My brother-in-law installed a woodburner in his basement to offset the price of fuel oil. The one he bought could handle coal so that's what he used. Last spring, he had no air. The system would run, but no cooling. I finally took the whole thing apart and the evaporator was coated in thick coal soot. I don't know how it got past the filters, but it did.
If the evap is freezing up, that's a sign of low freon.