Two weeks ago my water heater was not working at all.I've had the company that services my oil burning water heater service it and now they tell me I need a new one. They explained for now I would have to leave it off until hot water was needed. They said the inside is corroded and can't be left on. Why would it matter if the burner is on or not? Am I being taken? How do I find how replace it myself if needed?