A.O. Smith 505 Series 100, 705 Series 100 manual Water Quality and Scale, Lime DEPOSITED, lb/yr

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. *

LIME DEPOSITED, lb/yr

210

180

150

120

90

60

30

0

BASED ON 10 grains/gal HARDNESS *

180°F

170°F

160°F

						150°F
						140°F
						120°F
300 0	900	1500	2100	2700	3300	3900

WATER USAGE, gal/day

Figure “3”

*Source 2003 ASHRAE Handbook HVAC Applications

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