Understanding ASHRAE Design Days Fundamentals

ASHRAE Design Days

Energy Modeling requires Weather File and Design Days details in respect of the location where the building being considered for Energy Modelling is situated. Energy Modelling does carefully considers the weather data pertaining to that location. ASHRAE STD 169- 2013 Climatic Data for Building Design Standards provides weather and climate details about 3,012 locations outside the United States and Canada internationally. It has been observed that there are very limited descriptions and explanations on various terms and parameters used in Design Day files. The energy modelling program runs fine but the users must also be aware of the what goes in as the input to the energy modelling of the buildings. Warm season and humidity conditions are based on annual percentiles of 0.4  or 1.0% while cold season conditions are based on annual percentiles of 99.6 or 99%.  

99.6% Winter Design Dry Bulb Temp means that the outdoor temperature that your locations stays above for 99.6% of all the hours in the year, based on a 30-year average.

0.4% Summer Design Dry Bulb and coincident Wet Bulb Temps mean that the summer temperatures in an year for that location will exceed this design value about 0.4% of the time.

The term “design-day” is used very frequently in the HVAC industry. Design-day is used to describe a period of time with maximum conditions that a HVAC system was designed to accommodate and maintain the desired indoor temperature and humidity.  Design-day is used more frequently in HVAC discussions to refer to climatic design conditions. 

ASHRAE Design Days are generally provided by ASHRAE in their Design Day files for the selected location. The information can be very well used in the Energy Modelling exercise.

Daylight saving time is a consideration which energy modelling employs during generally summer time to take advantage of extended period of evening daylight in summers. The schedules are extended by 1 hr. Typically, regions with summer time adjust clocks forward one hour close to the start of spring and adjust them backward in the autumn to standard time. 

Dry Bulb Temperature: 

Its the temperature of the Air under normal circumstances measured with the help of thermometer.

Wet Bulb Temperature:

Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet cloth. The adiabatic evaporation of water from the thermometer bulb and the cooling effect is indicated by a “wet bulb temperature” lower than the “dry bulb temperature” in the air. If the air is humid then rate of evaporation would be less and cooling effect will also be reduced. In such situation the difference between Dry Bulb and Wet Bulb temperature will also reduce.

Dew Point Temperature

The Dew Point is the temperature where water vapor starts to condense out of the air (the temperature at which air becomes completely saturated). If the air is highly humid then in such case Dew Point would be close to Dry Bulb temperature. 

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