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8
Water Quality and Scale
A complete water analysis and an understanding of system requirements are needed to protect the
on-demand water heaters and water heating systems from scale. Water analysis shows whether
water is hard or soft. Hard water, unless treated, will cause scaling or liming of the heat exchanger.
The rate of scaling increases with temperature and usage because calcium carbonate and other
scaling compounds lose solubility (fall out of solution) at higher temperatures. For example, for every
20°F over 140°F, the rate of scale increases by a factor of 2 (See figure below). Reference target
water quality levels found in the operation / installation manual and consider water treatment if these
levels are exceeded. *
210
180
150
120
90
60
30
0
3900
3300
2700
2100
1500
900
300
0
WATER USAGE, gal/day
LIME DEPOSITED, lb/yr
180°F
170°F
160°F
150°F
140°F
120°F
BASED ON 10 grains/gal HARDNESS
*Source 2003 ASHRAE Handbook HVAC Applications
*
Figure “3”