Climate analysis - outdoor temperature - design impacts

This analysis is done by reviewing outdoor temperature and humidity conditions at the site and determining how those values compare to acceptable thermal comfort conditions. It may help answer such questions as:

• When is the weather comfortable?
• Do operable windows for natural ventilation or free cooling make sense?
• When will HVAC systems be necessary for providing heating and cooling?

Because the weather data is a large table of numbers, it would almost certainly be overwhelming to present numerical data to non-technical members of the design team. Include some visual representations of the data to help illustrate the key points.

🎓 Building Science and Design Concepts

• Operable windows can provide free cooling and natural ventilation with no need for HVAC systems when outdoor conditions are appropriate. Note that comfortable outdoor conditions don't guarantee that opening the windows will lead to comfortable indoor conditions too. It is important that the outdoor air flows in through the windows. Additional climate analysis of the prevailing wind conditions (see below) should also be done at this stage to determine the feasibility of natural ventilation.

• When it is too hot or too cold outside, operable windows should remain closed and HVAC systems should operate to maintain acceptable indoor comfort conditions. If no HVAC systems are installed, the windows may remain open, but the occupants may experience comfort issues during these periods.

• For buildings with operable windows and HVAC systems, it is important that windows are opened only when outdoor conditions are acceptable for comfort. Buildings that make use of operable windows and HVAC systems are said to have mixed-mode systems.
  • The HVAC systems serving spaces in the building with open windows should remain off, or in some cases, the fans can remain on to provide air movement but with the heating and cooling systems off. If left up to room occupants to turn the systems on and off based on the window open/closed status, there may be times when they forget to disable the systems. This would lead to situations when the HVAC system is on and the windows are open which will result in wasted energy use.
  • This can be mitigated by providing visual systems that indicate to occupants that they should turn on/off the systems, or by providing sensors in the windows that automatically disable the HVAC systems when open.
  • Some buildings with operable windows have windows that open automatically with actuators based on outdoor conditions. These systems are often controlled by the building automation system (BAS) in a manner where the windows open during comfortable conditions and the HVAC systems are turned off. When outdoor conditions are no longer within the acceptable comfort ranges, the BAS will close the windows and reenable the HVAC systems.

✏️ Analysis Steps

Select the appropriate data

The following variables in the weather file are used for this analysis:

• dry-bulb temperature
• wet-bulb temperature
• humidity ratio

Filter and review the data

• Filter the data to only include occupied hours.
• Determine which hours of the year fall within acceptable comfort ranges.
  • This step can be as simple as counting how many hours fall between a high and a low value of outdoor dry-bulb temperature. These types of range checks could be done in a spreadsheet, but many BEM software tools have built-in capabilities to perform these range checks automatically within the software.
  • A more detailed analysis that considers both temperature and humidity conditions could be performed by plotting each hour on a psychrometric chart. Performing this type of analysis manually would be cumbersome, but many software tools have built-in capabilities to display hourly data plots on a psychrometric chart.

📊 Present the Analysis

Chart showing the number of occupied hours outdoor temperature conditions are within acceptable comfort ranges. From Climate Consultant software.

Presentation of the analysis should clearly answer the question "how often is the outdoor weather comfortable?" This may be done by presenting a percentage of occupied hours in which the weather falls within the acceptable comfort ranges.

If the percentage is very low, then perhaps operable windows do not make sense. That is not to say that the building must not have operable windows, but their use may have adverse energy effects.

The following graphic shows a plot of temperature and humidity variables for each hour during the occupied building hours (filtered between 8:00 am and 5:00 pm).

From this graphic, we see that the outdoor conditions are within the acceptable comfort range about 15% of the time. We see that most of the uncomfortable temperatures occur because it is too cold outside. Very few hours occur when it is too hot.

This data suggests that HVAC will be needed for heating and ventilation during the heating season. However, the mild high temperatures suggest that natural ventilation and free cooling could be achieved by designing the building with operable windows.