I'm having trouble finding good information and ideas on this issue. The situation:
New house design, superinsulated and tight construction. Two levels, very small woodstove to go at lower level, chimney to run up through a chase in the upper level, through the attic, and out through the roof.
The woodstove won't get a lot of use and probably can't be used much until outside temps are really cold.
The problem: most of the time, when the stove is not in operation, but air outside is cold, the chimney will be full of cold air, and the chase will be cold. This will create a cold spot in the house and the associated heat loss. Design of a superinsulated house looks very carefully at even the smallest heat losses.
The initial solution: insulate the chase walls, protect the insulation with sheetrock. The chimney pipe vendors require a 2" clearance around the chimney pipe within an enclosure. I suspect that statements to the effect: "Don't worry; the pipe is designed for this, and just having 2" clearance covers you" are based on the typical uninsulated chase. I do worry.
The problem the chase insulation creates: Adding insulation to the chase walls will result in the air inside the chase becoming a lot warmer. Calculations based on an R4 chimney insulation layer, only a 2" XPS chase insulation at R10, and 400 degree F flue gas temperaure show that the air inside the chase could easily exceed 300 F. I'm not really comfortable with that.
Other possible solutions:
One, I could run the class A chimney up through an even larger diameter pipe in the chase, and duct combustion air down through the annular space, then down through a duct buried in a wall to the back of the woodstove. Possible problem: draft issues at startup.
Second idea: use triple wall chimney; duct the combustion air in from outside through a vent coming in low on the lower level exterior wall, perhaps up and back down through a loop of duct, to provide a trap for cold air infiltration during times when the stove is not used. It isn't clear to me from looking at Duravent pics just what the path of air would be in that outer cavity of the triple pipe. I heard one explanation, but it didn't seem right if my understanding of the triple pipe is correct. It seems to be a regular insulated class A pipe with an outer pipe around that. I need more info on this.
Surely someone has addressed this situation before. What is done that works well and meets all applicable codes?
New house design, superinsulated and tight construction. Two levels, very small woodstove to go at lower level, chimney to run up through a chase in the upper level, through the attic, and out through the roof.
The woodstove won't get a lot of use and probably can't be used much until outside temps are really cold.
The problem: most of the time, when the stove is not in operation, but air outside is cold, the chimney will be full of cold air, and the chase will be cold. This will create a cold spot in the house and the associated heat loss. Design of a superinsulated house looks very carefully at even the smallest heat losses.
The initial solution: insulate the chase walls, protect the insulation with sheetrock. The chimney pipe vendors require a 2" clearance around the chimney pipe within an enclosure. I suspect that statements to the effect: "Don't worry; the pipe is designed for this, and just having 2" clearance covers you" are based on the typical uninsulated chase. I do worry.
The problem the chase insulation creates: Adding insulation to the chase walls will result in the air inside the chase becoming a lot warmer. Calculations based on an R4 chimney insulation layer, only a 2" XPS chase insulation at R10, and 400 degree F flue gas temperaure show that the air inside the chase could easily exceed 300 F. I'm not really comfortable with that.
Other possible solutions:
One, I could run the class A chimney up through an even larger diameter pipe in the chase, and duct combustion air down through the annular space, then down through a duct buried in a wall to the back of the woodstove. Possible problem: draft issues at startup.
Second idea: use triple wall chimney; duct the combustion air in from outside through a vent coming in low on the lower level exterior wall, perhaps up and back down through a loop of duct, to provide a trap for cold air infiltration during times when the stove is not used. It isn't clear to me from looking at Duravent pics just what the path of air would be in that outer cavity of the triple pipe. I heard one explanation, but it didn't seem right if my understanding of the triple pipe is correct. It seems to be a regular insulated class A pipe with an outer pipe around that. I need more info on this.
Surely someone has addressed this situation before. What is done that works well and meets all applicable codes?
