u/jawfur

I recently had two new Goodman AMST (draw-through) air handlers installed. Since the installation, I have been getting recurring wet spots and stains on my ceiling (of the level below air handlers). Interestingly, the spots seem to move around on the ceiling of the level below, which I suspect is because the water is overflowing the internal pan and traveling across the closet floor or the emergency drain pan before dripping down. Both units' PVC by the drain pan have wetness at the bottom of PVC piping and are dry fitted currently.

There is no P-trap at the unit. There is a capped vent between the unit and the exterior trap; when I remove the cap and cover the vent pipe with my hand, I feel significant suction. The installer says the trap is exterior (underground), but the lack of a trap at the unit combined with the vacuum from the draw-through blower is preventing the unit from draining. The installer is suggesting to simply "glue the PVC joints," but I know that will not stop the internal pan from overflowing due to the negative pressure holding water in the pan which is probably causing the wetness and wet stains on my ceiling of the level below.

We are facing two difficult options due to the fact that there is only 1.5 inches of vertical clearance between the drain port and the platform:

• Option A: Relocate the Trap. Install a P-trap at the unit (before the vent) where it belongs. This would require digging up the exterior stucco or ground to eliminate the existing buried trap to avoid double trapping, which would cause an air lock. Furthermore, there is currently not enough vertical clearance for a standard P-trap under the unit and I cannot find any p-traps anywhere that would work with the single inch of clearance we have.
• Option B: Condensate Pump. Keep the existing exterior trap and install a condensate pump between the air handler drain and the vent pipe. The pumps require the PVC to be ~4 inches above the platform bottom, but we only have a quarter of that available. This would involve breaking part of the platform the air handler sits on. There is no more space because this is a compact closet that houses the air handler.

Questions

1. Since I have no vertical room for a standard trap or a typical Little Giant pump, are there ultra-low-profile "Mini" or peristaltic condensate pumps that are rated for central air handlers and short enough for the clearance of less than 2 inches?
2. For option B, can I leave the buried exterior trap alone? Are these two options in the right direction to fix this issue?
3. Is the "moving wet spot" common when an internal drain pan overflows and the water follows the slope of the subfloor or joists?
4. Why did my previous Lennox system have no problems with leakage despite the p-trap being exterior and the same plumbing setup? How are the physics 'changing' with this new system?
5. Should my installer have seen this coming beforehand and installed the air handler at a more elevated position to allow for proper plumbing adjustments?
6. General Advice? This is becoming a very frustrating headache and I am trying to be patient with the installer because they are an acquaintance but they are not listening to my technical reasonings above. It sounds like they are going to claim that this issue is outside of the scope of their contracted work and that he simply replaced the existing and nothing more.