Heat loss from a masonry chimney

[Nicholas]

Masonry chimney (2' x 4') vs 3 class A's going through the roof with about 8' height on the exterior.
This chimney "complex" would be in the center of the great room and service a masonry heater and 2 flues for the basement.

Now if I go with 3 class A's (kinda like this but 3 flues - http://www.turtlerockheat.com/tinmouth.html )
I will have to build my own ceiling trim/box, then build a Faux chimney box on the roof (10/12 pitch).

If I run the chimney all the way up, I will have a 8' heat sink sticking out of my roof.

Cost is not a factor, as I am the builder and this is a labor of love.

let me know your thoughts on what you would have if cost was not an object.

Thanks,
Nick

 

[LLigetfa]

Is the masonry chimney existing or new construction? Using insulated liners would negate the heat loss.

 

[Fsappo]

Masonry chimney (2' x 4') vs 3 class A's going through the roof with about 8' height on the exterior.
This chimney "complex" would be in the center of the great room and service a masonry heater and 2 flues for the basement.

Now if I go with 3 class A's (kinda like this but 3 flues - http://www.turtlerockheat.com/tinmouth.html )
I will have to build my own ceiling trim/box, then build a Faux chimney box on the roof (10/12 pitch).

If I run the chimney all the way up, I will have a 8' heat sink sticking out of my roof.

Cost is not a factor, as I am the builder and this is a labor of love.

let me know your thoughts on what you would have if cost was not an object.

Thanks,
Nick

I'm assuming this is new construction. Tell us more about the type of masonry heater you are planning on installing. Also what type of applinaces are going into the basement that will be attached to the dual 8" flues?

 

[Nicholas]

Is the masonry chimney existing or new construction? Using insulated liners would negate the heat loss.

This is new construction, in an existing house.
Not sure I was clear, I am not worried about heat loss of the flue gasses, as it will be either insulated pumice liners or class A.

I am worried about a 2' x 4' masonry structure sticking out the roof, conducting the cold back into the house.

3 flues - 6" for small wood stove set on the Masonry heater hearth, 8" for Masonry Heater, 8" for future basement.

Nick

 

[Battenkiller]

I am worried about a 2' x 4' masonry structure sticking out the roof, conducting the cold back into the house.

Technically, cold doesn't get conducted anywhere. Heat is conducted from a warmer mass to a cooler mass. Net effect is the same, as your chimney inside will lose heat to the colder outside portion. How much heat? Depends on the temperature differential and the density of the brick. In general, brick has low thermal conductivity compared to many materials.

http://en.wikipedia.org/wiki/List_of_thermal_conductivities

http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

 

[Martin Strand III]

It is not necessary to control the temperature of what ever type chimney penetrates and is outside your roof. You have planned two important factors which make this unnecessary: (1) your chimney is centrally located in the house and (2) a large heat bank within the house (masonry heater).

The portion of the chimney inside the building envelope will remain warm enough in between heating cycles with home heat plus heat bank. This is what is important, not what temperature the chimney is outside. For draft considerations, a large inside/outside chimney temperature differential is actually beneficial.

Therefore, I do not feel the material for chimney construction outside makes any difference.

I happen to have a faux chimney box outside on a 12/12 roof containing both Class A metal 6" and 8" metal pipe for a basement wood/coal stove and main floor masonry heater. No complaints. No problems. Both chimneys are insulated only where they penetrate the main floor and roof, otherwise single wall for maximum heat release.

Enjoy your setup.

Aye,
Marty

 

[Martin Strand III]

"[/quote]the exposed block chimney outside the heating envelope will absorb heat from the exhaust & diminish the draft[/quote]"

Would this be a theoretical or practical diminution of draft? And, if the latter, how much?

Aye,
Marty

 

[Nicholas]

Marty thanks for the reply.

The chimney "complex" comes up from the basement through the great room, but not in the center, It is 1/2 between the eave wall and the ridge.
Total chimney height for the great room heater is ~ 16' inside and ~8' outside. The basement heater add another 10' of interior.

I "plan" to have the first 8' as a masonry chimney (in the great rm) capped in a granite slab, then Class A from there up.
The portion above the roof will be in a chimney box.
I also plan to "sleeve" the class A in 12" black thin wall pipe, from the top of the masonry chimney into the roof chase.
The sleeve will be decrotive, and act as an insulation baffle.
BTW, the masonry chimney will be Chim-Tek insulated masonry liner.
Do you forsee any problems with ~8' of chimney on the exterior?

Thanks,
Nick

It is not necessary to control the temperature of what ever type chimney penetrates and is outside your roof. You have planned two important factors which make this unnecessary: (1) your chimney is centrally located in the house and (2) a large heat bank within the house (masonry heater).

The portion of the chimney inside the building envelope will remain warm enough in between heating cycles with home heat plus heat bank. This is what is important, not what temperature the chimney is outside. For draft considerations, a large inside/outside chimney temperature differential is actually beneficial.

Therefore, I do not feel the material for chimney construction outside makes any difference.

I happen to have a faux chimney box outside on a 12/12 roof containing both Class A metal 6" and 8" metal pipe for a basement wood/coal stove and main floor masonry heater. No complaints. No problems. Both chimneys are insulated only where they penetrate the main floor and roof, otherwise single wall for maximum heat release.

Enjoy your setup.

Aye,
Marty

 

[Battenkiller]

For draft considerations, a large inside/outside chimney temperature differential is actually beneficial.

The draft doesn't really know or care how cold the outer chimney itself is, it only cares about the temp of the outside air surrounding it. Any cooling of the flue gases anywhere along the way to the flue exit will impede, not aid, the draft. That's why masonry chimneys pull better after they have been warmed up from running for several hours.

 

[Martin Strand III]

OK. I retract my draft statement.

Aye,
Marty

 

[begreen]

He's referring to the differential between the interior house temp and the exterior ambient temperature. The greater the difference, the greater the draft. When these temps are close, say it's 60 outside and 70 inside, it takes a pretty good flue set up to draft well at the start.

Nick, here's some good reading about chimneys:
http://www.woodheat.org/chimneys/trichim.htm
http://www.gulland.ca/florida_bungalow_syndrome.htm
http://www.woodheat.org/chimneys/evilchim.htm

 

[LLigetfa]

Not sure I was clear, I am not worried about heat loss of the flue gasses, as it will be either insulated pumice liners or class A.

I am worried about a 2' x 4' masonry structure sticking out the roof, conducting the cold back into the house.

The amount of heat wicked out from the home would be reduced by the amount of heat pulled from the flue gasses. As mentioned already, masonry is an insulator albeit not as good as some. Three class A flues would also create a penetration through the envelope that cannot be be super insulated because of clearance requirements so I doubt there would be much difference either way.