No trap clean-out, no vent. This drain deserves an "F". It can "Air Lock" because there is no vent after the trap. With no overflow safety switch, no clean-out and no vent one is just asking for several thousand dollars of damage due to condensate water backing up and overflowing the cooling coil drain pan flooding out this interior furnace closet. I was giving an estimate to replace this system as it was not operating, but had it been operating and this was a service call I would have not left it running in this condition. I would have made corrections unless the customer refused to and signed off on this statement.
"Install not up to industry or manufacturer standards, not responsible for any water damage due to installation conditions."
How many years has this been like this? 20 or so and no one sees this? What an industry. I think plumbers know they need to vent drain lines that is like 101 plumbing.
Water liability now days is such a huge problem with mold and everything else that can happen there is no short cut to how we do it. Per Lennox factory specs I helped to design and illustrate I know how to do it right. Do this and condensate problems will be just about nil as long as you glue up the fittings and properly route the piping. If anyone is questioning the Rubitex insulation around the trap it is because the water in the trap is very cold and the trap would condensate being it is located in unconditioned space. The clean-out tee is one-inch so the Gallo Gun cone fits easily into the tee. See we have thought of everything.