Safe & chemical free way to clean AC coils?
Never say never or always because that will always get you in trouble.
Vinegar is an acid--it will eat the galvanized off the drip pan and can attack the fins so vinegar or acid-based cleaners are bad.
I don’t like bleach because it stinks and if not completely rinsed will have your house smelling like a swimming pool for days.
No boiling water the thermal shock can crack fittings.
Now that we talked about what you can’t or shouldn’t do... There are special chemical cleaners made for coils I use a foaming cleaner made by NU-Calgon called NU-brite. You will find coil cleaners at the big box stores made for coils these also clean the dust and grime out of the coil&amp; fins.
Mild dishwashing soap can be used with water but getting all the soap out can be the fun part there.
Never use a pressure washer on a coil or the fins can be permanently damaged. Same with compressed air. A gentle wash with a hose on the outside unit may be the best bet if you do not have the proper chemicals.
Simple green and water sprayed in with a pump sprayer similar to what you use for killing weeds (never use a sprayer that has been used with weed killer on the inside unit).
I have tried many things and found the best option is commercial cleaner. My 2nd choice simple green and water followed with a good clear water rinse.
Yes I have HVAC licenses and your advice is mostly good not to use chemicals that will react poorly with aluminum, or galvanized materials. I usually power down the system to clean them this allows the chemicals we do use to do there job breaking down tree sap, dust, oil dog/cat pee (on outside units) that will damage the fins also. Once I treat the coil I flush it with clear water sometimes a 2nd treatment is needed but not usually on a well maintained system as yours has been.
I use 2 different sprayers on inside units one for the chemical and one with clear water. Making sure the drain stays clear and doesn’t overflow the pan.
This should help return your system to peak performance. And don’t forget about your filter. Always a good time to change the filter after a coil clean.