Question: Two questions: Can I burn wood successfully/efficiently in a Harmon Coal stove (Mark II)? Can I successfully use a coal stove with a 6" vent connected to a 13" x 13" (O.D.) masonry flue? Answer: #1...Well, this depends. Newer stoves are sold with an EPA tag. If the stove says "coal only" then wood is only allowed to be used to start the coal fire. This is because the stove was never run through efficiency tests with wood as a fuel. If the stove is an older model, then you can surely use wood in it...it will not burn as well as a newer model, but may be fine for your off-season needs. #2...Depends, I've installed a lot of coal stoves into larger flues and most have worked fine. However, there are some concerns if the chimney is very cold and tends to downdraft. Coal puts so little heat up the flue that it possible for the flue to reverse while the stove is still operating. This would cause deadly coal gas to enter your home. With any coal stove, a CO detector is a must. Also, you want to be sure your chimney and stovepipe connections are very tightly sealed. No doubt that the best possible job is to reline the chimney with a 6" pipe.