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Two temperature example
1. A one hour demand of 75 gallons of 180°F water and 110 gallons of 140°F water has been
established. The lowest incoming water temperature is 40°F. The shortest time in any day in which
the demand will be repeated is 3 hours.
2. Convert the 180° water requirement into the equivalent of a 140°F water requirement to avoid working
with two different temperature rises.
Converting to a single temperature rise:
• Multiply the 180°F requirement by 1.4 in 100°F temperature rise applications.
a) This means 1.4 more water can be raised from 40°F to 140°F than 40°F to 180°F with the
same amount of energy.
b) Multiplier formula:
Hot - Cold
——————— = multiplier
Mixed - Cold
Example:
180 - 40 140
————— = ——— = 1.4
140 - 40 100
c) 75 gallons 180°F water required
x 1.4
105 equivalent gallons of 140°F water
• Add the converted 180°F water requirement to the 140°F requirement and proceed with heater
selection.
a) 105 + 110 gallons of 140°F water = 215 equivalent gallons of hot water required
at 100°F water temperature rise.
3. A State electric tank type commercial water heater will be selected for the job. (Any fuel or type of
heater could be substituted in this example.
Review SYSTEM TYPES AND APPLICATION beginning on page 11.
“Try” a CSB -120 with 24 kw input. This heater has 98 gallons per hour recovery at 100°F water
temperature rise and a 119 gallon tank. The heater will be operated at 180°F and equipped with a water
mixing valve set at 140°F.
4. Needed: 215 gallons for one hour
Subtract:
-98 gph heater recovery at 100°F rise
Equals 117 usable gallons needed from tank
Compare
tank 119 gallon tank vs. 117 gallon tank minimum
capacity:
NOTE: The 119 gallon tank capacity at 70% tank efficiency is equal to 83 gallons of usable hotw a t e r .
However, it is 83 gallons of 180°F water and therefore has the heat content equivalent of
83 x 1.4 = 116 gallons of 140°F water. Therefore the tank size is adequate (only 1 gallon short).
Compare tank size 1.21 hours vs 3 hours available.
vs recovery: (119 ÷ 98 = 1.21 hour)
Conclusion: The model CSB -120 with 24 kw input will do the job and should be the heater selected.
Field Assistance
Please contact your local State distributor, sales representative or the technical information center (See:
www.statewaterheaters.com for phone and fax numbers) if you need help designing a water heating system or selecting
the proper equipment for the job.