Why does the vent line fill with condensation and not drain

indicates blockage in the line back to the tank
could be a venting problem for the tank not allowing full fuel flow as indicated in your statement
check in the tank or filter for a black/grey sludge
That is a moss that grows from water in the tank and blocks lines , filter and eventually injectors. use a diesel fuel conditioner to remove it and always keep the tank full to prevent condensation that causes the problem in the first place

It should have a 3/4" drain hole right below where the refrigerant lines connect to the coil. If this is a new installation they may have not taken the "knock out" out of the hole. You may want to put a trap on the drain and fill it with water since the coil creates a suction and sucks air through the drain hole. I hope this helps you.

Tankless or hybrid water heater?

Tankless gas must have special fitting between water heater and vent that has a condensate line fitting. Hot toxic vent gasses traveling up vent pipe cause acidic water vapor to form inside vent pipe that drips back down vent and drips onto heat exchanger and other parts eventually burning hole through expensive metal alloy heat exchanger, rendering tankless computer water heater useless after electric circuitry gets wet and shorts out.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html#vent

Hybrid or heat pump type computer water heaters must have condensate drain connected to catch pan located just above storage tank and below compressor. Condensation can form on condenser coils and drip into catch pan where there is primary and secondary drain
http://waterheatertimer.org/Review-GE-Heat-Pump-water-heater.html#maintenance

Read product manual
Gene
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If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

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
Gene
hh

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

The accumulation of condensation in the vent piping is more than likly the problem. I am guessing the vent piping is either 3-inch or 2-inch. You need to splice in a "T" fitting reduced down to 3/4-inch with a 3/4-inch tubing attached and run this to a sutible drain. This will drain the condensation elimiating the accumulation in the vent assembly. Install this "T" fitting about 3-feet down stream from the vent motor.These supples and fittings can be found at a Home Depot or Lowes store.

This unit is designed to condense, I suspect that the drain is blocked by some debris. Remove the larger hoses from the burner box and inducer motor and blow into them to clear any debris. Plug the end of the hose so you don't force water into either the inducer or burner box. Make sure that there is a vent on the drain just as it leaves the furnace cabinet. This will assist the drain to work properly and not cause the condensation to go up in the pressure switch hose.

Your air conditioner evaporator condensate drain is blocked.
The condensate draintube should carry this water out through the floorpan to the ground.
Look for a kinked or blocked drain tube from under the A/C evap housing.

vent clear but vent opening arround vent frozen or coil is frozen eithor way you probably need a thermostat and a timer

This is condensation from your a/c running. This is the secondary catch pan which means the primary drain is clogged with algae and not draining, running into the overflow (what you see).

When they plumbed the main drain they should have left a "T" in the pvc line to allow for servicing it. A lot of people will pour a 1/2 cup of clorox into the line every 3 months or so to keep the algae clear. There is probably something more user-friendly at one of the big box stores.

I have a little air tank that I fill with air and disconnect the line and forcefully blow it out. Problem solved.

Good luck.

You have a clogged condensate drain line and the condensate is now being syphoned up by the blower and coming out the duct work.
Find where the condensate drain exits the house and you can back flush it gently with a garden hose. Then clean out the condensate pan and pour a mixture of warm water and baking soda into the pan down the drain.