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.

What metal parts? The drain pipe is all PVC all the way to ground outside.

Fumes that can back up into the AC unit.

Vinegar and caustic chemicals like bleach are a no no. Just flush with water and also suck it out with a wet vac. That’s all you need to do.