During a Professional Home Inspection performed by Blue Star Real Estate Inspections, one of the items checked is the HVAC condensate drain. Your HVAC System can produce up to 80 gallons of condensate per day. Condensation is produced by your HVAC system as it removes humidity from the air as it is conditioned. The best place for this condensation to go is the city sewer system. PVC piping is the most common material, but cast iron, galvanized steel, copper, polybutylene, polyethylene, ABS, and CPVC piping can also be used. Whichever piping material is used, it should be no less than 3/4″ Nominal Pipe Size, and should not change size throughout the length of the pipe. The piping should be sloped to allow gravity to drain the condensate from the pipe, unless a condensate pump is used.
The discharge location should ideally be tied into an existing bathroom or kitchen drain. Condensate should never drain into any vent stack. The illustration below shows acceptable condensate drain tie in locations.
On one home inspection that I performed, I came across a condensate drain line that discharged onto the ground directly outside the wall. The condensate pooled up at the foundation of the house. While keeping your foundation moist is a good idea, you do not want isolated water pools at any time. Serious foundation damage could occur.
The property in question above was on a septic system rather than city sewer. Routing condensate lines to the septic tank is acceptable. More than likely the owner wanted to avoid excessive septic cleaning/pumping.