19½" doesn't sound like nearly enough room. Imagine carrying a large, heavy box and wanting to put it on a shelf. The box would be against the body, as it should be for heavy objects, and you'd need to take into account the thickness of the average body and enough room to pivot to place the box on the shelf. Something like 30"-36" would be far more manageable and safer.

As for the distance between the shelving and boiler, you should check with the manufacturer and/or consult local code, especially if this is a commercial building or something like an apartment. Otherwise, I'd think a minimum of 36".