A.O. Smith On Demand 705 Series 100 Instruction Manual | Page 8

A.O. Smith Water Heater Instruction Manual - On Demand 705 Series 100.
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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

L

IM

E

 DEPO

SI

T

E

D,

 lb

/y

r

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”

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