Duct Design and Manual D

If you are going to replace your existing ducting for your new HVAC system you should really understand a few basic concepts of duct design so you can ask your prospective contractor the appropriate  questions. In addition a little history about duct systems in a majority of the homes in Southern California especially tract homes is in order. 


Undersized ductwork is the norm unfortunatley, along with oversized equipment. The result is an inefficient system, despite your equipment ratings, reduced comfort and increased utility costs. In many cases two systems were installed when it was only done for one reason, and that was one for upstairs and one for down. With modern technology we can install one system with variable speed condensers or heat pumps and a modulating zone system. By doing so you ultimatley reduce your repair costs down the road.


Manual D duct design simply put is a design standard for proper duct sizing which is usually performed on the computer using a program like Wrightsoft. The program analyzes each room, the  load or amount of cooling needed to keep you comfortable, and the required duct size to do so. It will design the whole duct system for the project. Obviously this may not be done on existing two story homes and this is when it is important for a contractor to know the basics so they can improve on the existing design. 


Proper duct design is also essential for your equipment to operate at 100% capacity. If you have a 4 ton system (meaning 48K BTU) you should get that capabilty, if not then you might only get 42K BTU or less.  



The picture above show the latest flex duct with a mylar jacket. One of the most durable flex ducts made as of yet. The only downside we have seen is it can conduct electricity so you have to be careful around open junction boxes in attics as we have found out from personal experience it will carry an electrical current. The most used versions are the R6 and R8 insulation values. The higher the R values the better the insulation. 

This is a recent picture of a duct system we replaced. The owner had no idea why their system didn't seem to cool very well anymore. 

This is one of the early versions of flex duct and the outer grey jacket was not UV resistant even in in direct outdoor light coming through a dormer vent, gable vent etc. The material just turned into a brittle paper like substance. After falling off the duct in many cases the pink insulation would come off leaving the inner liner exposed to the heat of the attic. 

Improvements were made and a black jacket duct that was UV resistant came out. However recently we have found the same effect in some cases on that flex as well but it doesnt seem to be as bad. 

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