On May 3, 10:56*pm, Red wrote:
House is built on a concrete slab. HVAC is in a centrally located
closet. *AC drain line is 3/4" PVC going into the slab. *After 40
years of use I finally had a drain blockage. *HVAC tech said the line
drains into the sewer line and it was probably blocked there and to
use pressure to blow the blockage out. *A plumber aquaintance said the
line drains into a dry well under the slab and to use a shop vac
suction to remove the algae blockage. *Who's probably right?

I tend to doubt drainage into a sewer line because of the possibility
of sewer gas/fumes getting into the house but at the same time if it
is, I hate to hook a shop vac to a line and suck sewerage into a 3/4"
line causing a bigger blockage.

Red

Clear it using a hose connected to mains water. Use pressure washer
if that fails.

Who cares where it goes so long as it works.