I believe the question was of combustion efficiency and the confusion was over CA. I find great use for the CA nearly every day. Though thermal efficiency can't be measured in the field it pays to know you stack temperatures (one aspect of the CA test). Suffice to say, atmospheric water heaters, including power vents are not efficient. We use a lot of condensing water heaters.
Thermal efficiency would be almost impossible to measure on a water heater. But on boilers and furnaces it is easy to measure, in fact it is the only efficiency that is possible to accurately measure.