Air conditioning systems in homes and small commercial buildings are often connected to the heat system and use the same vents and ducts. Why might this be the case?

Heating and air conditioning both need a way to circulate conditioned air through the building. Using the same vents and ducts avoids the cost and space needed to install two separate distribution systems, so sharing ductwork is often more economical. The systems generally operate at different times, not simultaneously, and hot and cold air do not behave identically under the stack effect.

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