Thermal Comfort

One of the most serious errors which can be made in modifying HVAC systems is to equate a change in dry bulb air temperature with energy conservation. It is worthwhile to recall that dry bulb air temperature is not the most significant determinant of thermal comfort during either the heating season or cooling season.

Since thermal comfort in the cooling season is most directly influenced by air motion cooling energy requirements can be reduced by increasing airflow and/or air motion in occupied spaces without decreasing dry bulb air temperature. During the heating season thermal comfort is most strongly influenced by radiant heating. Consequently, changes from forced-air heating to radiant heating can improve thermal comfort while decreasing heat energy requirements.

The total energy, or enthalpy, associated with a change in environmental conditions includes both sensible heat and latent heat. Sensible heat is the heat energy required to increase dry bulb temperature. The heat energy associated with a change in moisture content of air is known as latent heat. (See Figure 10.12)

Changes in HVAC system design or operation which reduce sensible heating or cooling requirements may increase latent heating or cooling energy which offsets any energy conservation advantage. This is particularly true of economizer cycles (ยง10.5.5). The use of cool, but humid outside air in an economizer can actually increase energy consumption if dehumidification requirements are increased. For this reason, the most reliable type of economizer control is enthalpy control which prevents the economizer from operating when latent cooling requirements exceed the savings in sensible cooling.

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