Energy calculations can be done using one of three basic methods:
- Degree-day procedure, which is the simplest but gives the poorest results.
- Bin method, which is relatively simple and accounts for the changing outdoor temperature and partial load conditions. This method is often used on single family residential applications.
- Hour-by-hour method using one of the many software programs available for these calculations. This method is often used for large commercial and institutional systems where considerable detail is required to make a good analysis.
The Energy Estimating Methods chapter of the ASHRAEHandbook of Fundamentals describes in detail these methods and includes examples. Another good reference is the IGSHPA Installation Guide.
Weather data software is available from ASHRAE and some equipment manufacturers. The bin method requires a load profile for the building. For most residential and light commercial buildings a simplified method of developing the load profile is often used.
Three key points are:
- The heating load should be calculated at the specified winter design outdoor temperature.
- The cooling load should be calculated at the specified summer design outdoor temperature.
- The assumption that neither heating nor cooling is required for a certain range of outdoor temperatures, typically from 62°F to 67°F.