ecco123 wrote:I always thought the bigger the chimney the better!
It's the taller the chimney the better, at least within reason. A large chimney only has greater capacity but needs that much more heat to draft well. That's why fireplace chimneys are so big. They need the capacity to handle a big fire and the big fire makes enough heat to make it draft. A chimney in the interior of the house will be warmer and draw better regardless of size. In the colonial houses of America the chimneys were always in the center of the house.
That you get smoke loading wood which has a hotter stack temperature and so better draft is a sure sign to at least suspect that the draft is not enough. Remember draft is created by hot air rising. If the chimney is cooling it too much it will not rise.
Any heating man should be able to measure the draft for you at the boiler breech and over the fire with a small hole in the loading door, or you can buy a manometer yourself for about $25 for the Dwyer Mark2 instrument. Then you will have definite figures to go by to make a decision rather than guessing.
There are simple things you can try to improve draft but lets get some figures first before making decisions. Measure the draft with the boiler running and with a simple magnetic thermometer the temperature of the smoke pipe about a foot after the boiler. Do this under typical running conditions.