How much weight can floor made of plywood over joists on concrete slab support?
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.