Restricting a chimney outlet..chimney reducing caps

 
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Post by Hoytman » Thu. Dec. 05, 2019 12:05 pm

Is it possible to restrict the outlet of a chimney by means of some sort of reducing cap on top of the chimney?

Here's the thought process...
Say you have a 6" stove outlet...and an 8" chimney. Rather than change the size of the entire chimney...I was wondering it the outlet itself could be reduced...sort of allowing the pressure to equalize in the chimney...the temperature to increase...therefore the draft to increase.

Just thinking hypothetically...


 
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Post by coaledsweat » Thu. Dec. 05, 2019 12:53 pm

The chimney should be larger than the stovepipe. Restricting the chimney will not increase draft, more than likely, it will reduce it.

 
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Post by Hoytman » Thu. Dec. 05, 2019 1:06 pm

Ok...
So you have a stove with an 8" outlet...someone reduces it down to 6" …

What's the difference between reducing it at the stove (they make and sell a lot of reducers to do this both at the stove and at the thimble), or the thimble...versus at the top of a chimney?

 
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Post by coalkirk » Thu. Dec. 05, 2019 1:12 pm

First of all if you have a stove with an 8" outlet you should not be reducing it to 6". You can't put 10 pounds of poop in a 5 pound bag. Next, why, since you apparently have a stove with a 6" outlet would you want to reduce the chimney from 8" to 6"? If your stove is not drafting well in that chimney there is something else going on.

 
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Post by Hoytman » Thu. Dec. 05, 2019 1:31 pm

Stop right there...
Don’t make this a personal discussion about me and/or my set-up. All the “you’s“ led to assuming I have a 6” outlet. Wrong! Let’s just not assume anything...drop the “you” language that way I don’t end up on the bar-b-q spit in this discussion.

At the beginning of this discussion I did say, “Just thinking hypothetically”.

Let’s continue...

People reduce stoves all the time...
Manufacturers of pipe sell these reducer/increases...
So let’s not tip-toe around that fact...
I don’t care what codes are...within the context of this discussion...

..
Last edited by Hoytman on Thu. Dec. 05, 2019 1:34 pm, edited 1 time in total.

 
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Post by franco b » Thu. Dec. 05, 2019 1:33 pm

Large fireplace flues have been reduced with a plate on top, and it seems to work to hold more heat to increase draft. Even better is to insert a length of smoke pipe hanging down from the plate into the chimney. I would not reduce an eight inch chimney.

 
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Post by Hoytman » Thu. Dec. 05, 2019 1:38 pm

...”chimney should be larger...” in which way?


 
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Post by Hoytman » Thu. Dec. 05, 2019 1:41 pm

franco b wrote:
Thu. Dec. 05, 2019 1:33 pm
Large fireplace flues have been reduced with a plate on top, and it seems to work to hold more heat to increase draft. Even better is to insert a length of smoke pipe hanging down from the plate into the chimney. I would not reduce an eight inch chimney.
Exactly what I was getting at.
Me and a fella was having this discussion and he said you could do it in a fireplace, but not a stove...giving zero explanation as to why it couldn’t be done with a stove.

I’m not saying yes or no. I am asking, “why”, and “because” doesn’t satisfy my curiosity.

Edit:
It is widely excepted that adding a cap to a chimney can increase chimney draft. So, does the cap not put back pressure on the chimney to do this? It would seem that is the case.

So why wouldn’t further restriction of the cap work in the same manor, up to a point?

Let’s say a couple buys a home with a newly built inside chimney of masonry with a clay liner...8”x8”...or 8” round.There is no stove in the home and the new owners have never burned any type of stove before. They buy a coal stove they think they like, but the installer won’t install the stove without them spending a couple more thousand dollars to install a new matching liner.

The owners have an issue with spending the additional money because they are not sure about burning coal...could be a wood stove and they may think they might like to switch to a coal stove later. Which stove they buy or end up with really has no bearing on what I am asking. The installer is adamant about reducing the chimney size before the install. Why is this scenario any different than that that Franco mentioned? Would a restriction at the top not work?
Last edited by Hoytman on Thu. Dec. 05, 2019 1:59 pm, edited 2 times in total.

 
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Post by franco b » Thu. Dec. 05, 2019 1:52 pm

Hoytman wrote:
Thu. Dec. 05, 2019 1:41 pm
Exactly what I was getting at.
Me and a fella was having this discussion and he said you could do it in a fireplace, but not a stove...giving zero explanation as to why it couldn’t be done with a stove.

I’m not saying yes or no. I am asking, “why”, and “because” doesn’t satisfy my curiosity.
Stove and boiler makers will often use the same size exhaust collars on their largest and smallest units. The smaller units probably don't need the large size and could be reduced. Using an MPD does just that.

Chimney makers such as Selkirk specify the BTU requirement for chimney sizes.

 
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Post by Hoytman » Thu. Dec. 05, 2019 2:00 pm

Franco I edited my post above.

 
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Post by Hoytman » Thu. Dec. 05, 2019 2:07 pm

coaledsweat wrote:
Thu. Dec. 05, 2019 12:53 pm
The chimney should be larger than the stovepipe. Restricting the chimney will not increase draft, more than likely, it will reduce it.
What does an mpd do? Reduce draft, reduce wasted heat, or both?

 
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Post by franco b » Thu. Dec. 05, 2019 2:16 pm

Hoytman wrote:
Thu. Dec. 05, 2019 2:00 pm
Franco I edited my post above.
Addressing the edit:

Installers generally want a liner because outside chimneys take so long to heat up and a liner helps that. Fireplace chimneys are just too big.

A rain cap stops the rain from entering. I never heard the claim that it increases draft, though maybe it does.

 
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Post by coaledsweat » Thu. Dec. 05, 2019 3:05 pm

Hoytman wrote:
Thu. Dec. 05, 2019 1:41 pm
Edit:
It is widely excepted that adding a cap to a chimney can increase chimney draft. So, does the cap not put back pressure on the chimney to do this? It would seem that is the case.

So why wouldn’t further restriction of the cap work in the same manor, up to a point?
Not sure where you got this but I doubt it to be an accurate description of what a cap does.

 
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Post by CoalisCoolxWarm » Thu. Dec. 05, 2019 6:29 pm

Keep in mind the risk of condensation with a reduced size cap AND cooler outside temps in direct contact. Think about cooling towers at power plants.

 
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Post by Hoytman » Thu. Dec. 05, 2019 7:58 pm

coaledsweat wrote:
Thu. Dec. 05, 2019 3:05 pm
Not sure where you got this but I doubt it to be an accurate description of what a cap does.
Well, I will agree.

However, to a small degree I can sort of see this happening. A cap is no more than a baffle in the stove, or a baffle/damper
in the pipe ..it’s just at the other end of the system. That may not be it’s intended purpose and certainly how high the cap is above the opening could increase or decrease any effect of being a baffle increasing pressure/heat/draft in the chimney, if it has any effect at all. It seems this idea could present some complexity and quite possibly some issues/problems.

Anyway, it was just something I’d been thinking about and considering from a different viewpoint.
My self, I don’t have a cap on my chimneys and many old homes in my community are this way. I didn’t have a say in it though...the house has been this way for 60+ years...the current clay liner has been like this for 40+ years and surprisingly the clay liner is in great shape near the top. It’s those just above the thimble that are cracked. That sort of defies and is opposite of what most sweeps/salesman/installers tell me the liner should be. There are issues with my chimney, but not from the effects of creosote buildup and/or weather as many people seem to agree can play havoc on clay near the outlet.


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