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2. Select a trial size heater 2 / .
3. Subtract the hourly heater recovery from the demand.
4. The difference in gallons between demand and recovery must come from the tank.
5. Multiply the difference by the number of demand hours. The result is the “usable” number of
gallons which must come from the tank.
6. Divide the “usable” tank gallons by .7 or .8 to obtain minimum tank size needed, see pages
7 thru 10.
7. Compare minimum calculated tank size with that of the “trial size” heater. If the heater tank is
equal to or greater than calculated tank size the selection is satisfactory. If not, adjust
recovery and storage as necessary, see page 10.
8. Divide the heater tank size by the heater recovery to be certain the tank will be recovered by
the time of the next demand. If not, adjust recovery and storage as necessary, see page 10.
1* / The demand could be in minutes or seconds. In either case all references to hours in the
procedure would revert to minutes or seconds. For example, a stationary rack type dishwasher
may have a 12 second demand period and an 83 second recovery period.
2* / Review PROFILES OF OPERATION, Page 5, as an aid in determining whether to favor
recovery or tank capacity in the selection of a “trial size” heater. Normally the hourly heater
recovery of the heater selected should not exceed the hourly demand. In this way the hot water
content of the tank will be put to use.
One temperature example
1. A two hour demand of 206 gph 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 8 hours.
2. A State gas-fired tank type commercial water heater will be selected for the job. (Any fuel or
type of heater could be substituted in this example.)
“Try” a Model SUF 100 -150. This heater has 171 gallons per hour recovery at 100°F water
temperature rise and an 100 gallon tank.
3. Needed: 206 gph for 2 hours
Subtract: - 171 gph heater recovery at 100°F rise
Equals: 35 gallons needed from tank, first hour
Multiplied by:
x 2 demand hours
Equals: 70 usable gallons needed from tank
Divide: 70 ÷ .7 = 100. gallons minimum tank size
Capacity 100 gallon tank vs.100. gallon tank minimum
Compare tank size vs.recovery:
Used 70 gallon. 8 hours is available to recover tank.
(70 - 171 gph recovery = .41, .41 X 60 minutes = 24.6 minutes needed to recover
70 gallons.
Conclusion: The Model SUF 100 -150 will do the job and should be the heater selected.
CAUTION: A two hour demand of 206 gph means that the 206 gph is spread throughout the
entire hour. It does
not mean that 206 gallons is dumped in 15 minutes and no additional hot is
used in the remaining 45 minutes.