The benefit of circulating out the volume of air in a building over the unoccupied hours at night is more than just relieving the conditioned spaces of air high in carbon dioxide content and replacing it with fresh, healthy air high in oxygen content. It can also be an energy- saving measure all year long.
Colder air holds less water vapor, so outside air can be easier to cool than warm inside air. Using circulating fans overnight to replace the building air with lower-enthalpy outside air reduces the latent load on the air handler the following morning. In dry climates, then, the air change can be done in the coldest part of the night to replace building air that is significantly warmer. This is also good preventive medicine against the sick building syndrome.
Any time the air in a building space is at a lower temperature than the ambient air (and consequently the average temperature of the building) the mass of the building relinquishes a part of its heat to the air mass. Cycling the building air out two or three times at regular intervals over the night (again, in dry climates) brings the building mass temperature down as far as practicable, so that the mechanical equipment does not have to do so the following day.
In the winter the best time to circulate air out of a building is just after the close of business. The outside temperature is warmest then, and the air is at its most humid. As a result, its enthalpy is highest, maybe even higher than the building air itself. Since very dry air in cold climates is an uncomfortable consequence of artificial heating, using outside air to boost the humidity is cheaper than doing it by artificial means. Also, more humid air usually feels warmer than dry air inside a building, so the thermostat can be lowered a few degrees without any noticeable effect on the occupants.
In very hot and humid climates it is not always possible to replace building air overnight with air that has a lower enthalpy. In such locations, an alternative is to run the air handler for a few hours in the middle of the night. If the unit is turned off an hour or so before the close of business, the building air will have heated up. Warm air can be dehumidified more efficiently across the cooling coils than cold air, so — with the outside air vents closed — circulating a few air changes through the unit efficiently dehumidifies the building volume. This also shifts the electricity use to outside of the peak demand period for even more savings (commercial utility rates charge more during peak use hours, i.e., regular business hours).
Later, in the coolest part of the night, as the building mass cools, its heat is radiated or convected away from the envelope. The dry air inside cools down faster and allows the building mass to be cooled even more, by the outside air from exterior to interior zones (perhaps with a little help from the air-handling equipment), bringing warmer air to the exterior zones and cooler air to the interior zones. This cools the entire building mass.
A final option is to install an additional row of cooling coils on an air handler—the more rows of coils, the higher the dehumidification—to be inactive during the day when cooling is the priority and active at night for dehumidification. Or the extra row can be cycled on and off during the day to provide dehumidification as required, such as an hour or so before the peak load is expected, so the latent energy in the building air mass can absorb the load by allowing the humidity to float upward. The extra row(s) of coils are not without a penalty, however, because they add extra static to the fan which requires more horsepower to overcome. Depending on the configuration of the system, this may or may not be a problem.
__________
Excerpt from Energy Conservation Projects.