HVAC systems like your home's air conditioning are built to improve and circulate air throughout your home. The air that is carried should be carried through a series of metal ducts that send the air to all parts of the home. There are also return ducts that take the air back in that is already in the house and supply it to the system for recycling.
Wall and floor cavities are usually filled with insulation, even if they also hold metal ducts. In older homes and where builders have taken shortcuts, return ducts may run through sections of the raw wall cavity instead of a proper duct. This is against most building codes for some very good reasons. Here are a few to help you understand why you shouldn't use a wall or floor cavity as an air conditioning return duct.
Difficulty in Sealing
The building cavities of your home are not designed to carry air through them. There are many small openings that altogether lower the air pressure going back into the AC unit. This has a detrimental effect on how much air is then pushed out through the rest of the system. Metal air ducts are designed to fit together tightly and seal. They are also sized correctly to the HVAC unit to keep air volume optimal. That is just not possible when using a building cavity as a return duct.
Dust and Contamination Issues
Building cavities are host to all manner of particulate from the breakdown of insulation and other building materials. Mice and pests also leave around a bunch of unhealthy debris in the building cavities that you don't want going into your intake ducts for two reasons. Firstly, all the particulate can overwhelm your air conditioner's filter. Secondly, the particulate itself wears out the internal components of the air conditioner itself.
Many homes tend to have small building cavities to minimize wasted space. Using this space as a return duct often provides too little air to the AC, leading to damage to the blower fan that circulates everything. If the blower cannot pull as much air as it needs from a properly sized duct, the motor will work harder to make up for the lack of flow, burning out prematurely.
Joist panning was an old-fashioned attempt at making a better cavity duct by installing sheet metal to help seal up the cavity. This practice was then stopped because the metal eventually comes loose and starts leaking air even with attempts to reseal it. Metal ductwork is less likely to come loose and easier to test for leakages and issues, so choose it for your AC installation instead of attempting alternative duct methods.
Reach out to a service such as STA SO COOL HVAC to find out more.