I'm trying to create a heat distortion effect by combining a particle layer over a background image. I can see the effect inside the compositor but when I render it only renders the particle layer. (seen bottom right) I check the Compositor option in "Post Processing" any help?
Whatever is connected to the composite node is your final render, the Viewer does not get saved using the output settings.
In your current setup you are feeding just one layer directly into the Composite (output) Node, bypassing any displacement.
Connect the output of the displacement to the composite node and be happy ever after.