You may want to consider installing a per-zone fuse. Using one single fuse for the whole controller makes it difficult to choose the correct fuse rating. Because some of the customers run multiple zones at the same time, which draws a lot of current, it’s very difficult for one single fuse to distinguish between whether the current surge is due to turning on multiple zones at the same time, or one solenoid is shorted. To solve the problem you can install a small fuse (like 0.5A or 1A) inline on each zone wire, that way, if a particular zone is shorted that should trip the fuse quickly and correctly.