Tankless Water Heater citric acid flush day! About 1.5lb citric acid to 3.5 gallons of water. Long ago I did the math to get the right pH and now I just trust it 😆⚖️A tankless water heater maintenance setup is shown. Two hoses are connected to the heater, with their ends immersed in a bucket filled with almost 4 gallons of water and 1.5lbs of citric acid crystals. An electric sump pump is in the bucket, connected to a power cord. The setup is on a concrete floor.