Two wall-mounted tankless water heaters with expansion tanks on an exterior wall
Generic photo: Downtowngal · CC BY-SA 4.0

Tankless water heater descaling (flush)

Scale insulates the heat exchanger, causing overheat codes and fluctuating temperature. Flush with a pump and descaling solution through the isolation valves.

Single source ~75 min easyBrands: All

Before you start

  • Turn off power to the heater and close the gas supply valve before flushing (gas: only close the appliance shutoff)
  • Scald risk from hot water in lines

Diagnostic steps

Unlock all 5 steps with expected values

Full library + AI assistant for $34.90, one time.

Get lifetime access

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

More water heaters guides

Guide · Water Heater

Heat-pump (hybrid) water heater: slow recovery, noise, errors

Hybrids need enough room air and a clean air filter, a working condensate drain and the right mode. Many complaints are mode settings or a dirty filter. Refrigerant faults are sealed-system work.

5 steps ~60 minmoderate
Guide · Water Heater

Hot water recirculation pump problems

Check timer/settings, check valves, air in the loop and the pump itself. On tankless units with built-in pumps, a recirculation code may point to air or a leak in the loop.

5 steps ~60 minmoderate
Guide · Water Heater

Thermal expansion tank: check and replace

On closed systems (check valve or PRV), heated water has nowhere to expand. An expansion tank absorbs it. Its air charge should match the static water pressure.

4 steps ~45 minmoderate
Tankless water heater descaling (flush) · OnCall Tech