How can one produce fire from surface? I'm able to produce fire by using a cube/sphere and choosing “source from objects”, but when I tried it with a grid it wouldn't work.

Of course, I can always change the height of a cube to make it similar to a grid, but I'm just curious about the general functionality of fire (and other fluids) if a flat plane or surface can be the source.

thanks!

Your object needs volume, a simple surface doesn't work. You can make a version of your object with volume to simulate the fluids, hide it and use the original surface for rendering.

there is Source From Surface tool on Volume Fluids shelf