Oftentimes HVAC units are put in the attic and the best practice when this is the case would be to include an auxiliary drip tray for air conditioning systems that is separately drained outdoors.
An HVAC system in the basement or garage will typically have one condensate drainage system. It can send this condensate into a floor drain or an electric pump to eliminate what it removes from the air. On a hot and humid day a large system can remove many gallons of water from the air every day, perhaps 10 or 12. That water has to go somewhere! If it has no where to go, it will choose to use your ceiling!
When a system is located in the attic, in former times, there was often not even a drip pan underneath!
Now all units will have such a pan.
But the condensate drainage can happen any of many ways.
There can be a single primary discharge line. This is not good enough should a clog occur.
If it leaks, that water will hopefully get captured by the drip pan, but that pan needs to send it somewhere.
Most HVAC contractors include one or both of two back ups.
Some put in a float device which, if the pan fills with water, theoretically will float up to a point where it turns off the AC and no more condensate is collected.
At that point an HVAC contractor needs to clean out the primary line and eliminate the water from the pan.
This is fine, but in my opinion not quite enough. In addition to, or even instead of, the float, a second discharge line should be installed into the pan. That way, if there is a clog and the pan fills up with water, this second tube can send it somewhere. Ideally it is discharged outdoors. Often the HVAC contractor will set it so it drips in front of a window or onto a deck where its discharge can be easily seen and the clogged unit will be detected and fixed.
This house has the second line installed in the pan. I like that! This is an older system and for them to have done this way back then made this installation way state of the art.
But there is a problem. The opening is clogged with insulation and debris.
I SEE THIS OFTEN ON NEW CONSTRUCTION INSPECTIONS, WHICH IS A VERY, VERY GOOD REASON TO HAVE AN INSPECTION ON NEW PROPERTIES!
The insulator sometimes blows in his product and disregards the HVAC drip pan. Such was the case with the system here. This pan has been clogged for many years and fortunately has not needed to discharge water!
My recommendation: check the HVAC system in the attic to make sure 1. that it has a drip pan and then 2. that the pan has a second condensate discharge line installed. And then be sure that it can drain water! If it has a float consider it triple protected!