convert gas heat to central heat & a/c

[socalsis]

Currently, My house has gas heat and gas water heater, only. I want to add a/c. I leaning more toward central a/c rather than the loud noisey window a/c. Even though I live in San Diego and wont use it very much. It nice to have it when you need it. Do I need to change the existing venting and duct when installing central heat and air.

[suemarkp]

It depends. The duct sizes you require depends on the airflow required. The airflow required is determined by doing a Manual J heat gain calculation and then assuming about 400 CFM per ton of cooling required.

Does it get very cold in San Diego? If not, you may have a small furnace with small ducts. If you want a very rough guess as to what size AC unit can be accomodated by your current ducting, figure the following:

70 CFM for each 5" metal or 6" flex duct to registers
100 CFM for each 6" metal or 7" flex duct to registers
150 CFM for each 7" metal or 8" flex ducts to registers