Appendix A: Rules for Air Flow Specification
It is important to specify airflows in the system completely and consistently. The rules for airflow specification are set out below.
Airflows may be specified by the following mechanisms:
- With a flow rate controller. Airflow is specified at any point in the system where a controller with its controlled variable set to flow rate is attached to the ductwork.
- With a percentage flow control controller. Airflow may be specified by a controller with its controlled variable set to percentage flow control. Such a controller may only be attached to the outlet of a damper set component. In this case, the flow entering the left branch of the damper set (often the outside air intake) will be set as a percentage of the flow leaving to the right.
- By the assumption of continuity across air handling components. The program will deduce the airflow at all points along a chain of air handling components given the flow at any point in the chain, on the assumption of flow continuity. This rule is subject to a qualification in the case of room components, as explained below.
- By addition or subtraction at a junction. At a junction where all but one of the flows are known, the program will deduce the unknown flow by addition or subtraction. You must ensure that, where a flow is to be deduced by subtraction, the result is never negative. If this situation arises, the negative flow is set to zero.
- For the current iteration scheme, every closed loop that can be traced in the HVAC airside network must pass through at least one assigned room component. To satisfy this requirement, the room component must have an actual space in the model assigned to it—i.e., it cannot satisfy this rule if it remains set as an adiabatic duct.
Important note: Airflow is not set by the fans or by any other type of component.
You should check before simulating a system that all flows in the system can be determined by applying these rules. Over-specification of flows is tolerated, but there should be no inconsistencies in the numerical values of flows.
In the case of rooms, the assumption of flow continuity is qualified by additional rules introduced to allow ApacheHVAC to interact with MacroFlo.
If MacroFlo is running in tandem with ApacheHVAC, it will detect any imbalance between the system flows entering and leaving each room, and make up the surplus or deficit with air flowing through the building through openings in the fabric (provided that suitable openings exist). In order to allow such imbalances to be set up, the following additional rules are applied relating to the assumption of flow continuity across rooms:
In a first pass, ApacheHVAC makes no assumption about flow continuity across rooms. Within this constraint, all possible flow deductions are made.
If some system airflows remain undetermined at this stage, a second pass is made in which the program allows room outflow to be set equal to room inflow. All possible flow deductions are then made under this assumption.
If some system airflows still remain undetermined, a third pass is made in which the program allows room inflow to be set equal to room outflow, or room outflow to be set equal to room inflow. All possible flow deductions are again made.
After this process, all airflows should be determined for a well-specified system. If this is not the case, an error message is displayed.
By delaying the application of the room flow continuity assumption, these rules allow flow imbalances to be set up to simulate a variety of mechanical ventilation and mixed-mode regimes.
When using ApacheHVAC and MacroFlo in tandem, it is important to note that ApacheHVAC can set flows in MacroFlo, but not vice versa. All ApacheHVAC flows must be set within ApacheHVAC.
There are instances when room flow imbalances may meaningfully be set up without invoking MacroFlo. If the supply rate for a room is greater than the extract rate, the deficit will tacitly be assumed to be lost to outside. Note, however, that if the extract rate is greater than the supply, the deficit will not be assumed to be made up with air from elsewhere unless MacroFlo is running and suitable openings are present.