Demand call for downstairs gas system from 2014 turning on and off. Upon arrival customer stated that the system has been running and cooling but will turn off and the thermostat will go blank for awhile and then it will start back up and run like normal until it shuts off again. During my inspection the customer took me to his attic to show me the system, while we were up there I checked the water safety switch to see if it was activating and sure enough the primary drain line was filled with water which caused the water switch to activate. The customer then took me to where the system drain terminated which was in an upstairs restroom. I explained to the customer that we need to flush the drain line to remove the clog but I can’t guarantee the drain line wont get clogged up again due to the water heater being connected to the ac drain line. Customer agreed to have his line flushed and for me to check the rest of the system to make sure nothing got damaged when it was turning on and