The usual tankless problems are scale in the heat exchanger, a clogged inlet filter, a flow rate too low to fire the burner, cold water sandwiches, and vent, condensate or gas faults that trigger an error code. Most trace back to water quality, flow or maintenance, and the code on the display narrows it down.
Why do tankless heaters scale up, and what does it cause?
Scale is the top maintenance problem. Hard water leaves mineral deposits inside the heat exchanger, and a tankless unit has narrow passages that clog sooner than a tank does. Navien's manual lists a maximum total hardness of 200 mg/l, or 12 grains per gallon, under the water quality levels it allows. Rinnai's diagnostic guide says hard water must be treated to prevent scale buildup and damage to the heat exchanger.
Both manufacturers build scale warnings into the unit. In the Rinnai guide, an LC code, or "00" when reading the maintenance history, means there is scale buildup in the heat exchanger that needs flushing. On older products the unit keeps running and the display alternates between the LC code and the temperature setting. On newer products the guide says a shutdown feature prevents damage from scaling up and locks the unit out until it is flushed and reset. A related code, 54, can also be caused by scale, and Rinnai's fix is to flush the heat exchanger with four gallons of vinegar following the owner's manual, then treat the water supply to prevent it from returning.
If you live in the Valley, this is the problem to plan around. Our page on how often to descale a tankless water heater in Phoenix gives a schedule, and how hard Phoenix water is has the local numbers. The flush itself needs a pump, hoses and service valves, and the unit has to be reassembled correctly, so most owners have it done during a service visit.
### What routine maintenance prevents most of these problems?
Most of the list above comes down to four habits. Clean the inlet water filter, since Rinnai's guide says to clean it on every service call. Flush the heat exchanger on a schedule, and Rinnai's FAQ recommends an annual flush. Keep the inlet air filter clean on models that have one, because a dirty one is on Rinnai's checklist for code 10. Finally, make sure the unit has shutoff valves on the inlet and outlet, and Navien's manual recommends manual shut-off valves on the cold water inlet and hot water outlet so the inlet filter can be cleaned without draining the house. If your unit lacks them, a plumber can add them during a visit.
What is the cold water sandwich?
The cold water sandwich is a burst of cold water between two bursts of hot. Navien defines it as the undesired introduction of cold water into the hot water supply line on occasions of frequent on and off operation. Rinnai's guide says one cause is ordinary use of hot water in a system without recirculation: when someone cycles the hot water on and off in a short period, some cold water can enter the hot line during the ignition sequence, which can cause a temporary temperature swing at the fixture.
Rinnai's FAQ says a one to two second temperature drop is normal without recirculation. A sandwich that lasts longer, or that happens mid-shower without you touching the valve, points to one of the causes in our page on how to fix a tankless water heater that goes cold mid-shower.
What happens when the flow is too low?
A tankless heater has a flow sensor that decides when to fire. Rinnai's FAQ says the unit needs a minimum flow to ignite and a lower flow to stay on, so a fixture that flows less than that will not trigger it. Rinnai gives ranges for its models of roughly 0.4 to 0.6 gallons per minute to activate and 0.26 gallons per minute to stay on. Navien's FAQ says that if there is no error code and no hot water, the flow sensor may be stuck, and the manufacturer's fix is to open and close the pressure relief valve to free it, if your model's manual calls for it. Navien then says to contact your original installer or an authorized service provider to service the sensor.
Low flow shows up in three ways:
- A shower with a tiny flow. Rinnai's guide says low-flow showerheads can cause trouble, especially where ground water is warm, and suggests removing the flow restrictor if possible.
- A clogged inlet filter. The filter at the cold water inlet catches debris, and both manufacturers say to clean it. Navien tells installers to instruct owners that the filter must be cleaned periodically to maintain proper water flow.
- Clogged fixture screens. Rinnai's guide says you should have at least 1 gallon per minute at every fixture, and to remove the restrictor and clean the strainer if you do not.
Navien's "A" models can avoid the minimum flow problem with a built-in recirculation mode. Its manual lists three advantages: it eliminates any minimum flow requirement, it eliminates the hot, cold, hot stacking of the cold water sandwich, and it delivers hot water to fixtures faster, so less water is wasted. The manual also says energy consumption increases because the water heater operates to maintain the water temperature within the circulation loop, and it points to Intelligent Preheating, a timer or the HotButton kit to limit that.
Which error codes point to a real fault?
Codes vary by brand, so the owner's manual is the last word. The Rinnai guide gives examples of common ones.
| Code | What Rinnai's guide says to check first |
|---|---|
| 10 | Air supply or exhaust problems. A clogged or frozen condensate drain line, a dirty inlet air filter, and a vent system with improper connections or clearances |
| 11 | No ignition. Gas supply and pressure, igniter, burner condition |
| 12 | Flame failure. Gas supply, vent restrictions, flame rod, gas pressure |
| LC or 00 | Scale in the heat exchanger. Flush it |
For a technician, that list includes measuring gas pressure at the unit, inspecting the igniter and flame rod, and checking that the gas line is sized for the load. Those are not homeowner steps. Never run a gas appliance with its venting disconnected or damaged, and leave vent work to a technician. Rinnai says to consult the manual for approved venting materials for your model. A clogged condensate line is one of the few you can check, since it stops the unit to prevent spillage. If you smell gas at any time, do not troubleshoot. Leave the home and call 911 and Southwest Gas at 877-860-6020. Our page on what to do if your house smells like gas covers the steps.
Should you repair or replace a struggling tankless unit?
If the heater is young and the problem is scale, a filter or a fixture screen, a repair or a flush is usually enough. If the heat exchanger is damaged, Rinnai says to replace it when it shows signs of damage from poor maintenance. Keep a record of each flush and service visit. Compare the cost of the repair against the age of the unit and your water quality, and see our page on whether to repair or replace a water heater. If you are still deciding between a tank and a tankless model, gas versus electric water heaters covers the trade-offs.
Thinking of switching to tankless in Mesa? Check the permit rule first in do I need a permit to go tankless in Mesa.
HQ Plumbing and Air is licensed and insured, takes emergency calls 24/7, and gives free estimates with an upfront price before work starts. See our water heater services.
Scale is behind many of these; see tankless water heaters and hard water. Each brand has its own code list. For A. O. Smith, see our page on A. O. Smith tankless error codes, and for Navien, see our list of Navien tankless error codes.