We can confirm if a recirculation loop, buffer tank, or an upgrade would stabilize your system. Small configuration changes, such as adding a buffer tank, can reduce cold-water bursts without replacing the main unit.

Safety notes worth reading

Never bypass safety devices. Do not jumper sensors or prop open covers to force operation. Tankless units rely on accurate sensor data and sealed combustion. If you smell gas, shut down the unit and call for service. If you see water leaking from the heat exchanger area, shut off water to the unit to prevent internal damage.

Our technicians are licensed and carry combustion analyzers, manometers, and electrical testers. We test before and after service so your system runs safely and within manufacturer specs.

When to book service now

If your tankless heater shows one or more of these signs, schedule a visit:

• Repeated temperature swings during normal showers or dishwashing
• Error codes related to flow, temperature rise, or ignition
• A year or more since the last flush, especially with hard water
• Noisy operation during startup or under steady demand
• Hot water that lags far longer than it used to

We offer same-day and next-day appointments across Modesto, Salida, Riverbank, Empire, and nearby communities. Search tankless water heater services near me and look for Knights Plumbing and Drain, or call us directly to get on the schedule.

Why choose Knights Plumbing and Drain

You want fast diagnosis, clear pricing, and a fix that lasts. Our team services all major brands and models. We carry the right pumps, solutions, gaskets, and sensors on our trucks so most descaling and repairs finish in one visit. We tune settings based on your real household use rather than generic defaults. That is why our Modesto clients call us back for maintenance, not repeat repairs.

We also keep the conversation simple. We explain what failed, why it happened, and what will prevent a repeat. If that means a six-month flush cycle, we set reminders. If a softener will save you money in the long run, we provide the numbers so you can decide.

Ready for steady hot water again?

If your tankless heater is turning showers into a guessing game, scale is likely to blame. A professional flush and tune will usually restore stable temperatures and quiet operation. If you are in Modesto or the surrounding neighborhoods and need reliable tankless water heater services near me, Knights Plumbing and Drain is ready to help. Call or book online today, and get your hot water working the way it should.