I had a forced air/ac unit installed a while back in my attic. This is a new home and I am just finishing it up. I note there are two 3/4" PVC drain lines coming from the drain pan in the attic. One goes outside and terminates above a window. I assume this is a secondary drain line and I should see nothing coming out of it unless the primary one clogs. I also assume this is why it terminates above the window which would make it easier for me to see anything draining from it.

The second 3/4" PVC line terminates beneath my vanity in the bathroom. I assume this is the primary condensate drain line? I have no idea how to hook this line up? I have been researching it on the net and find some information but no direct information as to how to make this connection.

Do I just hook it up to the LAVORTORY side of the P-Trap for the sink? Is there a special tailpiece to accommodate this line? Is a separate P-trap required for the condensate line? Do I hook it up similar to the dishwasher discharge line? Does it have to continue to be PVC or can I use a flex hose? I'm clueless at this point and cannot seem to find a definitive answer. Can you help?

You better be talking to your AHJ or plumbing inspector for the area in which you live. In this area they will allow no direct connection to the stack or other plumbing drainage areas.

That's a good point. But I'm pretty sure it is allowed. I live in San Diego, CA