Ever see Myth Buster's TV show? Those boilers and hot water heaters can explode, or blow out the bottom and pass through a floor and a roof and still shoot a hundred or more feet into the air. Most important thing I know of is a working pressure relief valve, maybe two, pointed or piped to discharge in a safe manner. If it's fuel instead of electric, you definitely want a "flame out" sensor, it's an electric eye, to cut off fuel flow.