Restricting a chimney outlet..chimney reducing caps

 
Hoytman
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Post by Hoytman » Fri. Dec. 06, 2019 11:01 am

Not sure if watching Gomer Pyle episodes qualify or not, Fred. I doubt it. Lol!

 
Hoytman
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Post by Hoytman » Mon. Dec. 09, 2019 1:11 pm

Alright, was reading another thread and found this...which has been my thoughts as well.
franco b wrote:
Wed. Feb. 20, 2019 1:09 pm
I just recalled that a member in the past solved his oversized chimney problem by simply placing a piece of sheet metal with a 6 inch hole in it, over the existing chimney outlet. It tended to hold heat in and maintain draft. Cheap to try and if it seems to work, more elaborate configurations could be tried as well like a section of smoke pipe extending down into the chimney through the sheet metal.


The above post was followed by this...
Pauliewog wrote:
Mon. Mar. 04, 2019 11:44 pm
This is exactly what I would do.

Running your stove in base burner mode is probably giving you an exit flue temperature under 120* F.

Since its dumping into a short uninsulated masonary chimney with all four sides exposed to the cold, with a large 8" x12" clay tile flue liner, I'm not at all suprised at the lack of a good draft.

I would cut a 6" hole in a 10" ×14" ×1/4" thick steel plate and weld a 4' section of 6" pipe leaving an inch or two sticking out the top, and stick the remaining 46" down the flue. Weld four pins, small blocks or angle iron pieces to the bottom of the plate to keep it centered in your flue.

I didn't notice if you had a mpd in your flue pipe ..... If not I suggest installing one.

As far as changing your 90* elbow to two 45*s or running it at an angle, in my opinion is not worth the effort.

This is a very inexpensive but highly effective an proven solution to slowing down the escaping flue gasses and retaining enough heat in your existing chimney enabling it to draft on its own.

Paulie

 
Hoytman
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Post by Hoytman » Mon. Dec. 09, 2019 1:39 pm

Of course, trying this for an anthracite stove should work. I would tend to think it may also work for wood, but have some reservations about creosote.

Then again, I may never burn wood again.

I may try and stick the pipe down the chimney as Pauliewog’s described, but I’m wondering if I could add a plate to the bottom of this apparatus as well...as a smoke block of sorts to prevent any smoke...thinking wood smoke...to prevent the smoke from getting between each plate. Of course, the entire thing could be lifted/removed from the chimney just by lifting it out.

Theoretically, this apparatus/baffle/reducer would allow the chimney to be warmer/hotter and hopefully increase draft.

I was thinking of this because my inside chimney is only 13’ high above the stove...10’ above the thimble. One person seemed to indicate my chimney may not be tall enough for a coal stove.

A manometer, a “Pauliewog-baffle”, and some testing and documentation might just prove to increase draft on my short chimney.

Hmmmnnnnn....

“Pauliewog-baffle”...

...remember you heard it here first. Lol!!!


 
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Post by Hoytman » Mon. Dec. 09, 2019 2:50 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.
Franco b,
My apology for quoting this a second time, but it does fall in line with the other two quotes I just posted.

I realize you said this earlier in the thread, and I did read it...several times in fact..., but for some reason when going back to re-read my own comments, the above words seemed to stand out even more after reading Pauliewog’s description in the other thread. Perhaps his comments in that thread are what prompted your comments in this one. Either way, it makes me much more sense now.

 
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Post by Pauliewog » Mon. Dec. 09, 2019 11:09 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.
+1

My dad repaired Radios and TV's back in the 60's and as a teenager I would often help him install rooftop TV antennas.

Some of the installatios were strapped to chimney's. On one occasion we installed the antenna on an exterior fireplace chimney with an 8"×12" clay flue.

The homeowner had a small oak style coal stove connected to the fireplace flue and mentioned to my dad that it worked fine when it burned wide open but gave him problems when it was idled down.

The following week he took piece of plate, cut a hole in the center and welded in a 6' long piece of 8" pipe that stuck out of the top of the plate about an inch or two.

We hoisted the pipe up, dropped it down the chimney mixed up a bucket of mortar and sealed it up.

A few years later I was with him on a TV service call and they were happy cozy campers. As far as I know that pipe is still in there. :D

Paulie

 
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Post by Berlin » Sun. Jan. 10, 2021 11:40 pm

There is *some* virtue to this. on oversized flues with low exhaust volume (air-tight stoves) I have observed on a 12/12 flue a convective loop of smoke exiting and outdoor air dropping down into the flue. This naturally creates turbulence and can reduce draft. I would suggest this is rare and there is a risk with substantially reducing the stack exit - you could pressurize the flue somewhat and make exhaust leakage through cracks in masonry etc. possible.

Chimney draft is a result of TWO things primarily: height and temperature differential. Height reqiuires you to consider the height of the stack above the highest indoor air space (bedroom or finished attic ceiling for example) not just the height of the stack above the stove or thimble. This is where people seem to have trouble; a building acts like a big chimney, negative pressure on the bottom and positive on top (due to ceiling /walls preventing air escape). There is a neural pressure plane in the house depending on how much and where air is entering and leaving; we don't really know where that is and it changes based on conditions. The height above this neutral plane is the effective height of your chimney. Because it changes based on the house and outdoor conditions, we should assume it's at the ceiling in the highest interior space for design purposes. The chimney and the house itself are competing with one another. On cold days or with high flue temps the chimney wins easily, but, to function well even in mild weather, the stack should be substantially taller than the highest indoor air space. A taller chimney will draft well on mild days and with a low temperature differential.

Chimney pots, appropriately sized reduce the flue slightly at the top and have a tapered top that improves draft in most conditions. I do not believe that chimney caps improve anything, usually they hinder it.

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