The user selects a service type from a drop down list of 20+ options located in the zone instance properties dialog. The service types are grouped into 4 different categories, and depending upon which category the selection falls under, will determine the type of calculation or output.
- Constant volume: A constant volume service type refers to a constant volume HVAC system, where the fan air volume output is constant no matter what the actual space cooling loads are at that point in time. The cooling calculation engine performs peak of the sum room load calculations, meaning that the engine determines at which month and hour of the year the sum of all of the individual room cooling loads for the zone is the greatest. This type of service type is utilized for simpler HVAC systems with little cooling diversity and no plans for variable air volume systems.
- Variable air volume (VAV): A variable air volume service type refers to a variable air volume HVAC system, where the fan air volume output may vary depending upon the diversity of the cooling loads in the individual spaces. The cooling calculation engine performs sum of the peak room load calculations, meaning that the engine determines at which month and hour of the year the peak cooling load occurs for each individual space. This allows the HVAC designer to properly size variable air volume boxes for each room(s) that it serves. Please note that the zone cooling load components will continue to display the constant volume (peak of the sum) values for the sake of properly sizing the fan or rooftop unit, while the individual space cooling load components will represent the peak values for the individual space.
- Hydronic: A hydronic service type refers to a hydronic HVAC system such as a chiller or boiler. By selecting a hydronic service type, some of the output results will display hydronic units such as gallons per minute (GPM) or liters/second (L/s) versus BTU/hour for conventional air system types. Any service type with the word "water", "radiator", or "hydronic" is a hydronic service type.
- Other: The reason the other choices remain is to support an export to other consuming applications.
The service type also specifies whether there is reheat applied (reheat is no applied by default, and only Service Types that include the word "Reheat" turn it on). Reheat is the process of cooling air to a lower set point that you want and the heating it back up before it's spread to the spaces. This can be useful when a particular relative humidity is required (for example in an emergency room or museum).