The flue gas moisture content must be known to determine the density and mass flow values ​​of the substances which are exhausted from the chimneys and which cause air pollution.

The air used for combustion often contains water vapor and is rarely dry. The amount of water vapor in the air varies depending on the temperature and relative humidity. This water contained in the air can absorb a certain amount of heat and cause heat loss, but in cold weather this loss can be ignored among other losses.

When calculating combustion efficiency and ideal air and fuel values, the latent heat loss due to the flue gas due to moisture is taken into account.

Heating and cooling systems are the areas where energy loss is the most experienced in residential and industrial facilities. It is of great importance for the energy efficiency and energy cost that the heating or cooling of the environment is done with the least energy.

Humidity is the amount of water vapor in the air. Absolute humidity, ie water vapor density, is the weight of water vapor in one cubic meter of air. The relative humidity is the ratio of the water vapor contained in the air at any temperature to the maximum water vapor that the air can carry at the same temperature. The specific humidity is the ratio of the weight of moisture contained in a gas to the weight of the gas.

Different methods and different absorbers are used to measure the moisture in the flue gas and to find the moisture value. Sensors used to detect moisture in the flue gas operate in certain dust rates and at high temperatures with certain error rates. When measuring the humidity in the flue gas, the cooling process is usually carried out in ice containers for the condensation of the flue gas to be measured and the amount of moisture collected is weighed. The moisture (ie the amount of water) taken into a measuring cup is measured by devices capable of measuring in micron accuracy.

The high humidity measured in the flue gas is indicative of inefficient energy from the combustion process.