VAV System

There are many different thoughts about this..........Mine is to design the system around worst case scenarios and while noting the best case scenario spec's. Then choose the VAV's and control system well. Finally knowing both ends of your operating conditions choose a VSD for the system and you will be able to accept load swings well.The VSD will reduce the noise that happens when all VAV's close down by slowing the fan speed. Don't forget to choose heat-strips for the areas of the building that will be susceptible to cool in the morning on one side and warm on the other and the mid day swing of both conditions.

thanks Mike, I apreciate that. That is on the supply air side. How about the return side. Should we use Return ducted system with return fan or just free air return to the AHU. If we use the ducted return, how are we going to control the fan? Should the fan use VSD as well.

Return air fans are not common on smaller systems. Larger system should be set-up for a "system" minimum flow rate and you should not need a return fan. I am forwarding this to the best a/c duct system control guy I know so he should chime in in a day or so.

thanks mike

Hi,

Cost is the main concern. Ducted returns are the best case. Return fans with vfds to control building static is also best case. Vfds for suply air static contol are required. Some question I would ask myself Is this a long single story building where the exteme peremeters may not get adequate return. How well insulated is the return air space. How much area is available in the return air space for ceiling plenum (structural steel etc.) Is this a high rise building. Will I use several smaller airhandlers centrally located within the zones it will handle or one large air handler. Hope this helps

The designer in my project( VAV) missed this point therefore some time he did it ducted while other is free return! I deleted the return duct assuming the return air will be pushed by supply air will as the suction in return side. But I do believe that return ducted type with motorised control damper is the best solution theoretically, motorised damper will be reset at commissioning time for several position, using Building management system can help to programed each VAV with associated motorized control damper to move in same time to keep air balancing in served room