A tankless flush, also called descaling, pumps a mild acid through the heater’s heat exchanger to dissolve the mineral scale that hard water leaves inside it. Navien recommends doing it at least once a year. HomeGuide puts a professional flush at $150–$350. The core of the job is a pump circulating vinegar or a descaler for about an hour on most brands, then a fresh-water rinse and a cleaned inlet filter.
What a flush actually does
The heat exchanger is the coil of narrow tubes where burning gas, or an electric element, heats the water as it passes through. Hard water carries dissolved calcium and magnesium. When that water is heated, some of the minerals come out as a hard white crust called scale. The USGS notes that hard water can cause mineral buildup in plumbing, fixtures, and water heaters.
Scale insulates the tubes and narrows them. The heater has to work harder to reach the set temperature, and hot spots can trip its overheat protection. A descaler is an acid made to dissolve that crust. White vinegar is the household version, and several manufacturers name it in their manuals.
Isolation valves are the pair of service valves on the cold inlet and hot outlet lines. Each has a small side port. Closing them cuts the heater off from the house plumbing so a pump can circulate solution through the heater alone.
Signs your heater is due
Many heaters tell you. These are the maintenance prompts named in manufacturer documents:
- Rinnai: code SS on SENSEI models means it is time for service and the heat exchanger should be flushed. Code LC on RE Series models means there is scale build-up in the heat exchanger and it needs to be flushed to prevent damage. See the Rinnai page.
- Navien: E760 is a flushing/service alarm (time for routine service). E016 and E030 list a flush among the fixes. See the Navien page.
- Noritz: C1# is a service reminder for periodic maintenance, such as flushing scale from the heat exchanger. See the Noritz page.
- Rheem: 1L means lime build-up has been detected in the heat exchanger, and code 88 is a reminder to flush the heat exchanger and/or replace the water treatment filter. See the Rheem page.
- Bosch: a Bosch bulletin says scale can cause lower flow rates, A7 and E9 codes and boiling sounds. See the Bosch page.
Without a code, watch for hot water that surges and drops, a slower flow at hot taps, or more than a year since the last flush. The error code hub covers the rest, and the no hot water page explains which symptoms a flush can’t fix.
How a technician flushes a tankless heater
Each manufacturer publishes its own procedure. In outline, they follow the same order:
- Shut down and isolate. Navien’s manual starts by turning off and unplugging the unit, then says to close the hot and cold water isolation valves.
- Connect the pump. Hoses run from the valve ports to a bucket with a small pump in it. Navien lists a submersible pump and a 5-gallon bucket. Rheem’s manual uses a submersible utility pump (Tankless Flush Kit RTG20124).
- Circulate the solution. The pump pushes vinegar or descaler through the heat exchanger and back to the bucket, over and over.
- Drain and rinse. Fresh water runs through the heater to push out the acid and loosened scale.
- Clean the inlet filter. Rinnai’s procedure says to remove and clean the in-line filter at the cold water inlet, and Navien’s says to remove the cold water inlet filter and clean out any residue.
- Restore and test. The valves go back to normal, air is purged from the lines, and the heater is restarted and checked.
How the brands compare
| Brand and document | Cleaning solution | Circulation time | Fresh-water rinse | Valve note |
|---|---|---|---|---|
| Rinnai, SENSEI manual | four gallons of undiluted, food-grade white vinegar | at least 60 minutes, pumped at four gallons per minute | 5 minutes | isolation valves are strongly recommended |
| Navien, NPE-2 manual and blog | an approved descaling solution or household white vinegar | at least 45 minutes | 5 minutes (manual) or 10 minutes minimum (blog) | the procedure begins by closing both valves |
| Noritz, NCC199 procedure | 1 gallon of calcium, lime and rust remover plus 1 gallon of water | 60 minutes | 3 minutes or more | isolation valves are necessary |
| Rheem, RTGH troubleshooting manual | 2 gallons of food-grade white vinegar | 45 to 60 minutes, depending on build-up and hardness | about 5 minutes | uses the Rheem/Ruud flush kit pump |
| Bosch, Therm service bulletin | straight white vinegar, or a commercial descaler mixed per its label | until the returning solution comes out clear | not stated | begins: shut off water with isolation valves |
We didn’t find a published flush procedure for Takagi or A.O. Smith. For those heaters, the technician should work from the manual for your model.
A warning on doing this yourself: Navien says incorrect descaling could void the warranty if the heater is damaged, and recommends hiring a professional.
Why Utah water makes this a yearly job
The USGS rates water with more than 180 mg/L of hardness as very hard. Many Wasatch Front supplies sit near or above that line. Highland’s city page lists 14 gpg. Provo’s 2025 report says 10–15 gpg. Spanish Fork’s water page listed 13.4–20.24 gpg, and Orem’s report shows 7–25 gpg across its system. In the south valley, Jordan Valley Water Conservancy District’s 2025 report averages about 10.7 gpg for the wholesale water that most cities there buy. (gpg means grains per gallon, a common hardness unit. The USGS scale uses milligrams per liter, or mg/L.)
Now compare the manufacturers’ limits. Takagi’s manual says hardness must not exceed 7 gpg in a single-family home. Bosch lists 100 ppm. Navien’s installation manual allows up to 12 gpg, and Rinnai lists 200 ppm. Much of the water above is over the lower limits, and some of it is over the higher ones. Rinnai adds that scale is accelerated by high temperature settings, so a heater set hot scales sooner.
That’s why scale damage matters for warranty claims. Rinnai’s manual says damage caused by lime build-up is not covered. The warranty guide quotes each brand. The Utah hard water guide and the flush interval guide go deeper on hardness and timing.
What a flush costs
The national cost guides agree on the low end and differ on the high end:
- HomeGuide: $150–$350
- Angi: $150–$250, with an average of $200
- HomeAdvisor: $150–$250
For comparison, HomeGuide puts a tank-style water heater flush at $75–$150, and Angi says a tankless flush runs $75 to $100 more than a tank flush.
The technician serving your area sets the price and confirms it when you book. Things that move it up or down:
- Isolation valves. If there are none, they have to be installed before the heater can be flushed. The isolation valves guide explains what that involves.
- How much scale there is. A heater that hasn’t been flushed in years may need a longer circulation or a second round.
- Add-ons. Replacing a scale-prevention cartridge, clearing an error code or fixing a leaking valve all add parts and time.
- Labor rates. Angi says plumbers charge $45–$200 per hour.
The flush cost guide breaks these down further.
DIY or a technician
A homeowner with valves already installed can flush some heaters. HomeGuide puts a DIY kit with pump, hoses, bucket and solution at $100–$300. The trade-offs are the warranty risk above, the gas and electrical shutdown, and knowing your model’s procedure. Utah Code 58-55-308(2) requires training and certification for anyone who handles the installation, repair, maintenance, cleaning, or replacement of a gas appliance. No Utah statute names flushing. The DIY vs professional guide compares the two in detail, including vinegar against commercial descaler. The Utah licensing guide points you to DOPL for edge cases.
What to ask the technician
When you call or send the form, we pass your details to an independent local technician. They’ll confirm the price and book a time with you. Before the work starts, it helps to ask:
- Which solution do you use, and how long do you circulate it on my model?
- Does my heater have isolation valves? If not, what does adding them cost?
- Will you clean the inlet filter and check for stored or recent error codes?
- Will you set or reset the heater’s service reminder to suit my water?
- Can I get a written, dated note of what was done, for my records?
If you want the flush to become a yearly habit, see annual tankless maintenance. If scale keeps coming back fast, read about scale prevention.
Flush service by city
Each city page covers that city’s water supply and hardness in detail.
- Lehi: water from seven wells and three springs plus district surface water
- American Fork: the city suggests a softener setting of 12.38 gpg
- Saratoga Springs: city water averages 8.3 gpg
- Highland: 14 gpg from the city’s deep wells
- Orem: 7–25 gpg across the system
- Provo: typically 10–15 gpg
- Spanish Fork: 13.4–20.24 gpg listed by the city
- Draper: WaterPro’s figure for its part of Draper averages about 10.8 gpg
- South Jordan: 7–10 gpg, per the city
- Herriman: local wells run 25–60 gpg
- Riverton: all district water, averaging about 10.7 gpg
- West Jordan: 93% of its water comes from the district