Hi Joe,

Thanks for the suggestion. I would like to find out a little more information on what you mean. There are typically two methods to determine the supply and return surface area. One is to use the percentage method and the other is to use the actual square footage measured. We currently used the percentage method by default which calculates the surface areas as a percentage of the square footage of the home.

We also change the assumptions around what percentage of the ductwork runs through the unconditioned space based on the region of the country. For example, in Florida we may assume the system is located in a garage and only 10-15% of the ductwork runs through an unconditioned space. In New England if a home is old we might assume the ductwork is 100% located in a crawlspace or unconditioned basement.

The only downside to using the percentage method is Manual J procedures use the same percentage for the supply and return. Do you see the supply-to-return ratios being much different that 1:1 in the field? If so, what would contractors like to see us use for a ratio. If they are 1:1 then the percentage method and the supply area method should yield the same result.

-Bill