It turned out the installers used the HVAC air handler and overflow pan as a trash can, while they were doing the installation. They never cleaned it out. And therefore once water starting running all of the debris flowed down the drain with the water and the clogged the drain causing the water to overflow in our attic.
The reason that the water leaked back into the return plenum and our onto the floor was because the air handler unit was not level. The air handler has a primary pan under the evaporator coil and if that is not level and it experiences a backup, it overflows into unexpected areas (like the return plenum). We cleaned out most of the debris from the secondary overflow plan earlier. The clog was inside the air handler, which they also used as a trash can. It was more insidious because we could not see what was inside there like I could in the secondary pan.
Luckily, we caught this before it caused major flooding in the attic and ruined the brand new drywall on the ceiling below. It did flood the attic some and caused mold to grow on the wood in the attic.
|Wet attic floor from the HVAC pan overflowing - we used a shop vac to suck up all the water. There was enough water to fill the shop vac container many times.|
|Soaked return plenum from the water backing up into it.|
|Moldy attic floor from the leaking water|
We called the HVAC company out to have them level the overflow pan, unclog the drain and remove the debris we couldn't remove ourselves, and replace the return plenum.
When they came out and did these repairs, they tore open a large hole in the supply duct in the attic. This caused the cold air to escape into the attic and not get to the house. As a result, the attic was cooler than the house and the AC ran continuously. They also moved the overflow pan for one of the HVAC units in such a way that the condensation was dripping onto the attic floor instead into the pan. Of course, they didn't notice any of these things; we had to find the problems once again.
So they had to come out once again to repair the things they damaged.