Using citric acid is the industry-standard, manufacturer-approved method to dissolve scale buildup on tankless water heater heat exchangers.
Because tankless water heaters flash-heat water on demand using tightly coiled copper or stainless steel tubes, they are highly sensitive to hard water scale. Even a razor-thin layer of calcium buildup inside the heat exchanger blocks heat transfer, causes the unit to overheat, throws error codes, and can ultimately burn out the expensive heat exchanger.
Why Citric Acid is Preferred Over Vinegar
While many older guides suggest using household white vinegar, top manufacturers (like Rinnai, Noritz, and Navien) explicitly recommend food-grade citric acid for several reasons:
- Higher Efficiency: Citric acid is a stronger chelation agent than the acetic acid in vinegar. It dissolves thick calcium carbonate scale up to three times faster.
- Odorless: Vinegar leaves a pungent, lingering sour smell in your hot water lines that requires extensive flushing to remove. Citric acid is virtually odorless.
- Material Safety: Citric acid is highly effective on mineral scale but incredibly gentle on the internal copper plumbing, rubber O-rings, and commercial seals inside the tankless unit.
The Tankless Closed-Loop Flush Setup
To flush a tankless unit, you must use a submersible utility pump and a 5-gallon bucket connected directly to the unit’s dedicated service isolation valves.
- Isolate the Unit
- Turn off the power: Unplug the water heater’s electrical cord or turn off its circuit breaker. 2. Turn off the gas/energy: Shut off the main gas supply valve running to the unit. 3. Close the main water lines: Close the main cold water inlet valve and the main hot water
outlet valve. (These are usually color-coded blue and red levers that stop water from entering or leaving the house plumbing).
- Connect the Pump and Hoses
- Remove the protective plastic caps from the service/purge ports located on the isolation valve kit.
- Connect a short garden hose from the outflow of your submersible utility pump (placed inside your 5-gallon bucket) to the cold water service port (blue). connect a second garden hose to the hot water service port (red) and run the open end of that hose back down into the 5-gallon bucket to create a loop.
- Mix the Solution
- Pour roughly 3 to 4 gallons of warm tap water into the bucket.
- Add 1 to 2 pounds of pure food-grade citric acid powder to the water. 3. Stir thoroughly until the powder completely dissolves and the water is crystal clear. [29]
- Run the Flush [30]
- Open both the cold and hot service port valves (the small T-shaped handles next to your hose connections).
- Plug in the submersible pump. The solution will pump into the cold side, force its way through the narrow coils of the heat exchanger, dissolve the scale, and exit out the hot side back into the bucket.
- Let the pump run continuously for 45 to 60 minutes. If the unit has not been cleaned in years, the water will turn milky white or cloudy as the calcium scale breaks down.
- Fresh Water Rinse and Restart
- Turn off the pump. Empty the used chemical solution from the bucket down a drain. 2. Close the cold service port valve (blue), but leave the hot service port valve (red) open. Disconnect the hose from the pump, but keep the hose attached to the red hot service port running into the empty bucket.
- Open the main cold water inlet valve to the unit. This forces fresh municipal water through the heat exchanger to purge out all remaining citric acid, dumping it into the bucket. Run this rinse for 5 full minutes.
- Close the hot service port valve, remove both hoses, and screw the protective caps back onto the ports.
- Open the main hot water outlet valve to the home.
- Remove and clean the unit’s internal cold water inlet filter screen, which often catches loose flakes during a flush.
- Turn the gas and electrical power back on.