Although this type of system can be beneficial for certain homeowners, there are some potential drawbacks. When installing a return air plenum system, it is absolutely essential that the system is constructed airtight without any leaks above the ceiling space; with return air plenums, there is less control over where the air actually travels. When installing return air plenums, all wiring such as telecommunication, electric and control wires must be plenum-rated.
Return air plenums, if not properly designed within the ducting framework, can cause outdoor air to be drawn into the building. Therefore, if the building is located in a humid climate, condensation of moisture can occur from the outside air. This can eventually result in mildew and mold growth within the building structure if a dehumidifier isn't used. It is also a possibility that return air plenums could lead to health issues within a building due to the possible buildup of dust and debris on ceiling tiles and other areas.
There may be building codes that specifically prohibit return air plenums. Most codes only permit the use of return air plenums if the materials that are exposed to the plenum satisfy particular smoke generation and flame spread limits.