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Water temperature 5°F less than set point?
Use Sealed System
Water temperature 1°F less than set point.
Small amount of water has been used (less than 10 gallons)?
Use Sealed System
Medium amount of water has been used (10-20 gallons)?
Use Lower Element
Water temperature 20°F less than set point?
Use Upper Element until 7°F less than set point, then:
Lower Element if medium flow during upper element on time
Sealed System is no water flow during upper element on time