I used to just drain my pre-heater when I shut down, cover all openings (float boxes and pans). Then one time I had to shut down during a boil to clean the syrup pan. I didn't drain the pre-heater in the hood. I ended up getting my first (and hopefully my last) ever ropy syrup. Now when I shut down, in addition to always draining the pre-heater I also drain my cold sap float box, then allow the hot from in the flues pan to fill the float box. Problem fixed!