DIY Network

All About Air Conditioning

An efficient system and well-sealed ducts are critical in keeping a home cool. See how a well-planned HVAC system can optimize comfort.

More in Electrical

HVAC is heating, ventilating and air conditioning

Enjoying a finished attic space in the summer sounds like a nice idea, but the room is like a sauna. Can anything be done to make the top floor as cool as the first and not cause the utility bills to skyrocket? Yes! Choosing an efficient system and making sure that ducts are well sealed are critical steps to keeping every room of the house cool.

Air conditioning is a component of the HVAC (heating, ventilating and air conditioning) system. The air-conditioning unit sits outside of the home and pumps liquid refrigerant (a liquid that cools something -- often referred to as "Freon") into the air-handling unit (AHU), the unit inside the home that generates heated or cooled air. The AHU generates cool air and forces it through the home via supply ducts and into rooms. (Ducts are passageways, usually tubular and made from sheet metal, flexible material or rigid insulation, that deliver air from the AHU throughout the home, and also return air from the rooms back into the AHU for re-circulation.) Meanwhile, the slightly warmed refrigerant travels back outside to the air conditioner, where it's cooled and re-circulated.

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. A state-of-the-art air conditioner is only effective if the air it cools reaches the family.

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 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, it's important to discuss ductwork sealing and location -- as well as the efficiency of the air-conditioning system -- with the builder when a home is being built. Do this before construction begins to ensure the 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 locations of the vents were chosen based on climate.
  • Whether the sizes of the vents were selected to adequately throw air into the room depending on their location.

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