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
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)
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).
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