ASI's Learn About HVAC Duct Work and Grille Sizing
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Duct Work and Grille Sizing Information with some rules of thumb
Duct work sizing on most homes is not correct and this is due to a couple of factors, the primary reason is cost and the other factor is misused "rules of thumb" in the industry.Supply Ducts
Many HVAC contractors have used a rule of thumb for duct sizing for many years and in many cases no actual duct calculations ever really took place outside this rule of thumb. The rule of thumb has to do with calculating the friction losses in any given duct.
Many contractors use a friction loss factor of 0.1 inches of water column for both the supply and return duct size calculations and in most if not all cases this leads to poorly sized ducts and causes uncomfortable areas as well as noisy grilles because the ducts end up too small. In our experience 0.06 inches of water column for supply ducts provides better circulation with minimal noise.
Return Ducts
Return ducts are sized incorrectly on a regular basis and many homeowners could save a considerable amount of energy by checking their return duct size. The industry standard for system total air flow is usually based upon the amount of cooling required, this is due to the fact that central air conditioning systems require a specific amount of air flow at all times to perform at their maximum efficiency. This industry standard is 400 cubic feet per minute (CFM) of air flow for each ton (Btu/hr) of cooling.
Again many contractors use the 0.1 inches of water column for the "rule of thumb" on their return ducts when in fact the friction losses should be closer to half that at 0.05 inches of water column. This, in effect starves the system for air flow when it needs it most, when it is running the air conditioning.
As an example for a standard 3 ton AC unit on a given home the return duct should be at a minimum 14.5 inches in diameter, granted the next standard size duct is 16 inches and that should be the size of the duct for proper air flow. Many homes will have a 12 inch or 14 inch duct for the return duct connected to what in most cases is the wrong size return grille.
Return Grilles
Return grilles themselves are another critical factor for air flow and again many in the industry use in an incorrect "rule of thumb" for sizing the return grilles of 1 square foot per ton or 100 square inches per ton. When in fact a grille will generally flow air through a grille at about 2 cubic feet per minute per square inch.
As an example lets look at that same house with 3 tons of cooling and assume it has a 20 inch x 25 inch return grille which has a total of 500 square inches of gross area. We then multiply the 500 square inches by the standard 2 cubic feet per square inch of air flow and we get 1000 cubic feet per minute (CFM). That is 200 cubic feet per minute (CFM) short of what is actually needed for a 3 ton system which is a minimum of 1200 CFM.
As a result many systems return ducts and return grilles are undersized by as much as 30% which means that you give up 30% of your air conditioning systems capacity.
Local Code Requirements
Recently in Southwestern Idaho new and improved HVAC code requirements provide that duct work be designed using standard methods of design, the "Manual D" among others. If enforced correctly this should lead to much better performance and efficiencies on new homes, however this does not correct all those homes out there now that were designed using these "rules of thumb" that have been passed down from generation to generation in the HVAC industry.
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