I think we have a clog of dried soap in our pipes, past the U-bend and down the pipes from upstairs to the drain at ground level.
How do I clear them?
I would skip all the chemicals and just use a drain auger on it to clear the build up. After you break through then you could probably rely on hot water to clean anything remaining out.
Any of the below options are good for a start without a more specific problem
I know I am replying to an old post but I have relative info. My mother has a soap shop and her lines backed up with soap after the P trap as described in this post. Imagine soap in your hands in the shower, soap is hard and even when submerged in water over long periods of time doesn't become soft enough to lose its structural integrity. Hot water won't suffer it any different or even acids. If you put all that down your drain you then have an impact of soap with dangerous liquids on top not doing anything. The ultimate outcome was to cut the lines and manually blow out the soap with an air hose. We tried all the methods above and when I cut the line the soap and then all the dangerous liquids rushed out into my working space. What I learned from this, don't put harmful ingredients or spare soap or hot pour soaps down the drain. Simple as that. If you do you ruin your drain lines.
ps. Augers will not work. They will bust up the soap yes but then the pieces will travel further down line and clog again.