Call Rheem and they will recommend calling installer since tankless computers are non intended as do-it-yourself.
If condensate drain is blowing out 'air' do not forget this air is deadly poison CO gas and will kill you, first giving you a headache that goes away when you are out of house. Remove affected people from home and put into fresh air and lay on side, never on back or they will suffocate. Call 911 immediately and most will recover.
Copy following link for resources and required maintenance for tankless computers that heat water: scroll down to venting and manuals and review each to check how vent lines are set up, and that condensate line is set correctly. However this information is NOT guaranteed to match each model made by Rheem as they advance their product marketing strategy to exclude as much safety information as possible for do-it-yourselfers.http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
Condensate lines can clog over time. Clean line with compressed air, and soak in clorox, and swing around overhead to remove anything inside line. This line contains bacteria so do it in neighbor's yard.
Not all conditions produce vent condensate. Condensate might not form in vent line if weather is dry and the vent has good draft. But later when conditions change, condensate might begin flowing.
Condensate is caused from burning hydrocarbon fossil fuels in any gas appliance. But problem is bigger with tankless since these appliance consume so much fuel per heating event since tankless are required to heat water rapidly as water passes by in a pipe. This is why tankless require larger incoming gas line, and larger vent line made from specific materials for the vent.
Among the long list of poisonous by-products being vented outdoors into the air our planet breathes is acidic water vapor. If conditions are such that the vapor condenses before reaching outdoor, then the acidic water gathers in the vent line and rolls back toward the tankless computer, and goes down into the condensate drain and then down into your pluming drain where it can corrode pipes and add acid by-product to other environments.
If the acidic water is allowed to fall back into tankless heat exchanger instead of down condensate drain, then eventually it will chew a hole through the special nickle alloy heat exchanger (imagine the hole it will chew through household drain lines), and the surprise will not be evident for a few years until leak develops and the tankless exchanger must be replaced at great expense, plus repairs to shorted motherboard and electrical sensors.
Just don't install tankless computer water heater is best solution for myriad of problems, maintenance, and expense of owning a device that must immediately monitor incoming and outgoing water temperatures and flow rates, incoming gas flow rate and pressure, outgoing vent flow and pressure, interact with a remote control device, while monitoring burn rates and combustion temperature using a computer motherboard that is not protected from line interference or power surge, etc
Just install tank-type heater:http://waterheatertimer.org/How-to-install-gas-water-heater.html