Within the Red Hat certification program, the RHCA is considered the most complete (and grueling) certification among the seven major Red Hat certifications. In fact, some equate the prestige of the RHCA to that of the Microsoft Certified Master and possibly the CCIE.
A plumbing fixture like a sink can not function properly without a sink vent. When a drain pipe drains it creates a negative pressure in the sink drain; the negative pressure will pull the water out of the p-trap of the sink that's draining and sewer gases can enter your home if a sink vent is not installed.
A return vent may be located on the wall or the floor, depending on the system. It is important to make sure that return vents are covered for safety and that the return air grille is periodically lifted and cleaned, along with the filter, to prevent dirt from entering the heating and cooling system.