fireplace chimney qusetion

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Ronald

New Member
Nov 22, 2009
8
central nc
I live in an old mill house with chimney located in center of house. Originally there was a fireplace on each side of the chimney. At some point the fire places were closed in with concrete. There is a 6" flue thimble in the center where I have hooked my wood burning stove. Recently my wife noticed that the concrete behind the wood stove is still warm for hours after the stove has been shut down and stove is cool. There is a void in the chimney below the flue thimble with no clean out. If this is a safety issue, what can I do to correct this. The wood stove is our only heat source and we are both are out of work. Not a lot of money. Have priced flue liner at a cost of $1500. The stove is a VC Acclaim. Can anyone help? Hope this is enough info. Thanks
 
The masonry once well heated will retain heat much longer than the cast iron of the stove. It's a matter of mass.
It sounds like you're concerned that creosote is accumulating below the thimble and igniting.
It may be that the fireplaces were closed off with cement blocks rather than poured solid with concrete. If that's so then you can install a cleanout on the opposite side of the stove or just remove the stovepipe from the thimble when cleaning so you can snake a shopvac hose in to remove the debris that falls.

You might do well to inspect the chimney from the top and evaluate the condition of the clay liner (if there is one).
Then if you can't see where the thimble enters remove the stovepipe from the thimble and feel for the bottom of the chimney if you can reach, or use a bent wire.
 
If what your concerned about it the warmth of the chimney and etc., my chimney is also indoors til it hits the ceiling [fireplace]. If I have a pretty good fire in the thing in the evening, then the following morning I can go back behind the fireplace and the stone is still warm. Normal.
 
There can be a saftey concern if your chimney doesn't have any airspace between the outside of the brick and the nearest combustibles (likely doesn't given age of the house). Over time, lots of thermal transfer can occur and damage to timbers can occur. That is where insulated chimney liners come into play - they let you have a code compliant wood flue with zero clearance to combustibles.

In the old fireplaces, flue temps were much lower than wood stove flues because there was lots of air dilution cooling the gases. With a modern wood heater, flue gas temps can reach 1200*F, and the brick will hold that heat for a long time. When you start to burn 24/7, that brick will get to some really high temps.

What you want to watch for is feeling heat behind the walls where the heat isn't from the stove, but from the flue gases. My chimney last year get quite warm behind the walls without the insulated liner, so I installed an insulated liner this year. I sleep much better now.

The clean out question is also well worth investigating.
 
Thanks for the help and advice. Checked all areas around chimney and found no heat buildup. Had no idea the masonry would stay warm for so long. I will go to back side of chimney and try to get access thru thimble in other room. There is a 4" thimble in the other room. Would like to get access to clean out anything that falls to bottom.
can't see all the way down flue because of an offset in the flue. Chimney is quite large, about 4 ft. square in the basement. Also you are correct about air space. Floor joist hug the side of chimney. When finances get better will have liner installed to increase performance of stove. Thanks again to all. Common since rules.
 
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