Tankless units are workhorses. They fire only when you open a tap and they can run for years with little complaint. That silence tricks many homeowners into thinking they never need attention. Then a shower turns lukewarm, or the unit throws a code on a Saturday morning, and the myth cracks. Yes, tankless water heaters need flushing. How often and how much depends on your water quality, your usage, and the model on your wall.
This guide explains why flushing matters, how long it actually takes, what can go wrong if you skip it, and the right maintenance rhythm for homes in Modesto, CA. You will also learn what you can safely do yourself and when to call Knights Plumbing and Drain for tankless hot water heater maintenance. We service Modesto, Riverbank, Salida, Empire, and nearby neighborhoods, and we know the quirks of Stanislaus County water.
Why tankless units scale up faster in Modesto
Tankless heaters push water through a narrow heat exchanger. That metal coil gets very hot during a draw, which encourages minerals to fall out of solution and stick. Modesto’s municipal water is typically hard to very hard. Across the city and surrounding county, hardness often lands in the 12 to 18 grains per gallon range. In homes near dry wells or older mains, hardness can spike even higher. Hard water and high heat create scale. Scale chokes the coil, adds backpressure, and tricks the control board into throttling the flame to protect the unit. The result is lukewarm water, long waits, or error codes.
I’ve opened brand-name tankless units in west Modesto that were only two years old and found the heat exchanger nearly white from mineral buildup. The owners had never flushed because the unit “seemed fine.” It seemed fine because the control logic masked the problem by lowering output. They were paying more in gas to get less hot water.
What flushing actually does
A proper flush circulates a mild acidic solution through the heat exchanger to dissolve calcium and magnesium deposits. We use food-grade white vinegar or manufacturer-approved descaler depending on the model and scale level. The pump runs the solution in a loop for 30 to 60 minutes. That restores waterway diameter and heat transfer. Done right, flushing often drops the unit’s combustion time per draw and improves temperature stability. It also reduces the whistle or buzzing sound you may hear when scale roughens the flow path.
Flushing is not a cure-all. It will not fix a failing flow sensor, a clogged inlet screen, a damaged gas valve, or a condensate trap packed with debris. It will, however, remove the most common cause of declining performance in our area.
Myth check: common misconceptions we see on service calls
- Myth: Tankless heaters are low maintenance, so you “set it and forget it.” Reality: They are low standby waste, not low maintenance. They still need annual service in hard water regions. Myth: If I have a softener, I never have to flush. Reality: A softener helps a lot, but it does not remove all scaling risk. Iron, silica, and leftover hardness can still plate the heat exchanger. We stretch intervals with softeners, but we rarely skip flushing entirely. Myth: Vinegar ruins gaskets and seals. Reality: Food-grade vinegar used as directed is safe on the materials found in modern tankless units. The risk comes from improper concentrations or leaving acid sitting for days, which no pro does. Myth: Any handyman can flush a tankless. Reality: A basic flush is simple. A proper service checks sensors, inlet screens, condensate lines, error history, gas pressures, and combustion. That extra work prevents repeat callbacks and protects the warranty.
How often should you flush?
In Modesto and nearby cities with similar hardness, once per year is a solid baseline. If your home uses a lot of hot water or you run the temperature setpoint high, consider every 9 months. Homes on effective softening systems can often move to every 18 to 24 months. Apartment turnovers, vacation properties, and low-usage dwellings can go longer in calendar time, but scale forms based on flow, temperature, and mineral load, not just days on the wall.
As a quick rule of thumb, watch for these triggers between scheduled visits: slower hot water delivery, inconsistent temperatures at different faucets, new error codes related to flow or flame, or a louder burner sound during showers.
What happens if you skip it
Skipping flushing drives a few predictable outcomes. The heat exchanger runs hotter for longer, which strains the thermal sensors. Combustion cycles grow longer and more frequent, which raises gas usage. The control board may start throwing codes for flame failure, vent restriction, or overheat. In extreme cases, the exchanger cracks or pinholes from hot spots. Replacing a heat exchanger costs far more than routine tankless hot water heater maintenance. We have seen units under five years old scrapped because scale was never addressed and the repair cost exceeded the value.
There is also a water pressure side effect. Scale sheds in flakes as it breaks loose. If the inlet screen and internal filters are dirty, those flakes can lodge and cause pulsation, pressure dips, or faucet aerator clogs. Part of a good service is catching and cleaning those traps before the flakes travel.
DIY flushing versus pro service
Many homeowners in Modesto handle a simple flush themselves. If your tankless has service valves installed on the hot and cold lines, the basic loop is straightforward. If it does not have those valves, stop and call a pro. Installing isolation valves requires draining, cutting, and soldering or pressing copper, or solvent-welding CPVC, depending on your system. It is a one-time upgrade that saves you time every year.
Here is a brief overview of the homeowner-safe version for units with isolation valves and no error history. If you run into a smell of gas, any leak, or repeated ignition failures, shut it down and call us.
Short checklist for a basic homeowner flush:
- Turn off power and gas to the unit. Close the hot and cold isolation valves. Connect washer hoses to the service ports and to a small sump pump and a bucket with 2 to 3 gallons of white vinegar. Open the service ports. Run the pump to circulate vinegar through the unit for 45 minutes. Flush with clean water for 5 minutes to remove acid and debris. Close ports and reopen main valves. Restore power and gas. Check for leaks, clear error codes if present, and run a hot tap to confirm steady temperature.
A professional service goes further. At Knights Plumbing and Drain, we measure incoming water pressure, inspect inlet screens, clean the combustion fan, verify condensate drain function on condensing models, check the vent terminations for obstructions, and review error history. We also test combustion and adjust gas pressures within manufacturer tolerances where applicable. The whole visit usually takes 60 to 90 minutes. If we find early signs of trouble, we fix them before they become Saturday-night emergencies.
The right descaler: vinegar or specialty cleaner?
White vinegar works for light to moderate scale. It is affordable and safe to handle. Specialty descalers clean faster and can dissolve heavy buildup that vinegar struggles with. Some brands include corrosion inhibitors to protect metals. On high-efficiency condensing units with stainless exchangers and stubborn scale, we often reach for a dedicated descaler. On older copper exchangers, vinegar usually does fine, provided you circulate long enough. Follow the manufacturer’s guidance. Using a harsher acid to speed things up can etch metals or pit gaskets.
Don’t skip the inlet water work
Flushing treats the symptom. Hardness is the cause. If your home sits in Modesto’s harder zones or you see frequent white spotting on fixtures, consider a whole-home softener or a scale-control unit. Traditional ion-exchange softeners reduce hardness and add longevity to your tankless, fixtures, and dishwasher. Template-assisted crystallization (TAC) systems do not remove hardness but change crystal structure to reduce sticking. They help but are sensitive to iron and phosphates. We can test your water and advise on a good match for your space, plumbing, and budget.
Even without a softener, a cheap but often overlooked step is replacing or cleaning point-of-entry sediment filters. A clogged filter starves your tankless of flow. We see low-flow complaints that vanish once we change a collapsed cartridge in the garage. If you have a prefilter, put a date on it and change it on schedule.
Local quirks that affect maintenance in Modesto, Riverbank, and Salida
Neighborhoods near older mains sometimes deal with fine sand or rust. That grit chews on flow sensors and clogs inlet screens. We pull surprising amounts of debris from units in East Modesto after main breaks or hydrant flushes. If your water suddenly turns cloudy and the unit starts throwing flow errors, shut it off and call us. We will flush the lines, clean the screens, and protect the heater.
On the west side, we see more high-hardness scale and silica, which bonds stubbornly. Those homes benefit from shorter flush intervals and, in some cases, tankless water heater near me Knights Plumbing and Drain a switch from vinegar to a stronger, manufacturer-approved descaler. Homes near orchards on well water sometimes have a mix of hardness and iron. Iron can foul softener resin and trip some scale-control systems. Testing matters before you invest in treatment.
Winter gas demand is another factor. During cold snaps, incoming water is colder, so the unit works harder. That is when marginal exchangers hit their limits. Scheduling tankless hot water heater maintenance in late summer or early fall gives you a buffer before the heavy season.
Signs your tankless is overdue for maintenance
A few signals show up again and again on calls in Modesto:
- Longer waits for hot water even though plumbing runs have not changed. Temperature swings in the shower, especially when someone opens a second tap. New noise: a hiss, whine, or flutter during a hot draw. Error codes tied to flow, flame failure, overheat, or venting. Sooting on the vent termination or visible condensate stains below the unit.
Any single item is worth a check. Two or more together mean you should schedule service soon.
What a Knights Plumbing and Drain service visit includes
We start with a quick conversation about your home, usage, and water treatment. Then we:
- Inspect the vent and air intake, clear blockages, and verify clearances. Check gas supply sizing and static and dynamic pressure when applicable. Verify cold-water shutoff function, inspect inlet screen, and check for debris. Descale the heat exchanger with the right solution and pump time for your model and water. Flush the system with clean water and test for leaks. Inspect and clean the combustion fan and check flame signal strength where the brand allows. Test the condensate trap and drain on condensing units and treat for slime if needed. Review the control board’s error history and update settings: temperature setpoint, recirculation schedule, or vacation mode. Document findings and give you a clear recommendation on the next service interval.
We respect your time. Most maintenance visits wrap in about an hour. If we uncover a worn ignitor or a failing sensor, we can often replace it the same day from the truck stock.
Warranty and code considerations
Many manufacturers state a maintenance requirement in the manual, especially for hard-water regions. Skipping documented service can complicate warranty claims. Also, Modesto code and manufacturer instructions require proper venting, condensate disposal for condensing models, and adequate combustion air. During maintenance, we verify those items. If something needs correction, we explain options and costs before we touch it.
For homes that never had isolation valves installed, adding them during service is a smart move. It keeps future flushes quick and clean and is cheap insurance against a messy teardown.
Practical ways to stretch the time between flushes
You can’t change your water source easily, but you can reduce stress on the heat exchanger. Keep temperature setpoint at a sensible number, often 120°F for most families. Ultra-high setpoints drive faster scale. Avoid over-throttling hot-water-only at fixtures; if you need a warm mix, open cold slightly rather than closing hot to a trickle. That keeps flow above the unit’s minimum, so it burns cleaner. Clean faucet aerators and showerheads; clogged outlets can trick the unit into short-cycling.
If your model supports recirculation, use scheduling rather than 24/7. Continuous recirculation raises run hours and scale potential. In homes with long plumbing runs across Del Rio or Village One, we dial in smart schedules that match your routine and cut needless burn time.
Cost and timing expectations
For most single-family homes in Modesto, an annual professional tankless service is modest compared to the cost of a breakdown. Prices vary based on model, venting, and access. Add-on costs include isolation valve installation if missing, or replacement of worn parts we discover during inspection. We can quote by phone with a few photos of your unit, or we can visit and give a firm price before we start.
Booking is flexible. We offer weekday and select Saturday windows. Peak demand hits during the first cool spell and during holidays. If you want priority timing, schedule preventive service before November.
A quick story from the field
A family in Sylvan recently called about lukewarm showers and a recurring 11 code on a five-year-old tankless. No softener. The inlet screen was packed with grit from a nearby main repair, and the exchanger was heavily scaled. We flushed with a stronger descaler, cleaned the screen, cleared a partially blocked condensate trap, and reset the gas pressures. The shower stabilized, and the code stayed gone. Their gas usage the next month dropped roughly 8 percent compared to the year prior, normalized for degree days. That is the kind of quiet win you feel in daily comfort and monthly bills.
Ready for dependable hot water again?
If you’re in Modesto, Riverbank, Salida, Empire, or the nearby county, Knights Plumbing and Drain can keep your tankless running strong. We handle annual flushing, full tankless hot water heater maintenance, water testing, and practical upgrades like isolation valves and recirculation tuning. Call us to schedule, or send a photo of your unit and location for a fast estimate. We’ll get you back to steady, efficient hot water and a system that lasts.
Knights Plumbing and Drain provides professional plumbing services in Modesto, CA, and nearby communities including Riverbank, Ceres, Turlock, and Salida. Since 1995, the team has delivered reliable residential and commercial plumbing solutions, from drain cleaning and water heater repair to leak detection and emergency plumbing. Homeowners and businesses trust their licensed plumbers for clear communication, quality service, and lasting results. If you need a plumber in Modesto or surrounding areas, Knights Plumbing and Drain is ready to help.
Knights Plumbing and Drain
Modesto, CA, USA
Website: https://www.knightsplumbinganddrain.com/
Phone: (209) 583-9591