We’ve addressed the importance of the load calculation and equipment selection processes in previous posts regarding quality installation.  Our next point of discussion is duct design, which is pretty simple, provided that information regarding the block load for the home as well as the individual space and/or room loads for the home is available.

The load calculation data gives us the information necessary to select equipment that will meet the capacity requirements for the geographic location in which it is being installed.  And in turn, the equipment data provides the sensible (and latent for cooling) output capacity, Cfm and static pressure available from the furnace or air handler that is selected to serve the load.  Once the pressure losses of all the secondary components that will be installed in the air distribution system are known, the available static pressure (ASP) can be calculated.  Secondary components include the evaporator coil, filter, dampers, air terminal devices, etc.

By creating a rough drawing or sketch of the proposed duct system (including equipment placement, supply and return trunks and branch run, and air terminal locations,) the total effective length (TEL) within the critical circulation path can be determined.  TEL is the combined length of the straight duct and the equivalent length of the fittings.

The ASP and TEL values are used to define the friction rate (i.e. the pressure drop in inches of water column per 100 feet of duct length) that will be used to determine the duct size for the supply and return ducts and the individual space and or room loads.  Friction rate helps ensure that the duct system will deliver the design Cfm at the static pressure available from the equipment.

The details associated with proper duct design are defined and illustrated within ACCA Manual D.  Manual D contains detailed explanations of everyone of the steps outlined above and the illustrations and examples make it easy to understand.  It also contains a significant amount of supplemental information that you will find useful in defining the best way to address duct design and comfort for your customers.

Regarding Manual J for load calculations, Manual S for equipment selection and Manual D for duct design:  Each of these manuals is sanctioned by ANSI (American National Standards Institute) as and industry standard for the residential HVAC marketplace.  These are the standards that we are all expected to follow.

Our next post will move from the design process to the physical installation processes associated with heating and cooling equipment.