Replaced customer supplied through the manufacturer gas control valve and burner assembly for two year old water heater. The water heater is still failing to properly ignite. Replaced the water heater completely, authorized by Rheem. The water heater is heating properly. The signs of a backdraft can be noticed. Both vents from the water heater and furnace are hot to touch. There is a possible blockage inside of the exhaust going out of the roof of the home.