[futuresoptions]

Sadly, when you install central heat only, the duct size is generally undersized for A/C. The reason this is, is because heat usually runs at a lower CFM / Fan speed and they have to make the duct smaller to keep the static pressure up. Depending on who did your install, I am going to guess that you would need new duct. A/C units usually run on high fan speed while heat runs on low speed. If you would like to know for sure, it is best to have a reputable company come out and inspect your duct work and see if it would supply the appropriate CFM needed for each room. It may be simple to work around and may only require a few more vents in a few places/rooms.