July 16, 2019 Bradford White
Do you know the difference between tank capacity and usable storage?
When a water heater's approximate set point temperature is satisfied and is in "standby" mode, there is approximately 70% of the total tank capacity available. For example, if a 100 gallon water heater's set point temperature is satisfied, there will be approximately 70 gallons of the stored temperature available for immediate use.
Knowing this helps with accurately sizing applications, especially when a specific "dump capacity" is needed.
You can use the formula shown below in commercial applications that require a larger dump capacity (car washes, large showers, etc.).
Example of a 100 gallon water heater with Incoming Water Temperature of 40°, Water Set Point Temperature of 160°F, and Delivered Water Temperature of 120°F:
1) (120 - 40) / (160 - 40) = 80 / 120 = 0.6666
2) (100 x 0.70) / 0.6666 = 105 gallons of usable storage
* Usable storage = 70%