Rheem 491 Owner's Manual And Installation Instructions page 19

WATER SUPPLIES
SATURATION INDEX
The saturation index 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. Where the saturation index is less than –1.0, the water is very corrosive
and the Rheem warranty does not apply to a copper sheathed heating unit. A
corrosion resistant heating unit must be used for the Rheem warranty to apply to
the heating unit.
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.
Where the saturation index exceeds +0.80, the Rheem warranty does not apply
to a standard watts density heating unit. A low watts density heating unit must be
used for the Rheem warranty to apply to the heating unit.
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 19. Refer to the cold water
connection detail on page 32 for the position of the expansion control valve.
Contact your Rheem Service Centre if a replacement heating unit is required.
SATURATION INDEX (SI)
ELECTRIC 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
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