This seems to be a common problem!
I really think there should be a strainer between the pump and the heat exchanger
. Even if the impeller is inspected or replaced on a schedule, all it takes is one over heating
from a closed valve or a plastic bag over the inlet, to burn the impeller enough to crack/burn the rubber.
Normally inspections are every 250 hours, and replacement at 1000 hours, or 1 year. And winterizing here in the PNW its common to remove the impeller and spray it down with silicon, if its fairly new, and replace in the spring. Carrying a couple spares is a good idea, too.