We have two identical Dovre 500s in identical rooms with identical flues and ventilation yet the one in the back room sucks like a good 'un and burns hot and clean while the one in the front room is sluggish, smokey and smelly, especially when it's not burning as drafts come down the flue.
I think I even posted a Q on here last year asking if anybody had any suggestions as to the reason. I believe it might be to do with the prevailing wind and the air flow over the roof. One thing is for sure; even the better stove is slow to light after a few days not lit because the whole flue and chimney cool down and it doesn't begin to draw and light easily until it's been burning for an evening or two.
Is your wood properly seasoned? If it's not, it will steam and the steam will be cooling your combustion down and damaging your flue by forming suplhuric acid. If you can hear hissing when you burn wood, it's too wet.
The other issue is the way you burn the wood. One or two big logs will smoulder at the edges but won't give out much heat. A woodburner needs a good bed of glowing embers radiating heat before it will get up to a good temperature and you are achieving proper, hot, clean combustion. You need to create this bed of hot embers by splitting the wood into smaller pieces or by starting the evening with a good layer of smokeless nuggets, which will raise the combustion temperature enough to deal with the big cold lumps of wood you throw in. Wood is a poor conductor of heat and you have to supply a lot of heat to a big fat cold log, especially a dense wood like beech or oak or ash before it will begin to burn. Once a heavy wood begins to burn it will liberate heat for much longer than a light wood like pine, which burns up in half an hour. I can't over-stress the importance of creating a bed of glowing embers; the radiant heat is what warms your room, not a couple of logs struggling in a cold draft from the bottom vent and sending everything up the chimney.
One word on stove size: the best advice we had when we bought ours was to err on the smaller size because that would mean we would be burning them harder with more oxygen and thus higher combustion temperatures, meaning less soot and less moisture in the flue. So often you see oversized stoves turned down too low with the window all sooted up because the combustion temp is too low and the fuel is smoking.
