Plenty of homeowners flush their own tankless heaters, and every major brand publishes a procedure. The work itself is not hard. The risk is in the details: the wrong solution, a skipped rinse or a missing valve can damage the heater, and most warranties won’t cover it. This guide compares the brand procedures, explains the vinegar vs descaler question, and helps you decide.
A few terms first. The heat exchanger is the set of narrow tubes where the burner heats your water. Scale is the mineral crust hard water leaves inside them. A descaler is a mild acid made to dissolve that scale. Isolation valves are the shutoff valves on the hot and cold pipes under the heater, each with a small service port for hoses.
What a manufacturer flush involves
The brands differ in detail, but the outline is the same. This is a summary, not a set of instructions. Your heater’s manual is the only procedure to follow.
- Shut the heater down the way the manual says. Gas, venting and electrical connections stay untouched. If you’re not sure how to shut your model down, stop here and call a technician.
- Close the hot and cold isolation valves so the heater is cut off from the house.
- Connect the hoses. One runs from a small pump in a bucket to one service port. The other returns from the second port to the bucket.
- Circulate the solution through the heat exchanger for the time the manual sets.
- Drain and rinse with fresh water to wash 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 service: close the service ports, open the isolation valves, purge air from the lines and restart the heater as the manual describes.
How the brand procedures compare
| Brand | Solution | Circulation time | Fresh-water rinse | Source |
|---|---|---|---|---|
| Rinnai | four gallons of undiluted, food-grade white vinegar | At least 60 minutes, pumped at four gallons per minute | 5 minutes | SENSEI manual |
| Navien | an approved descaling solution or household white vinegar | At least 45 minutes | 5 minutes (manual); at least 10 minutes (Navien’s article) | NPE-2 manual and descaling article |
| Noritz | 1 gallon of calcium, lime and rust remover plus 1 gallon of water | 60 minutes | 3 minutes or more | Noritz flushing procedure |
| Rheem | 2 gallons of food-grade white vinegar | 45 to 60 minutes, depending on build-up | About 5 minutes | RTGH troubleshooting manual |
| Bosch | straight white vinegar, or a commercial descaler at its own dilution | Run it until the returning solution comes out clear | Not stated in the bulletin we used | Bosch descaling bulletin |
| Takagi and A.O. Smith | No published flush procedure found |
The differences are bigger than they look. Rinnai uses twice the vinegar Rheem does. Noritz sets the shortest minimum rinse, and Navien’s own two documents disagree on rinse time. Bosch sets no fixed time at all, only a visual cue. That is why a generic “how to flush a tankless” video is a poor substitute for your model’s manual.
If you own a Takagi or A.O. Smith, we could not find a published homeowner procedure. Takagi’s own FAQ talks about periodic flushing by a plumber in exceptionally hard water, which is a hint that the company expects a professional to do it.
Vinegar or commercial descaler?
Both work by dissolving calcium and magnesium scale. The question is what your manufacturer approves.
- Rinnai and Rheem name white vinegar, and both say food-grade.
- Noritz’s written procedure uses a calcium, lime and rust remover mixed with water. Its knowledge base allows pure vinegar or a calcium, lime and rust product.
- Navien accepts an approved descaling solution or household white vinegar.
- Bosch recommends straight white vinegar, or a commercial descaler mixed to that product’s instructions.
Vinegar is cheap; HomeAdvisor puts it at $3–$6 per gallon. It is gentle and easy to find, and it is what most procedures assume. On heavy scale it simply needs more time, which is why Rheem’s manual ties circulation time to the amount of build-up.
Commercial descalers are made for the job and can work faster. The catch is that products and strengths vary, so the manufacturer’s approval matters. Use only a product your manufacturer allows, at the dilution it lists.
Whichever you use, the rinse matters as much as the soak. It washes out the acid and the loosened scale, so don’t cut it short.
The warranty risk
Navien is blunt about it: if descaling is done wrong and the heater is damaged, it warns that incorrect descaling could void the warranty, and it recommends a professional. Other brands exclude scale damage outright. Bosch, for example, states “damage from scale/mineral deposits is not covered.” So you can’t count on the warranty if a flush is skipped or done badly.
If you flush it yourself, keep a dated note of what you did, which solution you used and for how long. That record helps if you ever need to make a claim. Our warranty and scale guide quotes each brand’s terms.
Isolation valves come first
Every brand procedure assumes the heater has isolation valves. Noritz says isolation valves are necessary. Rinnai says isolation valves are strongly recommended. Without them there is no way to close off the heater or connect the pump.
If you look under your heater and see no valves with service ports, don’t improvise. Adding valves means cutting into the water piping. In Utah that is plumbing work, which the law treats differently from routine maintenance. Read who can work on a water heater in Utah, and see our isolation valves guide for what to look for.
When DIY is reasonable, and when to call
DIY can make sense when:
- the heater already has hot and cold isolation valves with service ports,
- the display shows only a routine reminder, such as Rinnai’s SS, Navien’s E760, Noritz’s C1# or Rheem’s code 88,
- you have your model’s manual and can follow it step by step,
- there are no leaks, gas smells or burn marks.
Call a technician when:
- there are no isolation valves, or a valve won’t close fully,
- the heater shows an overheating or flame code, such as Navien E016, which means the heat exchanger is overheating, or Rinnai code 14, which means the heat exchanger has overheated,
- a code comes back right after a flush,
- you own a Takagi or A.O. Smith with no published homeowner procedure,
- anything involves the gas line, venting or wiring,
- you smell gas. Leave the house and call your gas utility first.
What a professional visit adds
A technician brings the pump and solution, and should follow your brand’s procedure. Many visits also cover the inlet filter, a check of stored error codes, and a look at the venting and condensate line. You also get a dated service record without having to make one yourself.
The cost guides put a professional flush at $150–$350 (HomeGuide). See tankless flush cost for the full comparison. If you’d rather not do it yourself, book a tankless flush and descale.
Prefer to have it done? Call to book a flush.