HEAT MEASURED IN HVAC SYSTEMS BASIC INFORMATION

Sensible Heat
People, lights, motors, heating equipment and outdoor air are examples of substances that give off sensible heat. A seated person in an office, for instance, gives off approximately 225 Btuh of sensible heat into the conditioned space.

Enthalpy units of sensible heat are in Btu/lb°F. The change in the sensible heat level as measured with an ordinary thermometer is sensible temperature. Sensible temperature is measured in degrees Fahrenheit (°F) and it is indicated as dry bulb (db) temperature. Sensible temperatures are written °Fdb. For example, 55°Fdb.

Latent Heat
The definition of latent or hidden heat is: heat that is known to be added to or removed from a substance but no temperature change is recorded.” The heat released by boiling water is an example of latent heat.

Once water is brought to the boiling point, adding more heat only makes it boil faster; it does not raise the temperature of the water. The level of latent heat is measured in degrees Fahrenheit (°F) and it is indicated as dew point (dp) temperature (for example, 60°Fdp). Enthalpy is in Btu/lb°F. People, water equipment, and outdoor air are examples of substances that give off latent heat. A seated person in an office gives off approximately 225 Btuh of latent heat into the conditioned space.

Total Heat
Total heat is the sum of sensible heat and latent heat. It is measured in degrees Fahrenheit (°F) and it is indicated as wet bulb (wb) temperature. For example, 54°Fwb. Total heat level is measured with an ordinary thermometer; however, the thermometer tip is covered with a sock made from a water-absorbing material.

The sock is wetted with distilled water and the thermometer is placed in the air stream in the air handling unit or duct. As air moves across the wet sock, some of the water is evaporated. Evaporation cools the remaining water in the sock and cools the thermometer. The decrease in the temperature of the wet bulb thermometer is called “wet bulb depression.”

For room wet bulb temperature the wet bulb thermometer is typically in an instrument such as a sling- or power-psychrometer along with a dry bulb thermometer. Enthalpy is in Btu/lb°F. A seated person gives off approximately 450 Btuh of total heat (225 Btuh sensible heat plus 225 Btuh latent heat).