Very clever cluing, and the cheek to have fill like 2D! I love it! 3D is, I think, a variant in my dictionary—I can't recall ever seeing it singularly, but it was inferable and the surface earns the stretch. 4A's definition is SO good; another example of earning what you're doing. My only quibble is 1A; it feels like this should be [Hot gases: these keep you warm (5)] (note the mis-enumeration in your grid).