Rheem W2W Series Owner's Manual And Installation Instructions page 15

Where the pH is less than 6.0 the Rheem warranty does not apply to the water
heater. pH is a measure of whether the water is alkaline or acid. In an acidic
water supply, the water can attack stainless steel parts and cause them to fail.
Water with a pH less than 6.0 may be treated to raise the pH. The water supply
from a rainwater tank in a metropolitan area is likely to be corrosive due to the
dissolution of atmospheric contaminants.
SUMMARY OF WATER CHEMISTRY ADVICE AFFECTING THE RHEEM
WARRANTY
The water heater is not suitable for certain water chemistries. Those chemistries
are listed below. If the water heater is connected at any time to a water supply
with the following water chemistry, Rheem's warranty will not cover any resultant
faults:
Water Chemistry
Saturation Index (SI) < -1.0
Saturation Index (SI) > +0.4
Chloride > 250 mg/L
pH < 6.0
WATER SUPPLIES
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Component
water heater
water heater
water heater
water heater