This #10 Reduced Barrier & E-Z Clip Hose x 5/8" Heater Hose Adapter Fitting is necessary when using #10 Reduced Barrier or E-Z Clip A/C hose for heater applications. Although we prefer #10 Reduced Barrier or E-Z Clip A/C hose to conventional heater hose under the hood, we have to make a transition back to conventional heater hose to accommodate the 5/8" barbs on the heater control valve as well as the 5/8" barbs on the heater core.