We have two 40 gallon water heaters in our house and we do not get 5 years out of them before they develop leaks in the tank. Last relpaced heater made it 3.5 years befor failing. What could be going on? Our area has very hard water so we have a water softener. The heaters have dielectric unions on the connections and new ones installed with the heaters. Have an expansion tank installed in the system. Blow down the heaters every year. Any ideas?