Someone asked about blurring / adding noise to the distortion in post so I thought I’d post here too just in case people have the same question.
If there is no geometry in your scene, you can safely do this in post.
If you have geometry in your scene any noise / blurring or remapping in post can, for example, end up distorting objects that are in front of the volume that should have no distortion. The safe way to do this is to blur / add noise or remap values in your volume before it goes into the heat haze tool. This ensures the rays land correctly on the geometry.

Hope this is helpful

rOb