HVAC Design Notes

Last updated October 11, 2024
By Emma Howland

Background on Thermal comfort: There are many factors that affect how comfortable a room is-

  • humidity
  • mean radiant temperature
  • air motion
  • air purity

Thermal comfort factors can be very subjective to the individual and can vary depending on seasonal conditions, age, health, clothing, and activity of the individual. However, there is a certain comfort range that is has been tested for most North Americans.

Air temperature/mean radiant temperature: The human body receives radiant heat from or loses heat by radiation through the surrounding surfaces if their MRT is significantly lower or higher than the air temperature.

Air temperature/ Humidity: relative humidity is the ratio of the amount of water vapor actually present in the air in relation to the maximum amount that the air could hold at the same temperature. Humidity is more critical at high temperatures than within the normal temperature range.

Air temperature/Air Motion: Air motion increases heat loss by convection and evaporation. air velocity should range between 10-50 feet per minute but beware of creating a draft.

Heating and Cooling Loads

Heating load: the hourly rate of net heat loss in an enclosed space (Btu/hr). Primary sources of heatloss include: convection, radiation, and conduction through exterior wall, window and roof assemblies as well as floors over unheated spaces. Infiltration of air through cracks in exterior construction (ex. around windows and doors)

Cooling load: the hourly rate of heat gain in an enclosed space (Btu/hr). Primary sources of heat gain are similar to heat loss. Solar orientation of a building as well as solar radiation on glazing, the amount of building occupants and their activities, lighting and other heat-producing equipment, ventilation of spaces, and latent heat (requiring energy to condense the moisture in warm air so that the humidity in a space is not excessive) can also play a factor.

Calculating heating and cooling loads is detailed in the ASHRAE handbook.