Why Isn't A Weak Draft Better Than A Strong One?

 
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Devil505
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Post by Devil505 » Sun. Oct. 19, 2008 7:49 am

After starting up my daughter's new TLC (exhausting into a brand new masonry chimney) I can tell the draft is much stronger than my old (block) chimney & the baro damper will come in very handy, which leads me to few questions:

Since my chimney has a very weak draft (so far about .02.03 on the manometer) but I have no problem keeping my TLC running very low even on warm days, have never had a CO reading at all ( with the TLC) & seem to burn less coal (per winter) than many others here (slightly less than 2 tons of nut coal) ...A few questions:

1. Assuming your draft is strong enough to vent your stove & get rid of CO, why would you want a stronger draft?
2. Wouldn't it be smarter to fine tune a chimney from the outside (allowing a small amount of outside air into the flue) to adjust the draft to a minimally low setting to keep warm stove air from being unnecessarily sucked out of your stove, instead of using a baro damper to do this by stealing warmed inside air?

Bottom line question: Is there a benefit to having a stronger draft than is needed to safely operate your stove?


 
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Post by TimV » Sun. Oct. 19, 2008 8:22 am

Maybe its good to have extra draft available in case a variable like barometric pressure is screwy or air is heavy and smoke wont rise no matter what you do to open it up then it would be nice to have the "extra' available.
maybe its like 75 hp will make my 3.5 ton pickup roll down the highway but sometimes extra is needed to get me over that huge hill someone designed into my path for no good reason so that 250 hp is nice to have.
Variables is why I guess
:mad: dam keyboard is messing up agin its not my typos

 
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Post by Devil505 » Sun. Oct. 19, 2008 8:42 am

TimV wrote:Maybe its good to have extra draft available in case a variable like barometric pressure is screwy or air is heavy and smoke wont rise no matter what you do to open it up then it would be nice to have the "extra' available.
That would make sense Tim except for the fact that the colder it gets outside &/or the hotter your fire gets, the stronger your draft gets anyway. So, is the only benefit to a stronger than necessary draft that on warm days with a low temp fire, you have an extra margin of safety that you are paying for all winter with more coal usage to satisfy the "overkill" draft?

Edit: I'm thinking about something like a variable air valve on the chimney (outside) that can be adjusted to allow air into the flue, as needed, to set an ideal draft. (or closed completely for max draft) or a barometric damper mounted outside (protected from the weather) to do this.

 
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Post by gambler » Sun. Oct. 19, 2008 10:02 am

Devil505 wrote:That would make sense Tim except for the fact that the colder it gets outside &/or the hotter your fire gets, the stronger your draft gets anyway.
If you are running a low fire and it gets colder outside it can cool your chimney enough that you will have no draft. If you had a better draft to start with this would not be an issue.

 
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Post by Devil505 » Sun. Oct. 19, 2008 10:13 am

gambler wrote:If you are running a low fire and it gets colder outside it can cool your chimney enough that you will have no draft. If you had a better draft to start with this would not be an issue.
I think that if it get colder outside, your draft will actually increase due to the greater difference between the warm chimney air & the (now colder) outside air.
Maybe someone more expert on the physics involved here can enlighten us??

 
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Post by Bob » Sun. Oct. 19, 2008 10:13 am

Seems to me that the bottom line with draft is safety. You want an installation that will have adequate draft at ALL TIMES to remove products of combustion so that the safety of the occupants is not compromised. As pointed out in the above discussion the amount of draft varies dramatically based on outside temperature, humidity, wind, wind direction, strength of the fire , and probably a number of other variables.

Any mechanism to limit draft MUST be be reliable and work automatically in all conditions that occur while the appliance is operating. It is not clear what mechanism you contemplate when you say "I'm thinking about something like a variable air valve on the chimney (outside) that can be adjusted to allow air into the flue, as needed, to set an ideal draft. (or closed completely for max draft) or a barometric damper mounted outside (protected from the weather) to do this." but the description doesn't appear to be automatic in the same way as a barometric draft control.

 
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Post by Devil505 » Sun. Oct. 19, 2008 10:24 am

Bob wrote:It is not clear what mechanism you contemplate when you say "I'm thinking about something like a variable air valve on the chimney (outside) that can be adjusted to allow air into the flue, as needed, to set an ideal draft.
I haven't thought through what device I'm thinking of either yet. ;) (I am just thinking theoretically)

My thinking though goes along this line:

1. Since my chimney (masonry block built 26 years ago) undoubtedly is not air tight anymore (cracks, etc) it does not provide the same strong draft it did when first built.
2. Regardless of that weakened draft, I still have no problem keeping a low fire going even on warm days.
3. I never have any CO readings on my CO detectors
4. I seem to be using less coal, (per season) than is average

Why would I be better off with a stronger draft?


 
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Post by gambler » Sun. Oct. 19, 2008 10:43 am

Devil505 wrote:
gambler wrote:If you are running a low fire and it gets colder outside it can cool your chimney enough that you will have no draft. If you had a better draft to start with this would not be an issue.
I think that if it get colder outside, your draft will actually increase due to the greater difference between the warm chimney air & the (now colder) outside air.
Maybe someone more expert on the physics involved here can enlighten us??
Devil, I have lived it. When running my stoker in cool weather I can get away with running at a very low setting. When it gets cold outside I have to bump this setting up a notch because my chimney runs colder and reduces my draft. It happened to me last year when I had a reading of 12 on my CO detector. I checked my draft and it was below -.01 and it had previously never gotten below -.02 at the same stove setting. The only difference was the much colder outside temp.

 
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Post by Devil505 » Sun. Oct. 19, 2008 10:51 am

gambler wrote:Devil, I have lived it. When running my stoker in cool weather I can get away with running at a very low setting.
I'm guessing the difference is that your stoker uses a mechanically induced, forced draft, doesn't it? Where as my hand-fired performs with a purely natural draft.

 
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Post by gambler » Sun. Oct. 19, 2008 10:58 am

Devil505 wrote:I'm guessing the difference is that your stoker uses a mechanically induced, forced draft, doesn't it? Where as my hand-fired performs with a purely natural draft
Good point! I am probably able to have a lower heat producing fire than a hand fired stove.

 
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Post by coalkirk » Sun. Oct. 19, 2008 10:58 am

You can always mitigate a strong draft with a baro but you can't increase a weak draft without some mechanical assist such as a draft inducer. Given a choice, I'd take the strong draft any day. There are lots of factors that determine how strong your draft will be. A chimney within your home will almost always have a stronger draft than one on the outside of your home. The inside chimney stays warmer, hence stronger draft. The height of the chimney plays a major role. Taller is better. My chimney is outside and not very tall. The thimble is 5' off of the basement floor and that part of the home is a one story family room above. It's on the gable end but my total chimney height is probably about 18'. Consequently my draft is not real strong. If I had known 12 years ago when I built this family room that I would be burning coal in the second flue, I would have built the chimney taller.
Devil, I think in addition to the ideal layout of your split foyer for facilatating air distribution, you close proximity to the ocean must play a role in your low coal usage.

 
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Post by Rob R. » Sun. Oct. 19, 2008 12:05 pm

coalkirk wrote:A chimney within your home will almost always have a stronger draft than one on the outside of your home. The inside chimney stays warmer, hence stronger draft.
Agreed, a chimney located within the home will always perform better. I have a monster 3 flue chimney that runs up through the center of my house, it is roughly 35 feet tall. This summer when I was getting my new stove hooked up I noticed that with the thimble open, the chimney pulled a slight draft with it 80 degrees outside. The colder it gets outside, the stronger the draft.

 
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Post by LsFarm » Sun. Oct. 19, 2008 12:57 pm

Devil,, I consider your chimney borderline dangerous.. whatever is wrong with it to only pull .02-.03" may get worse.. cracks open, rusted cleanout door falls apart, what ever,, and you may end up with no draft and CO alarms going off.. and this always happens at 0400.

Personally I'd try real hard to figure out why your chimney isn't just as good as your Daughter's , I think you said they are similar height.? A tall chimney that has poor draft is a sick chimney... and needs help from Doctor Devil. :lol: :D :shock:

Your topography is different than you daughter's but you still should have a healthier draft.. Actually I'm amazed that you don't have burning issues with a hand feed with only .02-.03" draft..

Take care,, Greg L

 
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Post by Devil505 » Sun. Oct. 19, 2008 1:46 pm

coalkirk wrote:Devil, I think in addition to the ideal layout of your split foyer for facilatating air distribution, you close proximity to the ocean must play a role in your low coal usage.
I agree Terry & have no complaints!
LsFarm wrote:Your topography is different than you daughter's but you still should have a healthier draft.. Actually I'm amazed that you don't have burning issues with a hand feed with only .02-.03" draft..
Agreed Greg & I emphasize that my safety margins could be improved but I always have TWO Nighthawk digital CO meters working & have never had a reading since I bought the TLC. (My old Dutchwest "Federal" stove would occasionally register when the fire was low with warm outdoors temps)
This year a did replace the wall thimble but there may still be air leaks into the flue from it & from the missing cleanout door outside.

 
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Post by Bob » Sun. Oct. 19, 2008 2:16 pm

Devil505 wrote: Bottom line question: Is there a benefit to having a stronger draft than is needed to safely operate your stove?
I think the answer to your question is NO. As I tried to say above you want to be very careful not to make changes that could, under unusual or infrequent circumstances, result in less draft than needed for safe operation.

While in theory you might save some fuel by making modifications to reduce draft--or continuing to operate with a chimney in poor repair--in practice you might also end up with less draft than necessary for safe operation in some circumstances. A very bad tradeoff in my opinion!


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