Usable Storage

 August 1, 2016      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 “stand by” 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)

Usable Storage Formula with Blend Factor

  1. (Delivered Water Temp –Cold Water Inlet Temp) ÷ (Water Set Point Temp –Cold Water Inlet Temp)  = Blend Factor
  2. Useable Storage of Tank† ÷ Blend Factor  = Gallons of Useable Water at Delivered Temp

Example: 100 gallon water heater
Incoming Water Temp = 40°F; Water Set Point Temp = 160°F
Delivered Water Temp = 120°F

  1. (120-40)/(160-40) = 80/120 = .6666
  2. (100 x .70)/.6666 = 105 gallons of usable storage

†(Usable storage = 70%)