I'm installing a 3' x 4' masonry heater that will weigh 1700 - 2000 lbs in a room that is a converted garage. The floor is plywood over 2x4 joists directly atop a concrete slab. How much weight can this type of floor support? Thanks
You should be fine in terms of compressional strength, the more important issue is the point stress load. If the heater has feet of some kind they might punch right through your plywood if they're not over a joist. If you want to be safe you could cut out a square in the floor the size of the heaters footprint, then install blocking around the holes opening effectively creating a form. Then fill the void flush with concrete, just make sure to install sill seal first to prevent rot and provide a little room for expansion and contraction.
According to this table, the compressive strength of typical lumber when the stress is perpendicular to the grain is around 500 psi or more.
Let's say you have four 2x4 joists (on edge) each 3' long. That's a total surface area of the joists of 1.5" x 3' x 4 = 1.5 sq ft = 216 sq in.
You've got a 2000 lb heater, but 2000 / 216 << 500 psi. That's a factor of more than 50 times better than what you need.
The other answer regarding point loads is very true; however a masonry heater as far as I can think would be a uniformly distributed load. That said, the plywood subfloor would probably bow between the joists, so you ought to lay some additional support. 2x4s or even 4x4s perpendicular to the joists would probably do it.