Flush and descale tankless water heater

Why It Matters

Mineral scale builds up on the heat exchanger, killing efficiency and triggering overheating shutdowns. Untreated scale can shorten the unit's life by half.

Tools

• Submersible pump
• Two hoses with hose-bib fittings
• Bucket

Materials

• White vinegar or tankless-specific descaler

How to Do It

1. Close the cold water inlet and hot water outlet isolation valves, then shut off the gas or flip the breaker for electric units.
2. Connect the pump's outlet hose to the cold inlet service port, and run the return hose from the hot outlet service port back into the bucket.
3. Open the service port valves on both the cold inlet and hot outlet lines.
4. Fill the bucket with enough white vinegar or tankless-specific descaler to keep the pump submerged, then run the pump for about an hour.
5. Turn off the pump, close both service port valves, and replace the bucket contents with clean water.
6. Open the service port valves and run the pump for a few minutes to flush residual descaler from the heat exchanger.
7. Close the service port valves, disconnect the hoses, reopen the isolation valves, and restore gas or power.

Related Tasks

• Clean tankless water heater inlet filter Once a year
• Flush water heater Once a year
• Lift and release water heater pressure relief valve Once a year
• Replace water heater anode rod Every 5 years