Delivering the Air

Furnace ducts that deliver hot air in cold months can be used for air-conditioning.

If you have ducts. You should have your ducts inspected. Many times, modifications are required to accommodate the greater volume of air produced by air-conditioning. Ironically, older duct systems, which tend to be larger, work best when retrofitting central air-conditioning. (As the heating industry has learned more about the dynamics of airflow and sizing furnaces and delivery systems, ducting has gotten smaller.)

Here's a list of typical modifications that existing ducts require:

•Upsizing the furnace blower (rated in cubic feet per minute, or cfm) to move the cubic feet of cool air required for your house. An undersize blower won't move enough air through the coils, and they can freeze.
•Sealing the ducts to boost efficiency. This is a job best left to a pro, as an imbalanced system can lead to hazardous backdrafting during the heating season or pockets of warm air during the cooling season.
•Swapping out older supply registers for ones that allow a greater volume of air to pass. Old register grilles with slits just 1/4 inch wide offer too much resistance to airflow; new grilles can improve airflow by 15 to 25 percent.

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