Humidity ControlHave you walked outside recently and felt sticky as sweat starts to bead up on your forehead? In the summer, the relative humidity can seem unbearable. The HVAC field involves more than the acronym H (heating), V (ventilation) and AC (air conditioning) indicates. A key factor in human comfort is humidity control.    

There are two types of humidity that relate to the HVAC industry:

  • Relative humidity is the ratio of the amount of water vapor the air holds relative to the max amount of moisture (saturated) the air can hold at the same temperature.
  • Humidity ratio (absolute or specific humidity) is the mass of water vapor present in a unit mass of dry air.
Humidity Control

Humidity Control

For a better understanding, let’s look at the psychrometric chart below and take into account the winter and summer months. In the middle of winter, an occupiable space is going to be heated. The air temperature before being heated is represented by point 1 at 50°F and 30% relative humidity. After the air has passed over a heating coil, the dry bulb temperature increases to 70°F (point 2). Notice that the path from point 1 to point 2 is a horizontal line.  This shows that humidity ratio remained constant and simple heating occurred. Humidity ControlHowever, the relative humidity dropped from 30% to approximately 15%. The relative humidity can be increased by adding a humidifier to the HVAC equipment.  Point 3 assumes that steam was introduced to the airstream, increasing the temperature, relative humidity and humidity ratio. Humidification can also be accomplished by spraying water into the airstream. This may lower the temperature of the airstream from point 2, but the relative humidity and humidity ratio will still be increased. In the summer months, an occupiable space is going to be cooled. Point 4 shows the air temperature at 85°F and 80% relative humidity. As air passes the cooling coil, the temperature drops until its dew point is reached (point 5). The horizontal line indicates simple cooling and a constant humidity ratio.  However, once point 5 is reached, the relative humidity is 100% and condensate forms. Up to point 6, the temperature continues to decrease as the relative humidity remains 100% and the humidity ratio decreases. The dehumidification process takes place as the moisture is removed from the air in the form of condensate. Humidity ControlHumidity control is important for human comfort in the residential or commercial setting but can also aid in production in an industrial setting. Whether your next project is an office building, warehouse, hospital or restaurant, humidity control is a topic worth some thought and discussion.    

Allegheny Design Services

Allegheny Design Services (ADS), established in 2002, specializes in many different areas of structural and MEP engineering.  We provide reliable, responsible services of building system design and analysis. We are the link between owners, architects and contractors and help the architect realize their vision while providing simple, buildable solutions to the contractor. We pride ourselves in offering timely solutions to complicated engineering issues, whether structural, mechanical, electrical or plumbing in nature. For MEP and structural engineering services, contact Allegheny Design Services at 304-599-0771. Thanks for reading, Alex


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