Source of Design Weather

 

The source of design weather data can be chosen. The options to choose from are

·       acquire design weather data from the ASHRAE/CIBSE database

·       import from a custom database

·       use an older format Apache .apl file

·       manually input the design weather.

 

 

 

If the ASHRAE database is chosen, once Next > is clicked the following dialog will be shown. Here the option is given to use a design weather location different from your site location. This gives some flexibility for calculations. By choosing Select the location for the source of the design weather data can be chosen. The procedure for this is identical to choosing a location for the Location & Site Data, except that this time only locations in the ASHRAE weather database are listed.

If the location data was edited on Page 1 of the wizard the weather dataset will come up blank on this page. However the Select dialog will come up with the nearest ASHRAE location highlighted.

Once a location for the design weather dataset is chosen then the user has the choice to decide upon how the design summer and winter external temperature is derived.

To illustrate, a Percentile for Heating Loads weather of 99.6% defines an external winter weather design condition where the outside temperature is colder than the design value for 0.4 % of the time, and a Percentile for Cooling Loads weather of 0.2% defines an external summer weather design condition where the inside design temperature is exceeded for 0.2% of the time.

 

There are 2 options, Fundamental and Statistical.

 

This gives tabular weather data taken directly from the ASHRAE fundamentals. In the case of Cooling loads, only Monthly data is suitable for use with Apache.

The available percentiles are fixed as follows:

Heating Loads weather: 99.6% and 99.0%

Cooling Loads weather: 0.4%, 2%, 5% and 10%

 

The ASHRAE weather datasets allow these external design temperatures to be based upon a statistical treatment. This treatment analyses the weather dataset and returns the design external temperatures that are only exceeded by the percentage of the year or month that the user specifies.

The Outdoor winter design temperature is the hourly average temperature exceeded on the given percentage (Annual percentile for Heating Loads design weather) of occasions in the historical dataset. This dataset is stored as a set of binned temperature statistics. Linear interpolation is applied between consecutive bins.

The peak design dry bulb temperature for cooling load calculations is the hourly average temperature that is exceeded on the given percentage (Annual or Monthly percentile for Cooling Loads design weather) of occasions in the historical dataset. This peak temperature appears in the table as the maximum dry bulb temperature for the warmest month. The percentage of the time that this temperature is exceeded in the peak month is then calculated. This monthly percentile is then applied to the other months to derive the design maximum dry bulb temperatures for those months on the same statistical basis. Minimum dry bulb temperatures are then obtained for each month by subtracting from the maximum dry bulb temperature the daily temperature swing calculated for each month. In the case of the peak month the temperature swing is provided in the database. For the other months the swing is estimated by deriving the amplitude of the first Fourier component of the clear sky horizontal global solar radiation flux for the month in question and using this to scale the temperature swing recorded for the peak month.

The wet bulb temperature coinciding with the maximum dry bulb temperature (Twb at max Tdb) for each month is calculated as the mean wet bulb temperature coincident with the maximum dry bulb temperature, using the historical binned temperature data.

The peak dry bulb and wet bulb temperatures obtained by this method are in close agreement with those published in the ASHRAE Handbook of Fundamentals for the annual percentiles 0.4%, 1% and 2%.

The Handbook also publishes monthly dry bulb and mean coincident wet bulb temperatures for monthly percentiles 0.4%, 2%, 5% and 10%. There is no fixed relationship between monthly and annual percentiles in general, but for a given temperature the monthly percentile will be greater than the annual percentile (by a factor of about 12 for very low percentiles). The monthly percentile is displayed following retrieval of the design weather data, and a desired value for monthly percentile can be achieved by applying successive adjustments to the annual percentile.

For further information about ASHRAE weather data, see Chapter 14 of the ASHRAE Fundamentals 2009.

 

Once the percentiles have been set, click on Acquire design weather to generate design weather conditions. This data can later be viewed in the Design Weather Data tab.

Note that any edits to the location and/or the percentiles are only effective after an Acquire has been performed.

 

The second source of design weather is a custom database. This database is one created by the user once a current database, which has been edited to user’s requirements, has been saved. (The saving procedure would have been performed in a previous session, using the Add to custom database button in the Design Weather Data tab.)

 

Once Custom database is chosen, click Next > and on the following dialog, shown below, choose Select and select from the list of custom databases to choose the design weather.

 

 

The third source of design weather is to access the older format Apache APL file. This database gives the user access to the original design weather information supplied in previous versions of ApLocate.

 

After choosing Old format Apache .apl file select Next > and the dialog above appears.

To select an old format Apache APL file choose Select and then select the required weather file from the list which appears.

 

If user input is chosen the Design Weather will be left as the current default weather dataset and the user can edit the data as required, on the Design Weather Data tab.