Size of barometric Damper?

I am getting ready to install my kaa-2 boiler and questions about the size of and how to mount the baro damper.

I have a clay lined 8x8 chimney that is about 28' tall, sitting on the rock foundation of the home.

Currently the oil fired boiler is connected via 6" pipe. The top of the opening to the chimney is 9" to the ceiling of the basement.The oil fired boiler is going to the scrapyard.

According to http://www.fieldcontrols.com/draftcontrol.php#dctop I should use a 7" damper because the chimney is over 16' tall and the breech of the boiler is 6".

I'd like to use 6" double wall stove pipe and mount the 7" damper to that. I think double wall pipe should be used because the chimney opening is less than 18" to the ceiling. I don't want to chop a larger hole into the rock wall, though it may fit larger pipe. The opening to the chimney has some sort of soft material around the pipe.

Is it OK/normal to use a 7" damper on 6" pipe?

I think you'd be fine to use a 6" baro. Others may differ but I'd be surprised to learn that anyone here is using a 7" baro with 6" pipe.

I'd use a 6" Fields RC damper on 6" stovepipe. Buy a barometric damper to set it. If it turns out that you have a exceptional draw chimney a second damper can be installed in parallel. I suspect Fields is suggesting the 7" because it's damper area is larger than the 6" and under worst case conditions the full area might be needed. You could also just increase the weight if needed.

Machinist wrote:I'd like to use 6" double wall stove pipe and mount the 7" damper to that. I think double wall pipe should be used because the chimney opening is less than 18" to the ceiling. I don't want to chop a larger hole into the rock wall, though it may fit larger pipe. The opening to the chimney has some sort of soft material around the pipe.

I wouldn't use double wall insulated from the appliance to the chimney, especially with a 28' chimney. Shield the combustable surfaces instead. You will be giving up a lot of heat to the outdoors, I would prefer the heat in the basement myself.