Client called us out today due to a rotten egg smell from the 4 yr old water heater, and they were also getting some hot and cold crossover up stairs. Client does have a dedicated recirculating line and a recirculating pump with a check valve on the line. The crossover issue was happening due to the check valve on the recirculating line getting stuck open by calcification. We replaced that check valve then reconnected the recirculating line. The thermal expansion tank on the water heater had also failed, and we replaced with new thermal expansion tank. The rotten egg smell was due to the water heaters anode rod rotting away in the tank. We removed the old decayed anode rod, and installed new aluminum water heater anode rod. Turned water back on to the Qatar heater and filled it up and got all air out of it. Turned the breaker back on and water heater is heating. 1 yr parts and labor warranty on all work today.