Step-by-Step Tankless Water Heater Flush
Annual maintenance to keep your tankless unit clear of scale and operating efficiently.
General Procedure to flush a tankless water heater
(be sure to follow the instructions of your kit and/or water heater manufacturer)
​Remove the power to the water heater (unplug it, remove the electrical disconnect, or flip the breaker to off – which ever makes sense for your unit)
Shut off the gas to the unit (disregard if an electric tankless water heater)
Turn off the cold water supply valve to the unit, and turn off the hot water supply valve from the unit
Remove the caps to both flush valves
Connect the hoses from your kit to the flush valves (submersible pump supply to cold, discharge from hot)
Mix the descaler solution with water in the kit supplied bucket as directed by the instructions
Place the submersible pump in the supplied bucket, and place the end of the discharge hose into the bucket as well
Open both of the service valves connected to the unit – one for hot, one for cold. Do NOT confuse these with the cold water supply and hot water supply valves!
Plug in the pump and it should start running
Check that the solution is coming out of the discharge hose after 15 seconds or so
Run the pump with the solution through it for 45 minutes. This is the descaling part of the operation
After 45 minutes, unplug the pump
Disconnect only the kit supply side hose (the one with the pump) and replace the cap on the cold water flush valve
Turn off the service valve on the cold water side
Remove the end of the discharge hose from the bucket and direct it to a drain or other place that will handle a large amount of water. Note: the kit discharge hose is still connected to the water heater
Open the cold water supply valve to the water heater for two minutes or so to flush the unit out. The unit will not fire because the gas is still off and the unit is unplugged. The purpose of this is to clear the pipes of the descaler solution and any loose scale that may be remaining
After two minutes, shut the cold water supply back to off
Remove the discharge hose from the water heater and replace the cap on the hot water flush valve
Turn off the service valve on the hot water side
Confirm that both the hot and cold service valves are off and the caps have been replaced
Open the water heater hot water discharge valve and the water heater cold water inlet valve
Go to a nearby fixture and open the hot side of the faucet. Let this run for a minute until any air has blown out of the line, then shut it off
Open the gas valve to the water heater
Return power to the water heater (Reverse what you did to remove power - plug it back in, flip the breaker, or switch the disconnect to on)
Go to a nearby faucet and test for hot water. Confirm that the water heater fires up and that hot water begins running from the faucet - then turn off
Properly dispose of the descaling solution
Finished!
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