My Friend Doug Bowers had an interesting problem today. He sent me this email.
I have a schedule in which I do not want the CFM to show as text in the
schedule (I just want the numeric value). I figured that was easy enough to
do by just going into the schedule properties and changing the formatting
for that field. I selected the desired field, picked the Field Format
button, removed the checkmark for "Use project settings", and then changed
the "Unit symbol" from "CFM" to "None". It gave me an "Invalid Input" error
message and wouldn't allow the change.
I thought that was interesting, so I went in the Project Units for the
project itself and tried to change the HVAC Airflow units from CFM to
something else. Not only would Revit not let me change it to something
else, but if I just picked the OK button without changing anything it gave
me the same error message.
Here is the secret. Go into project units as Doug suggested and pick CFM, but this time first change the Rounding to anything but Custom first. Everything works as expected after that. For some crazy reason Custom is the default and it hasn't worked since at least 2010.
My suggestion is change it in your template and forget about it.