I guess that depends on the arrangement of the piping. On my unit, the rubber part of the device extends a few inches into the vertical exit part of the drain pipe. It seals off the horizontal drain connection from the AC unit. There is no way for any backflow to occur. The water can only go directly into the drain pipe and to the outside of the house.

On most units I have seen, there are 3 pipe connections above the vertical exit pipe. None of those 3 joints are glued together, so you can easily remove any part of the drain assembly to add water or to flush the system.

Pour water
 

I was told only warm water. Never vinegar. I use warm water only and I’ve done this for 15 years and no problems

Right after, no wait

It seems to me, that if you flush with water immediately after adding vinegar, you defeat the purpose of using the vinegar at all. The vinegar is gone, and it doesn't have any time to kill mold or anything else.

Don’t use vinegar! Use up to a gallon of boiling water.

The same here. Vinegar is not good for this system.

Recommended use is 3 parts warm water with 1 part of vinegar. Mix, pour into drain line.

Can you tell me why vinegar is bad for the system? I can’t find anything that contraindicates on the internet. I just keep hearing not to use it.

This Thread is a perfect example of…. Ask a question and get a multitude of answers.

Perhaps none in particular is wrong or right, but can be mighty confusing.

Just sayin’.

Munn's told me warm-to-hot water once a month. Shut off A/C and air handler at the circuit box first. Then pour slowly. I was informed to pour 1 gallon per month.

Don't use Dawn
 

Dawn dish soap will absolutely 100% clog your drain pipe. My last tenant did that and caused one hell of clog and mess!

Vinegar or a water/bleach mixture will not hurt your pipes.

2 shots of Tequilla, 2 oz of Lime Juice, 1 shot triplesec, mix in shaker with ice before pouring. THEN grab a gallon of hot water and dump it down the tube.

I bought the cheapest garden hose I could find (an old worn out hose would work as well) and cut it short (2-3 feet) on the end with the connector to the hose bib. I connect it to my regular hose with a pistol nozzle (threaded on spray end). Shove short hose down drain and squeeze nozzle with as much or little pressure as needed to flush A/C drain. I picked this tip up when I saw Munn's A/C guy use this to flush A/C on service call.

The theory is the fumes from the vinegar will come back up the drain tube and into your air handler where they will hurt the coils.

- Are you pouring enough vinegar to make many fumes?
- Is the vinegar concentrated enough after dilution with the water in the drain to make fumes?
- Are the fumes lighter than air such that they will work their way back up the tube?
- Is your house cool/warm enough that the air handler will not come on and disperse any possible fumes throughout the house before they can become concentrated enough to hurt the coils?

It's a theory, it makes some sense, it doesn't seem likely to really cause a problem. On the other hand, hot water is just as easy and a few pennies cheaper.