So, if I recall correctly, you discovered a stuck thermostat that was installed upside down and crooked. If it had been in straight, there would have been over heating and some boil over. Since it was in crooked, there was free flow and the proper operating temperature was never reached. Now that the new thermostat is in, you should have a good operating temperature and some increased pressure which probably affected the heater core. Your heater core was probably ready to fail.
