Since most boilers and water heaters vent with a conventional negative draft flue, it is important to ensure that enough heat remains in the flue products to maintain them as a gas for operation of the vent and discharge into the atmosphere.
However, as the efficiency of a boiler or water heater increases, more heat from the combustion process must be transferred into the water at the heat transfer surface. The relative temperature of the flue products decreases as the efficiency increases.