robsdesignForgot to mention, I use Mantra.
If you are using Mantra: Go to the render tab on the fire geometry. On Render Visibility, either type -primary or use the drop down "Invisible to Primary Rays (Phantom)"
If you are using Redshift: Go to Redshift OBJ>Visibility tab on the fire geometry and uncheck "Primary Ray Visible".
If you are using something else: I don't know.
Perhaps I did not put it correctly, I need the light created by the fire to be moved to the background, but the grid should not be visible.