Filter
Top Products
9
Eight hour demand period,
per hour capacity.
171 gph recovery Storage:
+ 8 gal storage
179 gal/8 hour 100 gallon tank
Recovery provides 96%
x 70% tank efficiency
of demand. = 70 usable gallons over 8 hours
70.0
_
8 = 7.8 or 8 usable gallons per hour
Here’s how it’s figured: 171 gph recovery + 8 gallons storage per hour =
179 gallons of hot water available per hour for 8 hours.
Thereafter, until the tank is reheated, only the heater recovery of 171 gph is available. The
heater recovery obviously provides the hot water in a long, continuous demand period. Without
any use of hot water during the recovery period the tank contents should be reheated within
about 25 minutes (70.0 ÷ 171 = .41 hour).
Select minimum recovery and maximum storage if the hot water demand period is less than 3 or 4
hours. Heater recovery must be sufficient to reheat the entire tank contents before the next demand
period.
To summarize:
“Short” Demand: “Long” Demand
• Min. recovery • Max. recovery
• Max. storage • Min. storage
Check for the possibility of any hot water needs occurring during the recovery period which could affect
the reheating of the system. Add heater recovery and/or storage tank capacity as necessary to handle
unusual conditions.
Equipment sizing calculations may lead to a combination of heater recovery and storage tank which is
not made. If so, both factors may be “adjusted” to favor one or the other as desired. Here’s how:
1. Where it is important that hot water temperature be maintained (as opposed to “within a 30°F drop”
being o.k.) increase recovery capacity in preference to increasing tank size. This will aid in maintaining
system temperature. Also, assume 10% less draw efficiency than if the 30°F drop was acceptable.
2. Where it is important to maintain water volume (for demands possibly in excess of heater recovery)
increase tank size in order to provide “instant” hot water.
Heater Recovery and Storage Tank Performance Comparison
These examples demonstrate the roles that heater recovery and storage tank capacity play over a demand
period. For example, a Model SUF 100 -150 which has an 100 gallon tank, when used for a one or an eight
hour demand provides:
One hour demand period
171 gph recovery Storage:
+70 gal storage 100 gallon tank
241 gal/1 hour
x 70% tank efficiency
Storage provides 30% of demand = 70.0 usable gallons
Here’s how it’s figured: 171 gph recovery + 70.0 gallons storage = 241
gallons of hot water available for one hour.
Thereafter, until the tank is reheated, only the heater recovery of 171 gph is available, The
heater tank obviously provides a good portion of the hot water in a short, intermittent
demand period. Without any use of hot water during the recovery period the tank contents
should be reheated within about 25 minutes (20 ÷ 171 = .41)