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Air Duct Systems

The tubes that your AC/Heat Pump pumps air through are called the "distribution system" And when it comes to saving energy and money, they're just as important as the heating and cooling units themselves.

Parts of a Distribution System

  • Air Handling Unit — This includes the furnace, AC, or heat pump, plus the compressor/blower that circulates the air.
  • Supply Ducts — These carry the air to the rooms. Each room has at least one supply duct and larger rooms may have more.
  • Return Ducts — These carry air back to the air handler. Most houses have one or two in a central area.
  • Plenums — These are boxes that distribute air to the ducts or registers.

All parts combined, the Distribution System is a network of tubes that winds its way through your walls, floors, and ceiling. Most distribution systems are made primarily of metal, fiberglass, duct board, and Flex-duct.


Flex-duct is a single piece of duct that connects registers to plenums or plenums to air handlers. There are few seams for air to leak through; however, if its inner or outer insulation is damaged, it becomes an efficiency problem. Also, runs of flex-duct that are too long can restrict air flow. Also, flex-duct can be crushed closed allowing little to no air flow. Proper design and installation are tricky and very important.

Metal and Fiberglass Board

These sections of duct are rigid, and several pieces are connected in a single run. The seams between pieces must be carefully sealed with Mastic or other appropriate sealant. Metal Ductwork should have at least two inches of fiberglass insulation with an attached metal foil vapor retarder facing away from the ductwork. Ductwork should be completely sealed before insulation is added, especially when located outside of conditioned space.

Energy loss through ductwork

Ductwork leaks and faults can waste up to 40% of the money you spend on energy to heat and cool your home — that could be around $1,000 a year! Ducts in the basement are better for the Earth and your checkbook than ducts in hot, humid attics.

When heated air flows through ducts, it can conduct into the surface of the duct and then out into the surrounding air. For this reason, well-insulated ducts are a necessity.

Heated or cooled air can also escape or enter through holes in ducts and through improperly sealed seams between sections of ductwork. Proper sealing and maintenance are therefore essential. It is important to make sure ductwork is completely sealed before insulation is added, especially when located outside of conditioned space.

If you have problems with air circulation or comfort, check all the ducts, or have a professional do it.