Do It Yourself: HOME BUILDING
 
This label appears on every air-conditioning unit and explains its efficiency rating. It's important to note, however, that even a high SEER unit may not perform well if the overall HVAC system, including how the air is distributed through the home, hasn't been well planned. As part of the HVAC system, ductwork plays a critical role in delivering cool air to the rooms. Would you buy a sports car that had skinny little tires? The same issue is at play with the cooling system. A state-of-the-art air conditioner is only effective if the air it cools gets to the spaces where you and your family are.

The HVAC contractor should perform engineering calculations to determine proper duct sizes according to how much air is designed to reach each room. Properly sizing vents, which are openings to a room, typically in the wall, floor or ceiling that allow air from the AHU to enter, helps to ensure good mixing of the air throughout the room. Installing dampers where necessary will help ensure proper flow of conditioned air to and from all rooms in the home. (Dampers are gadgets inside ducts that can be adjusted to lessen or increase the amount of air being delivered into a room.)

The size of each vent that supplies air to a room needs to be selected to deliver and distribute the air throughout the entire room depending on its location. So be aware if all the vent sizes are the same in each room! In addition, the location of the vents should be determined based on your climate. Think about the idea that hot air rises (and the lesser-known but equally true idea that cool air drops). Homes in the Northeast that need a lot of heating may do best with vents located low on the wall. Homes in the Southwest that primarily need cooling may do best with overhead vents or vents located high on the wall.

In addition, the tightness of ducts and their location are also important factors in designing a quality air distribution system.

If ducts aren't well sealed (that is, that a special sealant -- not duct tape! -- is used to seal all joints and seams), air can leak from the supply ductwork before it gets to the rooms. Also, leaks in return ductwork can capture outdoor air that is extremely hot and can warm up the cool air traveling through the ductwork. In addition, when ductwork is located in exterior walls or outside the insulation layer of a house, it's exposed to outdoor temperatures, which, in summer, can warm the cool air running through the duct.

As with the heating system, if you're having a home built it's important that you discuss with your builder ductwork sealing and location, as well as the efficiency of the air-conditioning system. Do this before construction begins to ensure your home stays cool and comfortable. Be sure to discuss the following:
What the SEER rating of the air-conditioner is and whether the indoor coil's nominal tonnage rating is one size larger than the nominal tonnage rating of the outdoor condensing unit.

Whether ducts were sealed with a UL-181 sealant (these are sealants approved by United Laboratories, and are the best-performing sealants for HVAC systems).

Whether the ducts are located inside the layer of the house that contains insulation.

How the location of the vents were chosen based on your climate.

Whether the sizes of the vents were selected to adequately throw air into the room depending on their location.