[bestgenman]

I want to correct my statement about the petcock.  The petcock in the hose would be in the same place as the top of the thermostat, that is, in the highest point.  The air is trapped underneath the stat and can't get through.  Yanmar and others use a tiny hole in the stat to facilitate air removal, that is why it is so slow.  I believe the best fix is to use one of the ports underneath the stat for a vent.  There aren't many ports underneath and most are used by the HWT switch and the temp gauge sender.  If there is another port, then certainly look into fixing a petcock there.  I believe this will correct the trapped air issue.