Heating, Cooling and HVAC

There are several issues to consider when adding central air conditioning to an existing heating system.

If a home has an older heating system with no provisions for central air conditioning, the ductwork may be smaller than what is required for air conditioning. Increase the fan speed to compensate for the ductwork. A larger motor often is required to achieve this higher flow rate.
In extreme cases, it may be necessary to replace the supply ductwork.

The location of the return-air registers also plays a role in comfort. In older homes, there were often no return-air registers installed on the second floor of a two-story home. It is difficult to cool the second story if this is the case. It may be necessary to install return-air ductwork.
Another consideration is the requirement for a floor drain below the furnace level. Air conditioners produce condensate when they operate. This condensate is the consequence of removing moisture from the air.

If a floor drain is not available below the level of the furnace, it is possible to purchase a small condensate pump set. For approximately $60, this set will pump the condensate to a convenient disposal site.

A final consideration is the arrangement of the ductwork at the furnace outlet. The ductwork around the furnace must leave sufficient room for the installation of the cooling coil. When installing central air, it is an excellent time to check the supply and return air ducts for leaks. Inadequate air flow across the cooling coil is the No. 1 cause for poor air-conditioning system performance.

Sources:http://www.engext.ksu.edu/ees/henergy/space/air.html#adding