As to the blowing out, I know 99% of the time it goes fine, but the drain lines are not always assembled well and capable of handling the pressure. Every once in a while, a line can come apart in a wall cavity, etc. (We had our own bad experience having them blown out when we moved in - they were badly clogged and exit at a bathroom sink. We ended up with black crud all over the walls of the bathroom. After that, I've been very diligent about treating the lines with vinegar every month, which has kept them clear for 5 years.)

Sorry, I realized after confusing you that my picture is not at a great angle. There is actually a p-trap between the Jones valve and the drain to lower floor. I've updated the pic to try to make this clear.

@jay613 - I've heard horror stories of damage to AC drain lines in wall cavities/etc that aren't well-assembled; they can handle draining just fine, but are not expecting any real pressure. And yes, you would cover the vent with your thumb if using the Jones valve, and add the vinegar there when doing preventative maintenance.

@PlatinumGoose - central TX. They do have sniffers and he could not find anything outside or in the attic (without cutting into the sealed part of the unit that contains the evaporator coil). I don't know if the life expectancy is 10 years as much as failure after 10 years isn't unheard of. It's a builder grade lennox unit.