The "Space Bar" is used a little differently in this version of Revit than its mates. As you already know, hopefully, it is used to flip certain elements and to rotate components. It is also used to toggle between Zoom/Scroll/Pan when viewing using the keyboard (SHIFT/CTRL) combined with the Middle Button or Right Click.
In Rmep it is also used to snap piping/duct to other related elements. It is used to match the size of the selected element and to orient to the correct workplane. It also assumes the same elevation of the element you intend to connect to. Here is an example of using the Spacebar to align a pipe with one side of a Double Wye Pipe Fitting.
Vertical duct/pipe (Risers) are very easy to place if you can remember to click the Apply button on the options bar.
When you sketch a horizontal run of duct or pipe and then need to create a riser pipe/duct you need only change the elevation and click Apply. Revit will create the transition/riser condition according to your M/P settings. Just be sure to allow enough elevation change to permit the insertion of transition fittings otherwise you'll get error messages.
If a warning appears informing you that the line is too short, you pressed Spacebar before you began drawing duct. Note that the Spacebar does not automatically specify the duct type. You must verify or select the duct type from the Type Selector on the Options Bar. Sometimes you need to sketch some extraneous duct to get a piece you really want and then delete the extra bits. Same for piping.
One tip for road...using Wireframe and/or Detail Level: coarse will improve regeneration times when sketching Duct and piping. Now, getting used to the presentation of these elements using those settings is another matter. Good Luck!!