The fuse in OpenSprinkler is rated 3A, so it’s possible that the amount of current was enough to burn the triac but not enough to burn the fuse. It’s always tricky to pick the fuse with the right rating — because we always have customers who want to run multiple zones at the same time, if we choose the fuse too low, it will falsely trigger; on the other hand, if it’s too high, it obviously won’t be able to protect anything. We’ve also seen cases where the power adapter’s internal fuse blew before OpenSprinkler’s fuse had a chance to blow. So in reality, it’s not possible to have one fuse rating that fits all situations. If you know roughly what target fuse rating you need you can replace the fuse yourself. It uses the most common type of fuse (5mmx20mm).
Given the symptoms of both your OSPi and OpenSprinkler, you should definitely perform a solenoid resistance testing. You may have a shorted solenoid or shorted wiring somewhere.