VBA Userdefined Function returns #Value! error

The Value error suggests to me some kind of math error in the function. The most likely place I can see is in the quadratic formula, and the most likely error there (since you are apparently only trying to find real roots and not complex roots) is trying to take the square root of a negative number. Since that involves more than just Q, it is hard to say for sure. Even if this isn't the immediate problem, you maybe should add some code to the function to deal with those cases where there is no positive real root.

Here's what I'd recommend. Set a breakpoint in the function so that the function will enter break/debug mode when it is called. Then you can use the debug tools (watches, immediate window) and step through the function one line at a time to see where the error is, and work on figuring out what the error is and when it occurs.