Water – How to deal with water damage from backed up AC drain

We have a central air conditioning unit that has one part of the unit in a closet. The blower connected to the ductworks and coils in this part of the unit, while outside there is a compressor and blower unit.

There is a rather long PVC drain line from the indoor unit to the outside. There is a catch connected to the drain via a small hole which opens up into larger PVC, which then goes down into the floor, and comes out about 6 feet lower on the outside of the house, where it then goes down another run of PVC to the yard.

The issue is that periodically this rather long piping clogs. There is not a large amount of catchment at the unit itself, and there is no indication if the drain is clogged, so it fills up and then starts overflowing into the intake vents space below the unit and the eventually works its way to the flooring where it proceeds to cause damage to the wood, causing it to curl upwards.

I am not sure what to do about this. When it happened the first time I was able to clean the drain line by attaching a vacuum to the PVC outlet, and then flush more water (by carefully pouring it into the catchment area and waiting for it to drain) until I was pretty sure it was cleaned out. There seemed to be some green stuff suggesting some kind of biological buildup, so I also flushed some bleach down, and have been pouring bleach down monthly.

I was not, however, able to figure out what to do about the damaged part of the floor. The local flooring store was not able to tell me anything – I took a sample of the flooring and they were clueless aside from redoing the whole floor with something completely different. I tried looking for handymen on Thumbtack but didn't get any responses from my inquiries that made me confident in wanting to let any of them come out and look at it. At the time only about 2 square feet around the closet were damaged, part of it under the door. I do not want to replace the whole hardwood floor (it's in a hallway adjoining two bedrooms and is wall to wall hardwood in the whole thing) so nothing was done.

Now however, despite the monthly bleach regiment the unit has backed up and overflowed again, this time damaging around 6 square feet of flooring. There was no green in the suctioned backup this time, or any obvious gunk, although it was a bit dirty looking (this could have partially been due to remaining stuff in the vacuum from a previous cleaning).

I have two questions:

1. Why does the blockage keep coming back? How can we prevent it from coming back since it is so hard to spot and damaging when it does?

2. How can we repair the flooring? I have tried prying up the loose wood, but the parts that are not wet are very securely glued and I don't know how to detach them without making a big mess (breaking the piece, leaving lots of remnants stuff attached which I have to somehow remove).

Pictures:
1. The floor in front of the air intake. The grate has been removed, as well as the flooring that would come up.

2. The floor near the hinge of the door of the utilities closet.
   

3. View of interior showing catch (black plastic) and exterior piping leading away from unit.
   

4.Closer view of catch, including opening to PVC piping.