Stove installation

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Desmond

Member
Mar 12, 2017
24
Kingston, Ontario, Canada
A friend of mine recently purchased a small cabin. The previous owner took whatever stove was in there. He want's to install a fisher baby bear. The current chimney is excellent. Good ceiling support, insulation shield, stainless steel double walled pipe and cap. However, it's 7" inside diameter. We would like to put a 6" T onto the back of the stove, a piece of 6" piece of pipe with a damper straight up into a 6 to 7 inch taper into the bottom of the chimney. Does anyone see any issues with doing that?
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,286
NE PA
Depends on the heated area of the cabin.
That will reduce the heating area the stove is capable of.
If you're pushing the capacity of the stove it could make the stove undersized for the heated area.

Allowing the hot flue gasses to expand allows them to cool. I've only physically measured the temperature drop from 6 inch increasing to 8. That drops the temperature by 1/2. That is a lot for a Baby Bear. The smaller the stove, the more critical the temperature drop becomes since the stove only radiates the excess heat from the stove surface inside.

The object is keeping the inner flue temp above 250*f to the top. So a 6 inch insulated flue is the best, being the same size as the stove outlet. Allowing the flue gasses to cool below 250*f allows water vapor from combustion to condense on flue walls making them wet allowing smoke particles to stick. This forms creosote. A larger chimney forces you to burn harder, leaving more heat up the chimney. In extreme cases the stove has to lose so much up the chimney it can no longer heat the area you're trying to heat. (not able to damper the flue) The height also has a lot to do with cooling as it rises.

The larger diameter chimney has more capacity to pass more BTU, so is for a larger stove. The larger the diameter, the more heat required to cause the draft. Just like a higher displacement engine is capable of more power, it requires more fuel to do it. 1 inch isn't that much, so it's not near as bad as going to 8.
The square inch outlet area in your case is 28.26 increasing to 38.46 square inches. So it does make a difference in efficiency.

You'll also find the smaller stove may flutter, not quite a "chuff". The flames may not be constant and smooth. The smaller intake isn't enough when you get the cubic inches of space in the chimney too high. I've tried baffles and opening the intake fully to prevent it, nothing stopped it entirely, and only had the issue with the Baby Bear testing the same chimney on other models. The rising gasses allow a slug of colder air to drop into chimney, rising again. As this cycle repeats the air slug gets down to the stove outlet giving it a burst of oxygen at the exhaust. Fuel, (yours being denser with higher BTU is a positive) altitude, inside pipe configuration and height, chimney height and weather conditions all play a part, so it's difficult to tell how well it will work in all conditions. Larger stoves are more forgiving since they have more heat to lose with less effect inside.

If problems, 6 inch double wall inside connected to the larger 7 inch chimney keeps the temp higher entering chimney.