Hi Eric,

I have done this as a regular part of maintenance every 6 month for the past 5 years. It is perfectly fine to use phosphoric acid or barnacle buster on the AC units. I don’t use BB anymore. I just buy phosphoric acid in the paint department of a hardware store or Sherman Williams for about ¼ of the BB price.

I use a set up just as you describe to re-circulate the acid. Be sure to use a filter in the bucket to catch the junk. I put a mesh balloon over the hose dumping into the bucket so the junk is not re-circulated.

If you use the concentrated BB which is about 17% phosphoric acid, use a solution of 30% acid 70% water in the bucket. There is a lot of water in the pipeline to add to the dilution.