Rheem 850 Series Owner's Manual And Installation Instructions page 21

WATER SUPPLIES
SATURATION INDEX
The saturation index (SI) is used as a measure of the water's corrosive or scaling
properties. The saturation index figures stated are calculated using a water
temperature of 80°C.
In a corrosive water supply, the water can attack copper parts and cause them
to fail.
In a scaling water supply calcium carbonate is deposited out of the water onto
any hot metallic surface.
Where the saturation index exceeds +0.40, the water is very scaling. An
expansion control valve must be fitted on the cold water line after the non-return
valve to protect and for the Rheem warranty to apply to the temperature pressure
relief valve and water heater cylinder.
Water which is scaling may be treated with a water softening device to reduce
the saturation index of the water.
Refer to the Saturation Index chart on page 21. Refer to the cold water
connection detail on page 36 for the position of the expansion control valve.
SATURATION INDEX (SI)
GAS STORAGE WATER HEATERS
WITHIN WARRANTY
SPECIFICATION
-1.0 0 +0.4 +0.8
SATURATION INDEX
(calculated @ 80°C water temperature)
very corrosive scaling very scaling
corrosive
21