Chimney feels warm through the wall

[heydan]

I have an internal brick chimney that goes from the fireplace in my basement to the fireplace (with insert) on the first floor then inside the walls through the second floor then exposed through the attic and finally out through the roof. It has three flues in it, each lined with ceramic tile (I think). The middle flue also has a full stainless steel liner going all the way from the first floor insert to the top. I was told this extra liner was not strictly necessary but would make cleaning easier and would provide some extra safety in the event of a chimney fire.

Today I noticed that when I'm burning my insert on the first floor I can feel some warm spots on a wall on the second floor where the chimney passes behind the wall. Going up into the attic, I was able to touch the chimney directly and I could feel a warm column all along its height corresponding to the middle flue, the one that serves the first floor insert. Interestingly, the outer edges of the chimney were cool -- the warm spot followed closely the path of the flue that was in use and the heat did not seem to dissipate throughout the rest of the chimney. Also, if I go down into the basement, the fireplace there feels cool. So the only noticeable warm spots follow the path of the smoke directly.

I like the fact that the chimney is conducting some heat from my first floor to my second floor. I'm not so happy about sending heat into the attic.

I was just wondering if this effect is normal and to be expected because I hadn't heard anyone mention it before.

I suppose it means my heating system has a big thermal mass that I wasn't expecting -- the first few hours of burning must work to warm the chimney until eventually the chimney starts radiating heat back into the house. This is good to know because I've only done intermittent burning so far and the rate of wood consumption seemed high to me. But if it's just because I'm repeatedly charging up that same mass with heat, I should get better results once we get to the point where I'm burning 24/7. Then I'll only charge it once and it should then keep radiating heat at about the same rate it's absorbing it. Right?

 

[Eric Johnson]

Yeah, basically.

A warm chimney is a good thing, since creosote tends to form when things cool off. Losing heat up the stack is a necessary (though at times depressing) fact of successful wood burning. The inside chimneys I've had in various houses over the years always felt warm-to-hot when I had the stove, furnace or boiler going. Theoretically, there should be enough insulation between the liner and the wooden frame of the house to keep the framing from overheating and catching fire, but even a well insulated chimney will radiate a fair amount of heat back into the living space, in my experience.

If the paint starts to blister up and/or peel off your walls, then I'd start to worry. But a wall that's warm to the touch won't hurt anything.

After burning steady for a week or two, you'll know what your wood consumption is. You can tweak things a little and maybe get it down some, but in my experience for any given set-up, it is what it is.

 

[Rhone]

It's inevitable, you have to figure you're probably sending 325F+ gases out the chimney for at least several hours, which will warm up the liner quickly and over time it will warm up the masonry.

The problem that I have is, that I don't have a sealed combustion boiler. When my insert warms up the chimney, my other flue the boiler is attached also warms and causes it to draft and pull in cold outside air into my basement. A "Vent Damper" would fix it, can't find any made for oil boilers, they're all gas.

 

[GVA]

It's inevitable, you have to figure you're probably sending 325F+ gases out the chimney for at least several hours, which will warm up the liner quickly and over time it will warm up the masonry.

The problem that I have is, that I don't have a sealed combustion boiler. When my insert warms up the chimney, my other flue the boiler is attached also warms and causes it to draft and pull in cold outside air into my basement. A "Vent Damper" would fix it, can't find any made for oil boilers, they're all gas.

Two places to check for that damper could be F.W. Webb or United Refrigeration WWW.uri.com they pretty much have everything or at least can send you in the right direction.

 

[hilly]

Heydan,
I've got a brick chimney that starts on the first floor, goes through the second and out. It is lined also and it feels warm to the touch as well. I got an infrared thermometer and even when it feels quite warm it is still only about 85 F. Right above the insert the brick is almost too hot to touch and it's only 140 F, so even that isn't hot enough to start wood burning.
As far as getting that thermal mass hot, once I burn all day the upstairs room the chimney goes through is in the 80's.

 

[elkimmeg]

You just described the advantages of an interior chimney it transmitts and radiates heat threw the second floor. Mine too is warm to touch 30 years later it still gets that way even tonight. If you can feel the warmth then the iner flue is doing it's thing and drafting correctly naturally the heat will continue into the attic space again that is an indication of a proper functioning draft