For the purposes of marking fire hydrants, a deficient fire hydrant is one that cannot provide a fire flow of at least 500 gpm at 20 psi residual pressure. Deficient fire hydrants (Class C hydrants) must be properly marked and identified. The public also needs to be educated in this area. A typical homeowner or business owner who sees a fire hydrant near their home or business, may not realize that the fire hydrant cannot provide enough water to extinguish a fire. In addition to giving the homeowner or business owner a false sense of security, inadequate fire flows result in higher property insurance premiums. Insurance requirements often state that adequate water flows must be available within 1,000 feet of structures to get full credit for fire hydrants.

Where fire hydrants are properly marked, most fire departments only connect to a Class C hydrant as a last resort because connecting into a red top hydrant may not supply basic needed fire flows and it can create back pressure situations.