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IDL - Bozeman eNEWS - April 2013 - Vol. 6 No. 4

Entries in Climate Design Tools (1)


Climate Design Tools

IDL-Bozeman is pleased to announce the latest addition to our website, a set of climate design tools originally created by IDL Boise. They can be found under the Software Tools, Climate Design Tools tab on the menu bar. With the permission of IDL-Boise, IDL-Bozeman has adapted these tools to reflect six reference cities in Montana. These include Billings, Bozeman, Great Falls, Helena, Kalispell, and Missoula. The creation of these tools was funded by the Idaho Power Company and was developed into two generations of spreadsheet calculators that are capable of analyzing building loads and the energy consumption impacts of a range of different design strategies. Currently, there are seven different calculation spreadsheets that span across two different generations of tool development.



Heat Gain Calculations:

Heat gain is the heat added to the conditioned space by infiltration, solar radiation, occupant respiration and lighting. This tool calculates the heat gain rate of a building, and is measured in Btus/hr/square foot. It will determine the cooling capacity requirement of either the building’s passive or active systems.   The amount of heat gain is calculated based on multiple factors including heat gains through the building envelope, air infiltration, ventilation, and the internal gains of a building from inhabitants and other types of equipment.  The heat gain calculation tool must be completed prior to the following three tools.


Cross Ventilation:

Cross ventilation is driven by wind and is dependent on operable windows. Once you have established the heat gain rate of the building, the cross ventilation tool can be used to establish a building’s cross ventilation cooling capacity.  These calculations are based on location and size of operable windows, average wind speed, and the temperature difference between outdoors and indoors.


Stack Ventilation:

Stack Ventilation relies on the principal that hot air rises, and depends on low openings to admit outside air and high openings to exhaust air. The stack ventilation tool is very similar to the cross ventilation tool and also requires the heat gain rate to be established first. This spreadsheet tool also utilizes opening sizes as well as stack height to determine the buildings stack ventilation cooling capacity. Stack ventilation is not as effective as cross ventilation. However one can input the cross ventilation capacity in this spreadsheet to determine the combined cooling capacity of a building.


Night Ventilation Thermal Mass:

Night Ventilation strategy maintains a building at temperatures lower than those outside by day and flushes the building with fresh air at night. This tool analyzes the volume and Btu storage capacity of the thermal mass within a building combined with an equation that utilizes hourly data from an average summer day.   Combining night flush ventilation with a thermal mass strategy can be one of the most effective forms of passive cooling.   




Balance Point Calculation:

Balance point is defined as the outside temperature at which the indoor heat gains balance building heat loss to maintain a desired indoor temperature. Like the heat gain tool, this calculation is driven by infiltration rate, solar radiation, occupant respiration and lighting. The key difference with this tool versus the heat gains tool is that weather data, including hourly information from TMY3 weather files, are embedded into the calculations based upon each reference city. The balance point calculated in this spreadsheet will be used in subsequent design strategy calculators to determine how much natural ventilation is possible, how much energy passive solar will save, etc.


Passive Solar:

Passive solar design is perhaps one of the oldest and most popular passive strategies utilized in contemporary practice today. The balance of design parameters within a passive system requires much forethought to truly optimize both heating performance without sacrificing cooling performance in a project. While there are many types of passive solar systems, i.e. Trombe wall, sunspace, roof pond, etc., this spreadsheet tool focuses on direct gain systems coupled with thermal mass. Critical design parameters explored through this tool include: load reduction measures, thermal mass amount and distribution, and glazing parameters and area.


Earth Tube:

Earth tubes are earth-to-air heat exchangers that capitalize on the moderate temperatures below ground to help reduce the heating and cooling loads of a building. They can be used to provide cooling in residential and commercial applications, or be used to pre-treat outdoor air for the ventilation requirements of commercial buildings. As air moves through a pipe, it transfers heat or couth to the surrounding tube material and soil before connecting into an HVAC system or being delivered directly to a space. The amount of heat transfer is contingent on the following critical design parameters: temperature difference between the ground and air, soil conductivity, pipe material, pipe diameter, tube length, and air velocity through the system. This spreadsheet calculator will help guide you through testing the effect of these parameters on heating and cooling performance.


These Climate tools are geared toward the conceptual phases of design and contain robust calculations that are quick and easy to use. They are designed to provide quick feedback loops that make direct correlation between design and performance apparent through instantaneous outputs in an intuitive spreadsheet input format. The 2nd generation of tools utilizes hourly TMY3 data for calculations specific to the six different reference cities in Montana. This detailed level of weather data provides a robust climate-specific analysis while opening the door to more advanced calculations for balance point, passive solar capacity, etc. The seven total spreadsheets provide many useful outputs which range from peak cooling load reduction, to heating energy/cost savings, to the quantification of natural ventilation hours for various design strategies.

We hope to offer some interactive workshops on the use of the climate tools in the near future, so stay tuned.

For more information about the production of these tools, please visit the Integrated Design Lab Boise.

Click here to download the climate tools for your region in Montana.