>12×14'
That's about 15sqm. Rule of thumb is you need 2.4kW of energy for every 8sqm, so what you are aiming is a firebox with alteast 6kw of heat output. Make sure you are not venting directly into the chimney, cause that's a lost energy. My furnace runs on pelletes, but i think the priciple should be the same - there are two barriers that seperate the firebox from the exhaust. Ofcourse i have inspection hatches at the top for cleaning, so that would complicate things alot.

If you plan to vent straight for simplicity, make sure there's a bend in the chimney louvres. I would suggest tapping into the alredy existing chimney if it is possible.