Chimney - quick heat up, or retained heat.

[Nicholas]

An insulated Class A gives you a quick heat up, but does not retain the heat, is cool after the fire is out, risking a "Tail out"
A solid masonry takes some time to get up to temp, but will keep the flue ward and a draft going, even as the fire dies out.

Which way would you go ?

 

[EatenByLimestone]

You didn't mention if the chimney would be outside our inside.

On the old house I built an external masonry chimney. In the new house there was an existing (external) masonry chimney that I dropped an insulated liner down. Personally, I'd go with the Class A. I like the insulated liner much better. It's easier to start and reloading isn't a problem. Thermal mass is nice, but if the mass is outside it doesn't do anything for you. I assume that there will be less creosote also, but I haven't cleaned it yet so I can't confirm anything.

Matt

 

[Backwoods Savage]

It all depends upon your situation. We put up a new SS chimney on the outside wall a couple years ago and have no problem with it drawing. You don't have anything posted to show folks where you are from but we are in Michigan.

 

[Nicholas]

Sorry guys,
Maryland
This will be an interior chimney, with about 8' above the roof line.

Nick

 

[Adios Pantalones]

So- what will the pipe height be- total? Any horizontal runs?

Insulated would be my preference if I had any concern. When the fire gets really low- it takes less heat from the fire to keep the insulated chimney drawing.

 

[firefighterjake]

I'm with BS . . . exterior SS insulated Class A Chimney . . . and no issues with draft.

My own take . . . if I had an existing masonry chimney I would use it . . . with an insulated liner. If I was building a new home I might put in a masonry chimney with an insulated liner purely for the look . . . but if I was retrofitting an existing home I would go the easier route and use Class A.

 

[Slow1]

So to jump in here with related question - if you put an exterior SS Class A on, is there any real practical benefit to 'boxing it in' other than the way it will look? Clearly it will add some insulating value and keep it from cooling as fast, but being outside the building envelope it won't be quite as warm as inside. Since exterior SS seems to work quite well without being boxed in it clearly is not necessary, but could it be helpful in any way?

 

[firefighterjake]

So to jump in here with related question - if you put an exterior SS Class A on, is there any real practical benefit to 'boxing it in' other than the way it will look? Clearly it will add some insulating value and keep it from cooling as fast, but being outside the building envelope it won't be quite as warm as inside. Since exterior SS seems to work quite well without being boxed in it clearly is not necessary, but could it be helpful in any way?

I was going to box mine in for the aesthetics . . . and figured it might help some with the draft . . . but after Season 1 I realized a) no one can really see the chimney from the road except for my next door neighbor and b) I'm getting a good draft anyways . . . and so I'm pretty content to just leave it as is.

 

[Highbeam]

There is very little redeeming value to the thermal mass of a masonry chimney. There many negative aspects of the masonry chimney such as cost, square shape harder to clean, cracks, earthquake stability, flashing is more difficult, maintenance on the mortar, etc. The only benefit is the aesthetics, they look cool.

I ripped one down to put in a steel chimney and have never been happier. If you want to avoid "tailing" or whatever that is then buy a stove that has thermal mass. A big heavy stove ideally made of stone. Or, you could just insulate your house very well so that the cooling stove doesn't cause a cold house right away.

 

[Nicholas]

HB,
Is that a Kioti ? I have a new CK20S, never happier
"Tailing out" is when the fire is just about out and your exterior chimney gets to cool, and the draft stops, then backwards draft.

I am putting in 2 masonry heaters and a wood stove, cost is not part of the equation , I just want the best product I can build.

Nick

 

[begreen]

I second that. There are no more masonry flues in this house. The last earthquake set the crown of the original chimney at 45° to where it started from. The only thing that stopped it from toppling was the rigid stainless liner in it. I wasn't going to risk burning in it again. Now we have a stainless, straight up, interior installation that I expect to outlast me. I like it and so does the stove. It's super easy to clean and tight, bottom to top. If the goal is thermal mass, I would put it into the stove and not the flue.

 

[Slow1]

"Tailing out" is when the fire is just about out and your exterior chimney gets to cool, and the draft stops, then backwards draft.

I would say that if that happens then your problem is not with the chimney, rather it is with the structure it is installed in. More specifically a serious case of "stack effect" or other problems kicking in. With even the slightest bit of warmth in that chimney you should be able to maintain a draft - certainly not have a backdraft. If you get a backdraft with the fire still burning then you have negative pressure in the stove room that is sucking air down the flue. Unless you have exhaust fans running (or other flues pulling air out) then you probably have air leaking out the attic or some such.

I am putting in 2 masonry heaters and a wood stove, cost is not part of the equation , I just want the best product I can build.

Now that must be nice - I'd love to have a masonry heater (or two). I hope to build my dream home around one some day - complete with an oven on the back side (where I plan to put the kitchen) for making some pizza and breads... Ahh, dreams of retirement. Back to work now.

 

[Highbeam]

HB,
Is that a Kioti ? I have a new CK20S, never happier
"Tailing out" is when the fire is just about out and your exterior chimney gets to cool, and the draft stops, then backwards draft.

I am putting in 2 masonry heaters and a wood stove, cost is not part of the equation , I just want the best product I can build.

Nick

Yes a Kioti, a CK30 that I've put 857 hours on. Great machine.

The masonry chimney is not the best product. Even if the cost factor is thrown out there are so many bad things about a masonry chimney that I would rather have a steel pipe. Even if the steel pipe cost more!

You won't ever have a properly installed steel chimney flow backwards unless you do it on purpose or don't provide combustion air. You would have to really try to make it happen. Further, you would not be any more or less likely to create a backdraft on a masonry system vs. a steel system.

 

[Battenkiller]

Insulated would be my preference if I had any concern. When the fire gets really low- it takes less heat from the fire to keep the insulated chimney drawing.

I have a masonry chimney. It holds a lot of heat, but as I understand things, an insulated chimney would draw better when the fire gets low. The walls of a masonry chimney never get hot enough to be close to the average flue gas temps needed for a superior draw, so it is always cooling the gases down to some degree. This gets to be a problem once the fire gets down to a low coal bed at the tail of an overnight burn. Once flue gas temps drop below 200ºF, the fire slows down and the draft gets worse and then the fire slows down more, etc. If I overload the stove, I wake up to a cold flue and a warm stove with too many coals for a good startup. This, to me, is a sure sign that my draft gave out sometime during the night.

The big plus with an internal masonry chimney is its ability to recover heat from the exiting flue gases. A conventional tile-lined chimney can boost efficiency by 5-15% if the surfaces of the chimney are in direct contact with the living space. With my basement installation, my whole house is kept at a boringly constant 70-72º because of this, regardless of the rather dramatic changes in stove room temps. I don't think this works as well with a non-cat EPA stove, and works even less with a cat stove due to their lower flue gas temps.


Hey... did I accidentally agree with Adios?

 

[Highbeam]

A conventional tile-lined chimney can boost efficiency by 5-15% if the surfaces of the chimney are in direct contact with the living space. With my basement installation, my whole house is kept at a boringly constant 70-72º because of this, regardless of the rather dramatic changes in stove room temps.

5-15% efficiency gain eh? I'm going to have to call BS on that. You know that the heat comes from somewhere right? Is the heat released from your masonry somehow different than the heat released from the equivalent single wall flue pipe? Thermal mass has an ability to absorb energy and release it slowly. It does not create energy or add to efficiency, rather, it stole that heat from the system when the fire was hot. I propose that there is no "efficiency" gained by a masonry chimney but what you are experiencing is the slow heat release benefits of thermal mass. This is why soapstone is such a nice stove material.

 

[Battenkiller]

I never said the chimney made the stove burn more efficiently. It makes the system more efficient, in that heat that would ordinarily go out the chimney gets absorbed by the mass of the masonry and gets radiated out into the living space. That is the whole idea behind masonry fireplaces. And I can burn hotter because of this, and that gives greater efficiency from the fuel.

The figure I used is just a number from the established research literature from way back before the days of EPA stoves. I have no idea exactly how much heat is reclaimed. That would depend on measurements taken in an individual installation. All I know is that I can run higher flue temps (getting cleaner burns and less creosote) than someone with an insulated metal chimney, and get at least some of the heat back rather than giving it all up to the great outdoors.

I heat my two-story house to a very even temperature using this principle. Air temp in my house was 72ºF when I went to bed last night. Upstairs from the stove in the basement, the wall in my kitchen that is adjacent to my masonry chimney was 104ºF on my IR thermometer when I woke up at 6 AM. Air temp was 70ºF. Didn't fill the stove until 9:30, and the air temp was still 70º.

 

[begreen]

Agreed that the chimney can be designed to give off some retained heat to the house. A class A flue passing through a second floor has some of the same potential in a ventilated chase. But it will depend on the location of the chimney. If it is exterior, then the brick (or metal) is heating mother nature. If it's interior, with a 1-2" space around it, chased within the house walls, but open to the ventilated attic space, the heat just goes outdoors. This is not uncommon, especially in older homes. It's the way our chimney was made. I was glad to pull it.

 

[Battenkiller]

BG, I'd be only too glad to drop a 6" liner down my chimney and attach a nice, new EPA stove to the business end if I could justify the expense. Maybe I could get by with only 3-4 cord of wood that I wouldn't even mind storing for three years instead of using the system I do. I'm pushing 60 now, and I don't know how many more years I'll feel like burning wood. I'd go to a pellet stove, but it just won't work with a basement installation. I really count on getting all that masonry as hot as I can to heat the whole house. A pellet stove would make the basement nice and cozy, but then I'd need another one upstairs. Ain't happening. I've spent 20 years tweaking what I've got, and it works real well for me... fall-bought wood and all.


If I was designing from scratch like the OP, I think I'd go with a metal chimney and a stove that ran cool flue temps.... probably a cat stove. Ultimately, I think that's the answer for true efficiency.

 

[Battenkiller]

Here's what my central masonry chimney does for my house:


Upper left is the thermostat between the kitchen and living room. A shade over 72ºF.

Upper right is my bedroom. Just under 72º.

Lower left is my office off to the side. 70º.

Lower right is my second floor spare bedroom. Door is kept closed all the time and heat off. The chimney runs right through it. 66º, heated almost entirely by conduction through the first floor ceiling and that masonry chimney (which feels warm to the touch through the wall, a good 15' up from the stove).


These temps are almost guaranteed to be exactly what I'll find at 9 AM tomorrow morning. A soapstone stove won't do that. In fact, the only room in the whole damn house that's not comfortable is the room in the basement that the stove is in.... 86.4ºF taken with the digital probe of my IR thermometer.

Outside temp right now is 32ºF (0ºC).

 

[begreen]

BG, I'd be only too glad to drop a 6" liner down my chimney and attach a nice, new EPA stove to the business end if I could justify the expense. Maybe I could get by with only 3-4 cord of wood that I wouldn't even mind storing for three years instead of using the system I do. I'm pushing 60 now, and I don't know how many more years I'll feel like burning wood. I'd go to a pellet stove, but it just won't work with a basement installation. I really count on getting all that masonry as hot as I can to heat the whole house. A pellet stove would make the basement nice and cozy, but then I'd need another one upstairs. Ain't happening. I've spent 20 years tweaking what I've got, and it works real well for me... fall-bought wood and all.


If I was designing from scratch like the OP, I think I'd go with a metal chimney and a stove that ran cool flue temps.... probably a cat stove. Ultimately, I think that's the answer for true efficiency.

Why fix what isn't broken. Your setup is working well. My reference is to the many homes that don't see as much benefit due to differences in masonry flue installations. Our chimneys vented the house from basement to the attic, with no firestop around them. The little heat radiated by the flue went straight up to the attic and out the vents. FWIW, our house is now similarly even tempered with a metal flue. I attribute that to it now being a much tighter house with good convective airflow.