In what type of fire hydrant is both the entire barrel and head below ground elevation?

The type of fire hydrant where both the entire barrel and head are below ground elevation is known as a flush hydrant. This design allows the hydrant to remain out of service and protected from the elements, which can be particularly beneficial in areas prone to freezing temperatures, as it minimizes the risk of water accumulation that could freeze and damage the hydrant.

Flush hydrants typically have a cover that is flush with the ground, making them less obtrusive and reducing the risk of accidents or damage compared to above-ground types. These hydrants are often used in locations where aesthetics are a concern or where the hydrant may not need to be accessed frequently.

In contrast, wet-barrel hydrants have their water source above ground, allowing for immediate access to water without needing to drain after use. Dry-barrel hydrants, while designed to prevent freezing by having only the top part containing water and a mechanism to drain the rest, still have a portion of their structure above ground. Flow hydrants, typically referring to any hydrant capable of discharging water, can also include both wet and dry-barrel designs, thus not fitting the specific characteristics of being completely underground like flush hydrants.