Abstract: A secondary valve unit is provided to interconnect a front/finish assembly and an existing wall outlet for supplying medical gases. The secondary valve unit includes a cylindrical body containing a secondary valve assembly to prevent gas leakage when the front unit that contains a primary valve is removed for services. The secondary valve unit is pre-assembled and threadedly connected to the wall outlet. The threaded connection is locked by an indexing plate which is attached to the wall and provides indexing connection with the front unit to avoid mistakenly connecting a wrong front unit for different gases. The secondary valve unit is especially used in renovations of medical gas systems in hospitals, and able to be installed to an existing wall outlet without breaking the wall so that the cost for renovations is significantly reduced. The connector assembly for gases using the secondary valve unit provides a great gas flow and achieves a more secure connection over a long period of time.