Manual D Calculations

Tekneca Manual D – Duct Design Services

Manual D is the third step in designing your HVAC system with ACCA’s System Design Process and is used to determine the overall duct layout and sizes for your HVAC system.  Having performed Manual J and S, we now know the load requirements and what equipment we are going to use, the next step is to design the duct system.

The duct system must be designed and sized correctly as it is extremely important to have the right amount of air supplied to each room.  A proper duct design allows the distribution of the required amount of heating and cooling to each room based on the Manual J load calculations and the equipment selected during the Manual S process.  Manual D is the industry standard for sizing HVAC ducts systems.

The Manual D process becomes extremely involved and uses values and formulas such as the friction rate equation, which takes into account factors like available static pressure, and total effective lengths of ductwork.  It then uses this friction rate to calculate amount of air flow and velocity.  It is combined with the calculations derived from Manual J and S, and then provides the duct sizes according to the design.

Why are Duct Design Calculations Important

Achieving occupant satisfaction is the principal goal of any HVAC design.  If the duct system moving the air is not properly designed then the equipment could fail more quickly, the energy costs could rise, there may be occupant health and safety risks, and the occupants comfort  might be sacrificed.

What Problems come from Wrong Sized Ducts

For a homeowner (occupant) to be comfortable within their home a duct system must be designed to carry the right amount of air, at the right velocity, and into the right room (area).  If the ducts are the wrong size then the incorrect amount of air will be delivered to the room (area) and may cause:

  • The air may be too noisy caused by high of velocity (speed) of the air flowing through and out of the ducts
  • The air speed may to slow and create a situation where the conditioned air will not circulate or mix well in the room
  • The blower fan may work harder, and possibly fail sooner, and also use more energy to operate and move the air
  • The room (area) will be to warm or to cool
  • Drafty air will make occupants feel uncomfortable and disturb them at what they are doing
  • Equipment safety devices may activate and stop operations
  • Unwanted air pressure differentials that may cause increased energy costs and decreased comfort conditions

Benefits of Correct Sized Ducts

  • Maximizes equipment performance, by matching equipment specifications to the project requirements
  • Air velocity speeds that are within standard specified limits, keeping the noise down to a minimum
  • Distributes air evenly and mixes air which prevents hot and cold areas in a room
  • Lowers energy costs (utility bills) for the heating and cooling of the home
  • Prevents equipment wear and tear by the units not operating too often and for too long
  • You can now correctly size supply registers and return grilles
ANSI Manual D Cover
Sample Manual D Report
Sample Manual D Report

Tekneca Manual D – Duct Design Services

A Tekneca, ACCA certified designer will perform the Manual D process for you, providing you with a schematic drawing of the duct system within your home.  The drawing will display items such as the HVAC equipment location, the supply and return duct line system, duct sizes, fittings, cfm values (amount of air flow), and notations.  To properly design a duct system the previous ACCA steps of Manual J room by room load calculation and Manual S equipment selection must have been completed.  The HVAC system designer also uses ACCA’s Manual T Air Distribution procedures to correctly select, size and locate constant volume supply outlets (registers) and return inlets (grilles).

The designer will use this information to properly design a duct system that will function efficiently and provide occupant comfort.

Utilizing the MiTek Wrightsoft software, Tekneca will send you not only the drawing but detailed reports regarding the duct system.  These reports give you information about your project and a breakdown of information and calculations about the system.  These drawings and reports are to be used for your blueprints, building permits, quoting, contractors, onsite installation, and more.

Want to see a sample of what you are getting.  Check out the Sample Manual D Report on this page. Depending on your project, your Manual D Report will include some or all of these items.

Remember, to achieve maximum efficiency and comfort, follow and perform the ACCA’s System Design Process steps, first Manual J, then Manual S, and then Manual D.