"Create Return Air System" Icon Not Available

[cporter.207875]

For some odd reason, my custom return air terminals and equipment do not provide the ability to create return air systems. I can't figure out why this would happen. The families list under the Unassigned Systems for Default Return Air in the system browser and I can connect duct to them. However, I can neither create return air systems with the families nor add them to existing return air systems. When I try to add them to an existing return air system I get an error message that says the family doesn't have the matching return air connector for the selected system. This is obviously not true, since they list under default return air and connected ducts color correctly based on my return air system filters. I edit the family to double check and the connector is set to Return Air.

OOTB families work correctly. So is there a button I've pushed to disable this function in my custom families? Anyone seen this?

[cporter.207875]

Turns out it was only the flow direction that was causing the problem. I was thinking of flow direction as referring to the direction of air (IN because it is coming into the connector from the space), but flow direction refers to the direction the flow value will propogate (OUT so it can be calculated upstream). Well, that's that...

[jseck]

Think about the connector arrow as a solid object that a duct/pipe will physically connect to. For example, consider an air handler with a chilled water coil. If you place your pipe connectors on the side of the unit with the connector arrows pointing away from the unit, the pipes can physically connect to the connector arrows, figuratively speaking. If you switch the direction of the arrows so they are pointing into the unit, there will be nothing to connect to. Thinking about it in terms of flow direction can cause confusion.

[cporter.207875]

You misunderstood the problem. I wasn't having trouble with connector's placement or which direction the arrow is pointing. That's a pretty intuitive set-up. Point the arrow toward the incoming duct... easy as pie.

I'll restate the problem for you. If you edit the terminal family and look at the connector's instance properties you will see a parameter for Flow Direction. I had my RA terminal's connector Flow Direction set to "IN" because that is the direction the air is moving (in my mind). Setting the Flow Direction with the wrong value disables the Create Return Air System icon, as well as the ability to add the air terminal to an existing Return Air System. Here's why: Flow Direction, as set in the connector's properties, is backward in terms of real conditions. A return air terminal's connector should set its Flow Direction parameter value as "OUT," because the air is leaving the terminal and entering the duct. Conversely, a supply air terminal's connector should set its Flow Direction parameter value as "IN," because the air is entering the terminal from the duct.

So it was my intuition that caused the problem. I assumed the Return Air connector should be "IN," since the air is entering the terminal from the space. With the same line of thought, I figured the Supply Air connector would be set to "OUT," since the air is leaving the terminal and entering the space.

In short, the Flow Direction parameter indicates the direction the air is going in relationship to the duct, instead of its direction in relationship to the space.

Do you see, now, how that could be confusing if you aren't thinking about it from the right perspective?