Richard
Here you see a classic example of a glorified helper acting like a chemist. I am a chemist, so here’s the scoop:
HCl (hydrochloric acid) is an aqueous acid, in other words, it is prepared by dissolving Hydrogen chloride in water. Since it is an aqueous acid, it will simply dilute with any water in the line. Therefore, it poses no significant risk to the plumber or the equipment.
Bud is right that hydrochloric acid is useful for dissolving calcium or magnesium salts that build up with time. However, EDTA (ethyldiaminetetraacetic acid) is better for getting rid of calcium, and is safer, but it takes more time to work.
The reason why sulfuric acid is used in drains is simple. Sulfuric acid (H2SO4) will turn organic matter to carbon, but will get hot and water will be released, along with some sulfate gases. H2SO4 does put the plumber and equipment at risk, so be sure to tell the plumber that the H2SO4 is in the line, in the event that the acid doesn’t work.
Good luck
