What does it mean if the calculated flow in a pipe is negative?
A negative flow value indicates orientation of the flow with regard to the orientation of the pipe itself. The flow arrow symbol on the pipe in the plan view always indicates the direction of flow, and the orientation of the pipe itself is indicated by the "Start node" and "Stop node" fields in the pipe properties.
If water is flowing from the "start node" to the "stop node", the flow arrow will point that way and the flow result value will be positive. If water is flowing from the "stop node" to the "Start node", the flow arrow will point that way and the flow result value will be negative. The absolute value of the flow result field is always indicative of the flow rate through that pipe, but a negative sign will be added to indicate direction.
The reason why this behavior occurs is because in some systems, flow can often reverse direction over the course of a day. Showing the negative sign in front of the calculated flow value is one way for the user to distinguish the current direction of flow. In general, it is recommended that you lay out pipes in the direction that water will generally/usually be flowing, so that the calculated flows will be positive for most pipes. If you need to reverse the orientation of the pipe, you can do so by using the "Node Reversal" field at the top of the pipe properties. The following wiki shows how this can be done.
Note that you can see the absolute value of flow through the pipe by looking at the "flow (absolute)" field in your pipe FlexTable. As you might expect, this basically removes the negative sign from the value seen in the "flow" result field.
How do I reverse the start and stop node for a pipe, conduit, gutter, etc...?