I burn my princess on low 99% of the time with very dry low btu woods like Doug fir and maple from a shed. The cat stays nice and hot, the probe for the flue meter stays brown and dry but the firebox can get pretty icky. The wet looking firebox deposits on the bare firebox steel can cause corrosion, especially in the corners, and this is what the deposits look like. To remedy this I’ll need to burn a half full firebox of wrist sized splits on high. That won’t clean the glass but it will dry out that tar stuff and allow it to flake off.