I ran across this one recently and wanted to confirm this is an improper installation. The water heater (2006) is naturally vented natural gas rated at 40,000 BTU/hr. The furnace (1967) is naturally vented natural gas furnace rated at 105,000 BTU/Hr.
The age of the furnace is a different subject. The water heater vent is approximately 12" BELOW the furnace in a common 8 x 8 flue. I called it as a safety problem since the smaller BTU appliance should be located ABOVE the larger BTU appliance. Your Thoughts?