I fully expect that at some point I'll need to break this page up into multiple pages each of them specific to areas of the application, but while the list is short I'm keeping it on one page.
Q: When I try to draw pipe only whole number show up in the size drop list. I've checked mechanical settings and it says that I have a number of non-whole number pipes defined. Why can't I use them?
A: Settings -> Project Units. Change the discipline to Piping. Pick the "Pipe Size" unit. Change the rounding to 'To the nearest 1/8"'.
Rooms / Spaces
Q: I've heard that I can use walls in a linked file as the boundaries for spaces, but I can't get it to work. What did I do wrong?
A: Select the linked file and go to Properties -> Type Properties and turn on the check box for Room Bounding.
Q: Can I add more demand factors to the list. The current list doesn't include everything I want?
A: Currently it is not possible to add to the list of demand factors.
Q: How does Revit MEP Calculate wire length on a circuit?
A: As described on this page
Q: What does the "Circuit Description" setting in Electrical Settings do?
A: Help is wrong. It doesn't do what help says. What it does do is control the formatting of the Circuit Description parameter that is on a wire. This was implemented to solve a very specific user request to be able to tag the # of poles and rating of a breaker on a home run (rather than the circuit numbers).
Q: How can I tag the 2nd connector on a device?
A: Simple answer... You can't right now. There are some hacks like putting in a 2nd device in the same place and drawing a "very short" wire that will work, but there ins't really a way to do this today.
Q: How do I know which connector is the primary connector?
A: The concept of primary connectors deserves a bigger discussion, but the primary one is the one drawn with an "X" in the middle.