We have about 25ft high ceiling with 4 HVAC units on the ceiling at our facility. And we have decided to install the ceiling 14ft high drop ceiling to save energy.The contractor extended the air outflow vents from existing units using duct which is about 15ft long.But didn’t connected intake vent from to the new ceiling level to the intake grid on the HVAC unit. He state that free-flow air will go through the existing intake grid on the unit.But I am concerned that the hot air above new drop ceiling will go through the intake grille in summer. Then the HVAC is not going to efficiently be able to generate cool air outflow.
This is whats know as free air return. The unit does not require duct back to the unit because it utilizes the ceiling space as a plenum. However, you are correct that you will loose some efficiency because in addition to cooling the finished space you will technically be cooling the ceiling space as well, because you are drawing your cooled air back through the ceiling space. Also, there are some code restrictions to be considered. For example anything in the plenum space has to be fire rated. ie.. piping cannot be PVC, t-stat wires need to be plenum rated, etc.. So... the ideal situation would be to duct the return air back to the unit for the best efficiency.
the returns need to be installed like your supply in the space to be conditioned and you need insulation on top of drop down ceiling other wize your waisting your time and money it will suck hot air and work harder thus your not getting anywere
