During the summer months the efficiency works the complete opposite. During the winter the heat supply is the key vehicle for the efficiency while the air return supports the circulation of warm air to be exchanged. In the summer the air return is the key vehicle for the efficiency and the supply supports the circulation of cool air to be exchanged. Here is how it works, the earth stays about 55-60° F at about 48” (this is a medium guide and may very depending on location) during the summer months. With an air return in the crawl space the home can utilize this free cool air in the home. When the air conditioner (A/C) comes on its job is to take the air from the home, cool it and then redeliver it. In the process the A/C will also dehumidify the air. The air in the home is pulled to the A/C by way of the air returns and then delivers the new cool air through the supply. We all know or should know that the air in the crawl space is cooler than the air outside. When the air is pulled from the home to be cooled it will take the warm air from the ceiling area of the home and mix it with the cool air from the crawl space. Since cold absorbs heat the cool crawl space air will lessen the work load of the A/C. This causes the A/C to work shorter cycles and run less often which impacts the electric bill saving the homeowner money. On a quick side note – the best solution for the home is to have both a dehumidifier and conditioned air in the crawl space. Here’s why, in the high humid months of summer the crawl space dehumidifier will both protect the crawl space from high moisture and help the A/C run even more efficient. The dehumidifier will lower the amount of moisture in the home therefore allowing the A/C to cool the air quicker and that means running even less. During the spring and fall months the outside temperature may be at a comfortable level and there is no need for heat or A/C. In these months the humidity can still be a problem and a dehumidifier can insure the moisture levels in the crawl space are kept under control. If the budget does not allow for both, conditioned air and a dehumidifier then the best route is to condition the crawl space. You will gain the energy efficiency and the furnace will control the humidity in the winter because it will cook the moisture out of the air and in the summer the A/C will step up and dehumidify the crawl space air. The only threat in this situation is the weeks or months the crawl space does not have conditioned air because of the comfortable outside temperatures. One way to help this is to turn on the fan to your furnace (no heat no A/C) to circulate the air in your home and crawl space. The fan by itself draws very little electricity and is an efficient way to move air.