6" vs 8" Chimney at 35ft height

[ABMax24]

We are in the process of installing a new Osburn Matrix wood stove into our basement and were originally quoted 6" chimney pipe, as it has a 6" connector on the stove. Due to a sale on the floor model we bought the stove first and brought it home with the intent to purchase the chimney later. Today we went to pick up the piping and the retailer is now strongly advising us to go with an 8" chimney to prevent excessive draft within the stove at an additional cost. The stove manual also says max 7" diameter chimney. Is this really an issue to be concerned with? Will 8" really have that much less of a draft compared to 6"?

Part of my hesitation with this is the majority of the chimney will be ran on the exterior of the house, 6" would get up to temperature faster, and with -40 winters getting a draft started rapidly to avoid draft inversion is of priority.

My other thought is draft is dependent on the differing air densities between the hot air in the chimney and the outside air, which is in turn controlled by the temperature of the flue gases. Does 8" loose heat that much faster to cause a reduction in draft? If so should I be worried about increased creosote buildup or even
condensation?

 

[bholler]

We are in the process of installing a new Osburn Matrix wood stove into our basement and were originally quoted 6" chimney pipe, as it has a 6" connector on the stove. Due to a sale on the floor model we bought the stove first and brought it home with the intent to purchase the chimney later. Today we went to pick up the piping and the retailer is now strongly advising us to go with an 8" chimney to prevent excessive draft within the stove at an additional cost. The stove manual also says max 7" diameter chimney. Is this really an issue to be concerned with? Will 8" really have that much less of a draft compared to 6"?

Part of my hesitation with this is the majority of the chimney will be ran on the exterior of the house, 6" would get up to temperature faster, and with -40 winters getting a draft started rapidly to avoid draft inversion is of priority.

My other thought is draft is dependent on the differing air densities between the hot air in the chimney and the outside air, which is in turn controlled by the temperature of the flue gases. Does 8" loose heat that much faster to cause a reduction in draft? If so should I be worried about increased creosote buildup or even
condensation?

Your dealer must have a bunch of 8" he wants to get rid of. No don't do it. It will not reduce draft it will just slow velocity but increase volume. You will most likey have to much draft regardless of 6 or 8. You will need a pipe damper or even 2.

 

[ABMax24]

Your dealer must have a bunch of 8" he wants to get rid of. No don't do it. It will not reduce draft it will just slow velocity but increase volume. You will most likey have to much draft regardless of 6 or 8. You will need a pipe damper or even 2.

I thought the same as well, but went to a different retailer and they offered the same advise.

 

[bholler]

I thought the same as well, but went to a different retailer and they offered the same advise.

Ok not sure where they learned about chimney physics but they are wrong.

 

[ABMax24]

Ok not sure where they learned about chimney physics but they are wrong.

I would agree, but I really begin to question myself when I'm told the same thing twice so figured I would ask here.

As far as dampers are you referring to a flue damper? I've run lots of wood stoves, just never with a chimney big enough to require anything other than the damper built into the stove.

 

[brenndatomu]

Your dealer must have a bunch of 8" he wants to get rid of. No don't do it. It will not reduce draft it will just slow velocity but increase volume. You will most likey have to much draft regardless of 6 or 8. You will need a pipe damper or even 2.

Ok not sure where they learned about chimney physics but they are wrong.

Truth ^ ^ ^

 

[bholler]

I would agree, but I really begin to question myself when I'm told the same thing twice so figured I would ask here.

As far as dampers are you referring to a flue damper? I've run lots of wood stoves, just never with a chimney big enough to require anything other than the damper built into the stove.

Yes a damper in the pipe.

 

[ABMax24]

Yes a damper in the pipe.

Thanks for your help. 6" flue pipe is what we will go with.

 

[ABMax24]

The stove is finally installed with 32ft of 6" exterior chimney and about 3ft of double wall inside. It pulls a decent draft but the damper on the stove keeps everything in check, you can almost snuff the fire with the damper fully closed. Will see how it works when we get -40 instead of the freezing temps lately but it looks very promising.