Tankless Water Heater Descaling in Phoenix

“My Navien screen says E760.” and “My Rinnai shows LC.”

Both makers built a scale warning into the display, and both of them mean the same thing.

Navien lists E760 as its flushing and service alarm, with the instruction to contact the installer for routine service. Rinnai uses LC codes to indicate scale buildup in the heat exchanger and says it needs flushing to prevent damage. Rinnai's codes can lock the heater out until it is reset, and on either brand, the scale remains until someone flushes it. Book the flush before an overheating code follows.

“The water just isn't as hot as it used to be.”

Scale and a dirty inlet screen: the gradual one.

Scale insulates the heat exchanger, and the inlet screen fills with grit and flakes. Both cut performance gradually, so many owners get used to lukewarm water and never connect it to maintenance. A longer wait for the water to reach full temperature belongs to the same pattern.

The flush

We follow the maker's procedure. Navien's own manual calls its flush somewhat complicated, and the circulation step alone runs at least 45 minutes, followed by a rinse and a cleaning of the cold-water inlet filter before the unit is restarted and the hot water checked.

Your part between visits

The air intake filter is the owner's job on a Navien, scheduled every few months. We will show you where it is on your unit.

Related work

Tank water heaters need a different kind of service: a sediment flush from the bottom of the tank, which our water heater maintenance page covers. If the flush turns up a fault that scale did not cause, our tankless repair page explains what comes next.

A flush dissolves the scale already inside the heat exchanger and restores flow and temperature. Next year's minerals start building the day it is finished.

A softener removes hardness minerals from the water. A salt-free conditioner leaves them in and is designed to reduce scale forming. We recommend salt-free, and we will explain what it does and does not do for your heater before you buy anything.

The makers' yearly maintenance still applies. Navien's annual recommendation and Rinnai's once-a-year flush are not waived for treated water, so keep the flush on the calendar.

Then the yearly flush does all the work against Phoenix hardness. That is when adding treatment is worth talking through, and we will lay out the options at the visit.

An inlet screen packed with grit and a heat exchanger that needs the full circulation time.

Flow and temperature usually come back. If the unit has run hot for a long time, we tell you what we see and what it means for the heater.

Water harder than the makers' limit

Phoenix's own 2025 water report lists total hardness between 172 and 302 parts per million (10 to 17.6 grains per gallon) and warns that hard water can scale water heaters. Navien and Rinnai both set their limit at 200 mg/L, which is the same as 200 parts per million, so the upper end of Phoenix's range is harder than either maker's spec. Both exclude scale damage from their warranties.

Outdoor units

Freezing nights are rare at Sky Harbor, but an outdoor unit's freeze protection depends on power, and Navien's advice for a unit that will lose power or gas for a long stretch in cold weather is to drain it. If you close up a seasonal home for part of the winter, bring it up at the descaling visit.

“Paul at American Discount Plumbing is very knowledgeable and reliable. He came out to our house right away to check out our hot water heater that was leaking. He quoted us a great price (compared to another large plumbing company) on both the parts and installation for a tankless water heater and also a whole house water treatment system. He was very pleasant to work with and always showed up or called when he said he would and was careful to make sure the job was done right. We ended up having him also replace a 2nd water heater in our RV garage as a preventative measure. The tank would not flush as there was too much sediment build up from not flushing on a yearly basis. My mom also had her hot water heater replaced by Paul and he installed and plumbed her BBQ island sink. She was very pleased with his work also. We definitely recommend Paul and American Discount Plumbing.”