What purpose does this part serve?

bb12385 · December 2025

Hi, I’m trying to figure out if this is a secondary drain pan and what purpose it serves? I reached out to my local HVAC contractor about it but have not heard back.

I am no expert, so please forgive me if this seems ignorant. Wondering if it should have been removed at the time of install?

Visible signs of mildew and moisture behind it and it appears it cannot be removed without having the lines disconnected first. I just reinstalled the blower motor and fan after doing a deep clean so I’d like to make sure this pan isn’t contributing to moisture buildup.

Thank you in advance for your time and expertise!

Ironman · December 2025

It’s a horizontal drain pan for when the AHU is installed in that position.

Since the unit can run with it when it’s in that position, it does no harm to leave it in when the unit is vertical. In fact, it can actually help hold the insulation from coming loose.

hot_rod · December 2025

Is it the drain pan for when the unit is mounted horizontally?

EBEBRATT-Ed · December 2025

Mult position Air Handler

pecmsg · December 2025

That entire unit is one big mold factory.

Download the manual and see if it must be removed or it can stay.

ratio · January 1

I've yet to see instructions to remove the horizontal drain pan of a multi position coil.

The mold does seem excessive though. What's the air flow, ∆T & leaving temp/RH? Perhaps airflow is too high & moisture is coming off the coil. Got good filters installed, that are changed regularly?

pecmsg · January 1

https://forum.heatinghelp.com/discussion/comment/1880239#Comment_1880239

I believe Trane had a model that required removing it when used vertical, others recommended removing if not used.

Check the IOM.

mattmia2 · January 1

maybe set up the t-stat to do a post purge to dry things out a bit.

Big Ed_4 · January 1

The drainage is all wrong … You need a trap for the low pressure drainage and the vent belongs down stream of the trap … I am assuming it is leaking in the crawl space …

Happy New Year

mattmia2 · January 1

https://forum.heatinghelp.com/discussion/comment/1880313#Comment_1880313

right, because the evaporator is on the inlet of the blower, the condensate can't drain without a trap. if the drain is open to the atmosphere at the pan the suction from the blower will pull air in and keep the condensate from draining.

bb12385 · January 2

Thank you all so much for your feedback! Unfortunately there seems to be a shortage of qualified and thorough HVAC techs in this area, at least in my experience. Upon further inspection, I noticed a lot of mold on the coils above the blower fan mostly concentrated to the left side of the unit (which is directly in line with the additional drain pan). I cleaned those coils as much as I could but noticed even more mold above them on the walls of the duct.

I just did a deep clean on the return plenum and I am in the process of installing foil backed foam board in that area to better seal it off. The filters are changed once a month and I use 20x30x1 MERV 11 filters. I also cleared the drain line from the outside of the house with a shop vac. I suspected something was off with the drain line also. There is a trap in the bottom section below the evaporator coil but it gets clogged and does not seem to drain effectively leading to the pan overflowing and leaking below.

I have attached some before and after photos of what I have cleaned so far. I am planning to have the ducts cleaned but I want to make sure I resolve the issue that's causing the mold beforehand.

Thank you all again for your time and insight.

BEFORE: