This is a problem I have quite often. When modeling a duct system, I sometimes run into a tight situation where the length of a fitting is preventing me from making a connection/tap.

For example, I may have to tap a round duct into the top of a main duct. I do this by elbowing down and tapping into the top of the main duct. The main duct is at a set height and cannot be shifted. When I tap into the main duct I get an error because there is not enough room for the elbow, the short length of duct, and the bellmouth tap fitting. I work around this by temporarily lowering the main trunk and then tapping into as mentioned above. I then adjust the parametric Takeoff Length of the bellmouth fitting (let's say from 3" to 2") which would then theoretically give me enough room once I return the main duct to the correct height. However, after adjusting the dimension, when I try to move the main trunk back up, the adjusted dimension reverts back to the default length of 3" and thus error returns.

Any help on this would be greatly appreciated!