How to Flush a Rinnai Tankless Water Heater

Flushing a Rinnai tankless water heater requires closing the isolation valves, connecting a submersible pump and two hoses to form a closed loop through the heat exchanger, circulating a food-safe descaling solution for 45 to 60 minutes, then rinsing with clean water before restoring power. No plumbing license is required for units with accessible service ports. Rinnai recommends annual flushing in hard water areas to maintain efficiency and warranty coverage.

Key Takeaways

• Rinnai recommends annual flushing in hard water areas - twice yearly above 180 mg/L.

• Use only NSF/ANSI/CAN 60 certified descaling solutions. Rinnai's warranty requires manufacturer-approved, food-safe products.

• The flush circuit runs: pump outlet - cold-water service port - through the heat exchanger - hot-water service port - back into the bucket.

• Run the pump 45 minutes minimum. Extend to 90 minutes if the LC error code triggered the flush.

• The Rinnai LC code is a direct scale alert - flush immediately when it appears.

Why Rinnai Tankless Water Heaters Need Regular Flushing

Rinnai tankless water heaters heat water on demand by passing it through a compact heat exchanger. In hard water regions, dissolved calcium and magnesium deposit as limescale on the heat exchanger surfaces every time the water is heated above 120 degrees Fahrenheit. Rinnai heat exchangers are engineered for high efficiency, which means the internal channels are narrow by design - scale narrows them further, restricts flow, forces the burner to run harder, and raises gas consumption.

The U.S. Department of Energy estimates water heating represents approximately 18 percent of residential energy use. Scale buildup can reduce heat exchanger thermal efficiency by 10 to 20 percent. Rinnai's warranty documentation specifies regular descaling as a required maintenance task in hard water installations. Skipping it can void warranty coverage on the heat exchanger - the most expensive component in the unit.

The EPA Ground Water and Drinking Water program documents that over 85 percent of US homes receive hard water. In high-hardness states - Texas, Arizona, California, Nevada, and Florida - meaningful scale can form within six months. If your local water hardness exceeds 180 mg/L, twice-yearly flushing is appropriate.

What You Need to Flush a Rinnai Tankless Water Heater

Rinnai tankless water heaters include flush ports (service isolation valves) on the cold-water inlet and hot-water outlet connections as standard on most models. These are the connection points for the flush kit hoses.

The Chromex Tankless Descaler Kit includes everything required for a complete flush: a submersible utility pump, two service hoses with standard 3/4-inch fittings, a 5-gallon bucket, and a descaling solution certified to NSF/ANSI/CAN 60 for potable water systems. If sourcing separately, use a pump rated at minimum 500 GPH and a food-safe descaling solution with NSF/ANSI/CAN 60 certification.

Rinnai recommends using only manufacturer-approved, food-safe descaling solutions. Never use bleach, muriatic acid, or any product not explicitly rated for potable water contact - these corrode copper heat exchanger components and void the warranty. Browse the Descaling Solutions collection for NSF/ANSI/CAN 60 certified formulations in residential and professional concentrations. The Tankless Water Heater Maintenance collection covers the full range of Rinnai-compatible flush tools.

How to Flush a Rinnai Tankless Water Heater: Step by Step

Step 1 - Locate the Flush Ports

On most Rinnai models, the service isolation valves are located on the cold-water inlet pipe (blue handle) and the hot-water outlet pipe (red handle) directly at the unit. Each valve has a removable port cap - these are the connection points for the flush kit hoses. Rinnai RU, RS, and RVR series all use this configuration. Confirm both valves are present and accessible before gathering your tools.

Step 2 - Shut Down the Unit

Set the Rinnai controller to the service or maintenance mode if available, or power off the unit at the circuit breaker for electric models. For gas models, set the gas valve to the off position. Do not flush with the unit in an active state.

Step 3 - Close the Main Supply Valves

Close the main cold-water supply valve that feeds the unit. Also close the hot-water outlet valve. Open a hot tap briefly to relieve residual line pressure, then close it.

Step 4 - Prepare the Descaling Solution

Fill the 5-gallon bucket with 3 to 4 gallons of clean water. Add the descaling solution at the manufacturer's recommended dilution ratio. Follow the concentration instructions on the product label. Submerge the submersible pump fully in the bucket.

Step 5 - Connect the Flush Hoses

Remove the port caps from both Rinnai service isolation valves. Connect one hose from the pump discharge outlet to the cold-water service port. Connect the second hose from the hot-water service port back into the bucket. The circuit runs: pump outlet - cold-water port - through the Rinnai heat exchanger - hot-water port - back into the bucket. Confirm both connections are secure before powering the pump.

Step 6 - Open the Service Ports and Run the Flush

Open both service isolation valves. Start the pump. Run for a minimum of 45 minutes for a unit on a regular annual schedule. For units that have not been flushed in two or more years, extend to 60 to 90 minutes. The solution will turn yellow or brown as mineral deposits dissolve. If Rinnai's LC error code triggered the flush, run for the full 90 minutes.

Step 7 - Rinse the Heat Exchanger

Power off the pump. Dump the spent solution, refill the bucket with clean water, and run the pump for 10 minutes. Repeat once. This removes residual descaling solution before the unit is returned to service.

Step 8 - Restore the Rinnai Unit

Close both service isolation valves, remove the hoses, and replace the port caps. Reopen the main cold-water supply valve and the hot-water outlet valve. Restore power or gas. Return the Rinnai controller to normal operation. Run a hot tap for two minutes to confirm normal flow, temperature, and pressure before resuming household use.

How Often Does a Rinnai Tankless Water Heater Need to Be Flushed?

Rinnai recommends annual flushing in hard water conditions as a standard maintenance interval. Most Rinnai models also include a built-in scale alert - the LC error code - which activates when the unit's flow sensors detect a scale-related restriction. If your Rinnai displays the LC code, flush immediately regardless of your last service date.

The U.S. Department of Energy recommends following the manufacturer's maintenance schedule to preserve both warranty coverage and system efficiency. In areas with water hardness above 180 mg/L, flush every six months. Between 60 and 120 mg/L, annual flushing is sufficient. Below 60 mg/L, every two years may be adequate.

For a complete annual maintenance kit covering both tankless descaling and tank water heater corrosion protection, the Complete Kits collection includes all required tools. For tank-style heaters alongside a Rinnai tankless unit, the Water Heater Maintenance collection covers anode rod protection and sediment flushing.

Signs Your Rinnai Needs Flushing Now

The Rinnai LC error code is the most direct indicator - it signals that flow restriction from scale has reached a threshold the unit's sensors detect. Other signs include reduced hot water flow, longer wait times at far fixtures, higher gas bills without a usage change, and rumbling or popping sounds during operation. If the LC code persists after a standard flush, a second flush cycle or a stronger descaling solution may be needed.

Frequently Asked Questions

1. What descaling solution is safe to use in a Rinnai tankless water heater?

Use a food-safe descaling solution certified to NSF/ANSI/CAN 60 for drinking water treatment chemicals. Rinnai specifies food-safe, potable-water-rated solutions only. Never use muriatic acid, bleach, or any product not explicitly approved for potable water system use. The EPA WaterSense program supports the use of certified, manufacturer-approved maintenance products for residential water heater systems.

2. What is the Rinnai LC error code?

The LC code is Rinnai's scale alert. It indicates that limescale buildup has reached a threshold affecting unit performance. Flushing with an approved descaling solution clears the code in most cases. If the code returns shortly after flushing, the scale buildup may require a second flush at a stronger concentration.

3. Do all Rinnai models have flush ports?

Most Rinnai tankless models installed after 2010 include service isolation valves as standard equipment. Older models may not. If your unit lacks these ports, a licensed plumber can install isolation valve kits before your first flush.

4. How do I know if my water is hard enough to require regular flushing?

Water hardness above 60 mg/L (milligrams per liter) is sufficient to cause scale accumulation in a tankless heat exchanger over time. Most municipal water suppliers publish annual water quality reports that include hardness readings. If your hardness exceeds 120 mg/L, annual flushing is recommended. Above 180 mg/L, flush every six months.

Ready to flush your Rinnai? The Chromex Tankless Descaler Kit includes the pump, hoses, bucket, and NSF/ANSI/CAN 60 certified descaling solution - everything needed for a complete flush in one kit.