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State commercial tank type water heaters, hot water storage tanks and water heating systems using
tanks have assigned tank efficiencies as follows:
Gas and Oil-Fired Tank Type Heaters
• Use 70% tank draw efficiency for all one and two temperature applications. For example, a gas
fired Ultra Force
®
SUF100-150 model has an 100 gallon tank:
• 100 x .70 = 70.0 usable gallons of hot water available within 30°F temperature drop during the
demand period.
• Conversely, if 70.0 gallons of usable hot water were needed from the tank over the demand
period, the minimum purchased tank size would be:
70.0 ÷ .7 = 100 gallons
Note: Storing water below 140°F may require more storage capacity.
• If the input of the heater is satisfactory for recovery purposes but the tank size is not, an
auxiliary hot water storage tank may be piped into the system to increase the amount of available
hot water during the demand period. State instruction manuals show the details.
Electric Tank Type Heaters
• Use 70% tank draw efficiency for all two temperature applications. For example, a model CSB
- 52 has a 52 gallon tank:
52 x .70 = 36.4 usable gallons of hot water available
within 30°F temperature drop during the demand period.
• Conversely, if 36.4 gallons of usable hot water were
needed from the tank over the demand period, the
minimum purchased tank size would be:
36.4 ÷ .7 = 52 gallons
• Use 80% tank draw efficiency for one temperature
systems in the same manner as described for two temperature.
• As in the example of gas and oil-fired tank type heaters,
and auxiliary tank can be used to supplement the heater capacity
if necessary. However, it should be noted that State commercial
electric water heaters are available in tank sizes to 120
gallons. Booster size heaters may also be connected to
auxiliary tanks of any size. This would permit fuel conversion
at a later date by heater substitution.
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Auxiliary Tank (Unfired)
• As explained previously, auxiliary tanks are used to increase the hot water storage potential of
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gas and oil-fired an electric tank type heaters. Also, auxiliary tanks are used with gas copper
heat exchanger type heaters in applications requiring stored hot water.
• Use 70% tank draw efficiency for all two temperature applications.
• Use 80% tank draw efficiency for all one temperature applications piped according to State
instruction manuals.
Heater Recovery Plus Storage Tank Equals Demand
As previously explained, select maximum recovery and minimum storage if the hot water demand
period is longer than 3 or 4 hours. Storage must be sufficient to handle any peaks within the
demand period.
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