I have a Lennox complete heat sysem in my house and have had lots of trouble with it initially but has been very good for four years or so... Today I found the drain valve leaking so I drained the unit and replaced the valve and refilled the unit, when I tried to restart the unit I get "limit switch open" error message. When I first tried restarting I got sounds like grinding after four or five restart attemps these sounds stopped? Any thoughts?