ACCA Manual D Duct Design (3rd Edition)

The residential duct system is designed to match the heating and cooling equipment blower capacity.  The Manual D duct layout provides the contractor with a "duct blueprint".

How to accurately design HVAC duct based on ACCA Manual D

• Undersized residential HVAC duct makes the temperature rise through the furnace too high
  
• Undersized duct systems impacts cooling capacity by freezing the coil
  
• Proper residential duct design software ensures that the needed amount of conditioned air is delivered to each room
  
• Undersized heating and cooling ductwork results in unacceptable noise levels
  
• Oversized furnace and A/C duct will result in too low velocity and rooms not receiving adequate comfort
  
• Calculate properly the available static pressure (ASP) - Information on blower and all air side pressure drops for the coil, air filters, registers and grilles
  
• Calculate total effective length (TEL) - This is the longest supply path + the longest return path + fittings
  
• Calculate friction rate value based on available static pressure - per ACCA Manual D the friction rate must be more than 0.06 and less than 0.18 (0.06-0.18 FR)
  
• Design adequate return air paths for all rooms
  
• Size duct based on needed CFM and friction rate for each room
  
• Verify duct air flow rates and velocities are within ACCA Manual D recommendations
  
• Provide residential air distribution system layout with duct sizes
  
• Provide correct locations of HVAC returns (regarding high or low position)
  

ACCA Manual T Air Distribution (1st Edition)

Properly sized grilles and registers are essential to proper room comfort.  You can have a properly sized HVAC system, proper equipment selection and a properly sized duct system but improperly sized grilles and registers could ruin all that hard work.

How to accurately size HVAC registers and grilles with ACCA Manual T:

• Verify supply register and return grille face velocities
  
• Verify supply register throw and spread (direction of jet)
  
• Verify pressure drop produced by terminal devices (grilles and registers)
  

Avoid having Goldilocks ductwork.  Confirm your HVAC duct designer uses ACCA approved residential Manual D duct sizing software and theory to size your duct work.  You are investing a lot of hard earned $$$ - with proper heat loss/heat gain results (Manual J), you can select a properly sized system (Manual S), and proper duct sizing (Manual D).  Then your Comfort System will be Just Right

What is Manual D?  Manual D is the industry standard for sizing home HVAC return and supply duct & registers.  Manual D duct design distributes the correct amount of heating and cooling to each room based on the Manual J load calculation results. The furnace and A/C selected during the ACCA (Air Conditioning Contractors of America) Manual S process will determine the duct CFM for your exact humidity and winter and summer design temperatures.

Be sure to check out our webpages on What is Manual J? and What is Manual S? - for other questions about the ACCA Manual J, S & D Process

Nationwide ACCA Manual D Duct Design Service - Call 801-949-5337 Today!

What is Manual d duct design?

ACCA Manual D Residential Duct Designs - the 3rd step in the quality hvac design process

Developed by Hank Rutkowski

Why is a residential Manual D duct design important?  You're ready to replace an old HVAC system or perhaps you're building your dream home.  Before purchasing ANY HVAC equipment, there are 3 important HVAC design factors to ensure optimum comfort in your home for many years to come.  This webpage addresses the 3rd step - sizing HVAC supply & return duct properly using ACCA Manual D duct sizing techniques developed by Hank Rutkowski.

Why do many permit offices require an ACCA Manual D duct design before issuing an HVAC permit?  Because Manual D Residential Duct Systems has a proven track record over many decades just like the ACCA Manual J load calculations & Manual S HVAC sizing.  In proper duct sizing techniques, your HVAC designer will utilize:

• Layout of your home from your floorplans
  
• Manual J Room X Room load calculations results to ensure the correct heating and cooling is supplied to each room
  
• Correctly sized supply registers and return grilles based on Manual T air distribution
  
• Properly sized supply and return main plenum based on velocity (fpm) and friction rate.  Too high a velocity produces noise issues.  Some contractors use the same size branch duct in all rooms while the bath needs very little conditioning and the Great Room requires 2-3 times as much
  

The Manual D procedures creates a "duct blueprint" that code officials can inspect, a home owner can review and an HVAC installer can use.

Residential HVAC ductwork that is too large will result in rooms becoming uncomfortable - This duct work is TOO Big!  Duct work that is too small will increase your utility bills and perform inefficiently - This duct work is TOO Small!

What does a Properly designed Residential HVAC Duct system provide or prevent?

1. Duct work designed by Manual D is critical for proper performance of your heating and cooling equipment
   
2. A less expensive system with a proper duct design often performs better than a high SEER system with a poor duct design
   
3. A poor duct design results in much higher heating and cooling utility bills
   
4. Poorly designed duct layouts will require your HVAC unit to run longer and more frequently
   
5. An improper duct design can result in inadequate air flow causing HVAC compressor issues and can cause the A/C coil to freeze up
   
6. Uneven temperature distribution can result in a Goldilocks home - some rooms will be too hot in the summer and other rooms will be too cold in the winter
   
7. Because the unit is working harder to overcome the duct design issues, the unit may not last as long as normal
8. If the heating and air conditioning ductwork is poorly sealed or has leaks, moisture and mold can develop due to condensation in the unconditioned attic or crawlspace
   
9. A quality HVAC design, selection and installation will increase the comfort of YOUR family and the value of YOUR home - Insist on an ACCA Certified Manual D duct design!​
   

Why Should You Insist on a Proper Residential Manual D HVAC Duct Design?

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Like everything, there is a right way and a wrong way.  Many contractors use improper duct sizing techniques because they want to save materials, time and labor.  Their shortcuts result in a spider web of duct work of varying sizes and lengths.  Supply registers deliver inconsistent comfort and the returns are often undersized.  Do you really believe all your rooms are receiving the required CFMs consistently?

If the ductwork is not installed properly you cannot expect your comfort system to be effective.  Your system may be properly sized using ACCA certified Manual J load calculation results, and you may have selected a great HVAC unit, but your poor residential duct design will result in HVAC ducts that don't deliver the required airflow to each room.  Or sometimes contractors will add an addition onto an existing system and the new heating and cooling system is too small to cover both the existing space and the new addition.

Some symptoms of a poor duct work design are:

• Your air conditioner runs continuously
  
• Your energy bills seem unusually high
  
• Some rooms may seem very cold while others seem very hot
  
• Your air filter never needs to be changed and always looks new
  
• Occupants seem to have a higher than average incidents of sore throats and respiratory issues
  

Many municipalities and permit offices are requiring an residential ACCA certified Manual D duct design along with a Manual J load analysis and Manual S HVAC equipment sizing to meet code requirements.  These residential HVAC duct designs accurately size the duct work needed for your properly sized heating and cooling system.

Some things to look for in a good home HVAC duct design are:

• Supply trunks that are equal distances apart (spread out evenly along the supply plenum).  This provides even heating and cooling distribution and ensures each room maintains about the same temperature.
  
• Correct number of registers, correct diameter of supply trunk.  Too many supply ducts and the CFMs will be too low delivering too little comfort to each room.  Too few supply ducts and the CFMs will be too high delivering too much comfort to each room.  Improper duct sizes will distort the flow to each room.  For example, if a bedroom has too small a supply branch while the bath's is too large, the bedroom will hardly get any air while the bath will get far too much.
  

Improperly sized return trucks:

• Coil could freeze up if undersized
  
• Compressor is forced to overwork
  
• House will take much longer to achieve the desired comfort level
  

When your home duct system is properly designed, air will distribute evenly throughout your home.  Each duct will carry the proper amounts of CFMs.  Save yourself money, time and aggravation.  Invest in an ACCA Manual J load calculation and Manual S equipment sizing to ensure you select the properly sized HVAC unit and an ACCA Manual D duct design and enjoy years of comfort.  Your home will no longer be a Goldilocks home - instead it will be Just Right!