Product review (1 stars): Not an exact replacement for AC-818.  Does not fit properly.
This filter, at 7.75" X 9.5" X 1.75", is not the same size as the original AC-818, which measures 8.125" X 8.375" X 2".  It is really a model HC-819 replacement.  In order to fit it into a humidifier that is supposed to have the AC-818, it has to be stretched in one direction and squished or trimmed off in the other in order to fit.  It still leaves a little open space that lets air around the narrow dimension, which reduces the efficiency of the humidifier.  Get the correct filter from "Exact Fit Parts" on Amazon.