Interior Fireplace insulation

[Tom Gallo]

I have an interior fireplace with three flues, fireplace and water heater in basement and fireplace on main level.
The flue is clay, 13" by 13" surrounded by 4" by 8" by 16" solid block. I assume there are two layers of block because the overall width is 31.5". Around this is red brick, 4" by 9". Between the brick and the block is a 1.5" gap on one side and a 3" gap on the other side. This brings the overall width to 44.5".
I would like to fill these gaps with sand or something to try and pull more heat out of the flue.
Comments, better ideas, always welcome. Thanks, Tom

 

[gzecc]

Not sure what you want to do. A nice insulating high temperature insulation is Roxul.

 

[Tom Gallo]

Thanks for the comment Gzecc,
I am trying to get better conductivity between the block and the brick facing to get more heat out of the system and increase the thermal mass.

 

[mellow]

Can you give us more info, is this for a open fireplace, insert, free standing stove? Got some pictures of it?

 

[Nick Mystic]

I don't think I would alter the way your chimney was built for a few reasons. Number one, there could well be a good reason for this air space. A recent post talked about a problem with condensation dripping down the inside of his flue. Perhaps this air space your system has help control the movement of such moisture. This is just speculation, but my point is there could be a good reason for why your chimney was built the way it was and changing it could cause an unforeseen problem down the road. Second, I would be surprised if you got a noticeable increase in the amount of heat your chimney radiates with the change you mention. Yes, you will add some thermal mass to store more heat, but it will come at the expense of more creosote build up inside your flue. As it now stands your exhaust gasses only need to heat up the clay tile flue, so the tiles heat up relatively quick. Once that sand is added you create a physical connection between the tile liner and all the remaining cinder blocks and bricks. Those components will suck up a lot of heat (your goal), but it will mean the clay liner stays relatively cool much longer, thus the added creosote build up.

 

[Tom Gallo]

Can you give us more info, is this for a open fireplace, insert, free standing stove? Got some pictures of it?

Sorry I do not have pictures of the "guts". This is a fireplace with an insert, Fisher papa bear. I filled in the remaining spaces between the insert the the fireplace with about 40 firebrick to hopefully increase the thermal mass. Would like to get more thermal conductivity between the block and the brick facing. The gaps are 1.5" and 3".
From the attic I can remove some insulation and see the gaps and could fill them. Between the outside of the flue and this gap I want to fill is about 9". This 9" is 4" of solid block and I guess the other 5" is a second row of solid block and a 1" gap. I would have to break into the chimney to know for sure.

 

[Tom Gallo]

I don't think I would alter the way your chimney was built for a few reasons. Number one, there could well be a good reason for this air space. A recent post talked about a problem with condensation dripping down the inside of his flue. Perhaps this air space your system has help control the movement of such moisture. This is just speculation, but my point is there could be a good reason for why your chimney was built the way it was and changing it could cause an unforeseen problem down the road. Second, I would be surprised if you got a noticeable increase in the amount of heat your chimney radiates with the change you mention. Yes, you will add some thermal mass to store more heat, but it will come at the expense of more creosote build up inside your flue. As it now stands your exhaust gasses only need to heat up the clay tile flue, so the tiles heat up relatively quick. Once that sand is added you create a physical connection between the tile liner and all the remaining cinder blocks and bricks. Those components will suck up a lot of heat (your goal), but it will mean the clay liner stays relatively cool much longer, thus the added creosote build up.

I have that fear too. A bit more data: Between the outside of the flue and this gap I want to fill is about 9". This 9" is 4" of solid block and I guess the other 5" is a second row of solid block and a 1" gap. I would have to break into the chimney to know for sure. So I think the construction is a 13" flue, 1" gap, 4" solid block(guess), 4" solid block( I can see), 1.5" to 3" gap, 4" brick facing that stops in the attic. Maybe I should try to get a temperature in the gap with a hot fire to see if any heat makes it that far.

 

[mellow]

I don't think you are going to see the gains you are expecting. You would be better off to install a newer insert with a stainless steel liner and blow that heat into the living space, not try to capture it from the chimney. You are robbing pete to pay paul and with lower chimney temps you are going to see more creosote buildup, especially with burning with an older insert.

 

[bholler]

Is there a stainless liner attached to the insert? Those gaps are in there to allow for the differing expansion rates and to stop heat transfer They are there for a reason i don't think you should mess with it and absolutely don't fill the gap around the flue tiles they need to expand freely