A tankless water heater needs its heat exchanger flushed of scale, its inlet water screen cleaned, its air intake and vent checked, and its condensate drain kept clear on condensing models, plus a yearly professional inspection. In hard-water Phoenix, scale is the main job, so flush schedules run tighter.
What does a tankless water heater need done to it?
The manufacturers agree on the core list, though the exact steps differ by brand and model, so your manual wins over any web page.
| Task | What it involves | Who usually does it |
|---|---|---|
| Flush (descale) the heat exchanger | Circulate white vinegar or approved descaler through the service valves, then rinse | Handy owner or plumber |
| Clean the inlet water screen | Shut the isolation valves, unscrew the screen, rinse, reinstall | Homeowner |
| Clean the air intake filter | Power off, pull the filter, wash, dry, reinstall (indoor units) | Homeowner |
| Inspect the vent and terminal | Look for blockage, damage, debris | Plumber if anything looks off |
| Check the condensate drain | Confirm condensate flows and the drain end is open | Homeowner or plumber |
| Check burner flame and dust | Clear blue flame; vacuum dust from burner and fan | Plumber |
| Test the pressure relief valve | Scald risk from discharge | Plumber |
| Annual inspection | Combustion, gas, venting, water connections | Qualified professional |
Rinnai's N Series manual says the water heater "must be inspected annually by a trained and qualified professional." Navien says annual maintenance is recommended. Noritz's maintenance guide says most manufacturers recommend professional maintenance at least once a year. That yearly visit is the backbone.
How do you flush and descale a tankless water heater?
Follow the model's approved descaling solution, isolation sequence and circulation time. The cited Rinnai instructions are one example, not a universal procedure. Rinnai's manual has you turn off power, close the shutoff valves, connect a pump and hoses to the service valves, pour four gallons of undiluted food-grade white vinegar into a pail, and circulate it for at least one hour. Rinnai's flush page adds a warning: use only vinegar, and "never use chemical solutions of any kind." Noritz recommends a 60-minute flush with white vinegar and warns that CLR-type products can be harsh on the copper heat exchanger.
The valves matter. Rinnai says it "strongly recommends installation of isolation valves to allow for flushing of the heat exchanger." If your unit was installed without service valves, a plumber can add them, and that is worth doing before the first flush rather than after the first error code.
How often is a separate question, and it depends on your water. Our page on how often to descale a tankless water heater in Phoenix covers the schedule, so we will not repeat it here.
Why is scale the main job in Phoenix?
Because the water here is hard, and scale is what tankless heaters hate most. Noritz's own marketing manager said maintenance frequency "all depends on the hardness of the water," estimating a very hard water area could mean servicing once a year and a soft water area four to five years. Phoenix sits at the hard end. Our pages on how hard Phoenix water is and how hard water damages plumbing have the local numbers.
Ignore scale and the warranty can follow it out the door. Rinnai's manual states that damage caused by lime build-up is not covered by the water heater's warranty. A whole-house softener changes the math. See whether water softeners work with Arizona hard water if you are weighing one.
Temperature swings, weak flow and error codes are the usual signs scale has crossed the line. Cold water mid-shower on a tankless is a classic, and common tankless water heater problems lists the rest.
How do you clean the inlet water filter and air filter?
Inlet water screen. Rinnai's manual says to close the cold and hot water isolation valves, put a bucket under the filter at the bottom of the unit, unscrew the filter, rinse off debris, reinstall, and open the valves. Navien says its units have water screens on the cold inlet, and its NPE-Advanced models have a second one on the recirculation inlet. Rinnai's flush procedure also has you pull and clean the in-line cold water filter at the end of a flush.
Air intake filter. Indoor Rinnai units have one behind the front panel. Rinnai's steps are: power off, pull the filter, scrub with mild dish soap and a soft brush, rinse, dry with a lint-free towel, reinstall. Its manual is blunt that the unit should not run without the filter in place. Navien says to clean its air screens with a toothbrush and cold water and let them dry fully. Outdoor units differ.
What about the vent, condensate drain and burner?
Check them for blockage and damage. Rinnai calls for periodic cleaning and inspection of the vent system, including the screens in the vent termination, and says a vent that is blocked and cannot be easily cleared should go to a qualified professional. Venting is a safety system, since it carries exhaust out of the house. Our code page on water heater venting under UPC 510 explains the rules behind it.
Condensing tankless units also make acidic condensate that has to drain away. Rinnai's manual says to make sure condensate is flowing and the end of the drain is open to the atmosphere. Navien says its condensate trap sits at the bottom of the unit and can be cleaned by removing a wire clip and pulling out the cup, and to make sure the o-ring is seated when it goes back.
Where a neutralizer is installed, its media and drainage requirements need their own documented maintenance check.
Rinnai also says the burner flame should be "a clear, blue, stable flame," and that the fan and motor should be kept free of dust with an annual cleaning. A yellow flame or a dusty burner is a technician job.
Routine fan care is also different from a brief Rinnai fan after-run when the tap closes. That operating explanation does not replace the inspection or authorize opening the fan compartment.
What do error codes and service reminders mean?
They mean the unit's sensors caught something, and scale is a frequent cause. Some models show a service reminder. Rinnai's manual describes a "Service Soon" (SS) code that signals time to flush, and warns that failing to flush will damage the heat exchanger. Codes differ by brand, so use the right list: Rinnai error codes, Noritz codes 11, 90 and 99, Rheem tankless codes and Takagi codes 111, 121 and 991.
Smell gas? Leave the house and call the gas utility or 911 from outside. Our page on what to do when the house smells like gas has the steps. Do not troubleshoot a gas smell yourself.
What can you do yourself, and when should you hire a plumber?
Perform only the owner-maintenance tasks your model manual permits, such as an exterior check or accessible screen cleaning. Hire a plumber for the annual inspection, the burner and combustion check, vent problems, gas connections and the pressure relief valve. Rinnai's manual says testing the relief valve should only be done by a trained and qualified professional, since discharged water can cause severe burns instantly. Warranty is a practical reason too. Navien warns that damaging a unit during routine maintenance may void the warranty, and recommends a certified technician to protect it. If you do the flush yourself, follow the manual and keep a note of the date.
HQ is licensed and insured, and gives free estimates and upfront pricing before work starts. If you want the annual service or a first-time flush with added isolation valves done properly, that is a job we can look at.