Remember, warm sap turns into bad sap and bad sap gets nasty. I always flush my preheater out with water after shut down. I rinse my head tank out and then I put some water in it to flush out the preheater, letting this water drain out into a pail to dump. Your rig and the sap in it stays warm for some after shutdown and that allows sap to "stew" in the pipes if it's left in there.