Woodstove chimney chase construction
I’ve been trying to find information on construction of a chimney chase running up the inside of a new house. The superinsulated house will have two living levels, with a small woodstove on the lower level. Combustion air will be ducted in from outside directly to the stove. The stovepipe will connect to a properly rated insulated chimney pipe running through a chase within the upper level to the attic and through the roof.
Most likely the woodstove will see only occasional use, even in the winter. The house, being superinsulated, won’t take much to heat, even during very cold weather. When not in use, the chimney will be full of cold air, so there is a need to avoid cold spots where it passes through the upper level.
My concern is both heat conservation and fire safety. The chase will be adjacent to one wall in the upper level. I envision having 2″ or more of foam insulation lining the inside of the chase, covered with sheetrock for ignition protection.
If the chase is closed at floor and ceiling level of the upper living level, and there is a high insulation level on the outside of the chase, then a period of use of the stove will result in rising temperature within the chase, despite the insulation within the chimney pipe.
Where can I find information on proper design of the chase for this situation?
One thought I had earlier was to have the insulated chimney pipe run up within a larger diameter outer pipe. Combustion air would flow from the cold vented attic space just inside this outer pipe. At the bottom of the chase, an outlet from the outer pipe would connect to a pipe running down through the floor and wall behind the stove on the lower level, then out and directly into the stove. Certainly this downflow of combustion air would keep the air within the chase from rising. However, I don’t know if this would prevent proper drafting or would be counter to whatever code applies.
Suggestions or comments?
Edited 9/7/2008 12:39 pm ET by DickRussell
Replies
I think you are largely overthinking this thing.
You main Q - where to find infoi on chase design - is from the manufacturer of the pipe you intend to use.
There are two basic kinds of metal flue pipe for wood stoves. Both are generically called 'zero-clearance' but both require 2" separation from flammables.
One is a mineral fibre ( used to be asbestos) packed tight between an inner wall of stainless steel, and an outer wall that is either stainless or galvanized.
The other is a triple wall pipe that has perforations at joints so that air can flow in the chambers between the three walls. As combustion exhaust heat rises in the flue proper, the inner wall chamber is heated and the air in it rises. This creates a suction draft at the base, and draws replacement air from the outer chamber. Then cold air spills in the outer chamber at top and rolls down to the base support of this kind of chimney system keeping the outside wall of it cool and safe.
If this is an interior chase, I see no reason to insulate the walls of the chase. Just use draft stop at each floor level
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I've looked some of the installation information, and there isn't a whole lot of info on what I need. Most just talk about connection through attic floor (stove in room below it) or through sidewall to an outside chase. Other sites speak of issues with having an outside chase.I'm reluctant to skip insulation of the chase walls, since the cold air normally in the chimney pipe would lead to a cold chase. This strikes me as contrary to the principles of a superinsulated house. I think I have two ideas pulling in different directions. Needing insulation to prevent heat loss most of the time and maybe needing to skip it to prevent heat buildup in the chase a small fraction of the time.I've looked at triple wall pipe section pics, and I couldn't see an awful lot that explains the operation. The cold air that spills into the outer chamber at the top - is that from the attic space, above roof level, or within the chase? It can't come from inside the chase, unless the chase either is open to the attic or else leaks badly.I would think that others who have done well insulated houses have solved this problem already.
The cold air enters a triple wall outer chamber at the very top of the pipe - above the chase at same level as where the smoke comes out.I see why i was not understanding you. In my thinking, any time you would be likely to have cold air in the chase space, you would be burning the stove and heating the pipe - normally.But you are going to such extremes to super-insulate that you do not plan to use the wood stove very often. With that in mind, no harm in insulating the chase as long as you shield it with shetrock over the foam.
The chase will not et too hot simply because the pipe is designed to handle the heat.As a matter of fact, normal operating temperatures will be around 185 to 400 degrees F, but the pipe is designed to handle temperatures up to about 2400°F for short periods - so that it will not fail in a chimney fire.Be sure to keep your chimney celan! using it only occasionally is one of those things that typically leads to greater creosote buildup and a higher likelyhood of chinmey fire.
And if you do have one, be sure to fully inspect the metal chimney. What commonly happens is that the high heat makes the inner SS liner soft and maleable, while the fast draft of the escaping gases creates suction towards the center, which collapses that inner lining so that it blocks the flue.Next fire you light cannot vent out.
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what is the name brand of the pipe you are using?
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You're free to insulate the chase walls. What I meant was that the air gaps relieve you of any obligation to dissipate heat yourself. The assembly is UL rated regardless of what's outside the 2" clearance. The duratech instructions (http://www.duravent.com/?page=1a.php) give an interior chase diagram on page 4.
Piffin, no particular chimney vendor has been selected as yet. I have looked at the Simpson site a fair amount.KHW, I clicked on that link you provided, looked through various instruction documents there, but couldn't find a page 4 showing chase construction. The instructions do say that when the chimney passes through living space, it must be enclosed, with a 1-hour fire rating for less than four stories.In any case, perhaps the key point of my concern is the phrase "designed for" in connection with that typical 2" clearance to combustibles requirement. I imagine the basis for that design is what might be a "typical" chase, a sheetrocked enclosure, but uninsulated.Heat will move outward from inside the chimney, through the insulation between inner and outer pipes, at some presumably small rate. That heat must be dissipated through the walls of the chase at the same rate for there to be a steady temperature profile. An uninsulated chase will do this easily. Add a lot of insulation to the walls of the chase, however, and the temperature of the inside surface of the insulated wall of the chase will scoot up dramatically. The thicker the chase insulation, the closer its inside surface temperature will approach that inside the chimney.That's why I wondered about another means of removing the heat lost through the chimney wall.Perhaps I'll send a note to the Simpson support line, noted on their site.Edit: one source I just looked up gives the maximum service temperature of XPS foam as 165 F. Unless I take some steps to remove heat lost through the chimney wall to the chase, I'll have to figure on some other type of chase insulation, I would think.
Edited 9/8/2008 9:15 am ET by DickRussell
Yes, it's probably best to contact them directly. I think the maximum temp the NFPA allows on adjacent materials is 195F, so the foam could be damaged under extreme conditions. If you want, you may be able to calculate the heat transfer directly. The insulation on the pipe is an inch of packed rock wool. The pipe is supposed to handle a continuous flue temp of 1000F, so if you can figure out the r-value, etc., you might get an idea.The manual I mentioned can be found by clicking "Catalogs and Instructions" and then selecting 'Duratech 5-8"' under the "Installation Instructions (pdf)". Sorry I couldn't link it directly. It doesn't give many details, but shows the top and bottom fittings and the fire rating you mention.
" I imagine the basis for that design is what might be a "typical" chase, a sheetrocked enclosure, but uninsulated."I believe you are doing too much assuming here. It matters not whether the chase is insualted or not. Many times they are. The proper pipe will control how much heat it radiates into the chase. The design requirements are to remove flamable materials from contact with any part of the chimeny flue pipe that might possiblly reach a temperature ( in the range of 425°F or so), where spontaneous combustion can happen. The 2" takes care of it
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Your right as usual.
Following the required clearences is really all one needs to focus on.
Short of that a call to the manufacturer as a paranoya measure is all that's necassary.
I did exchange a couple of emails with Simpson on the subject, but the reply wasn't much more than a statement saying I just had to maintain 2" clearance, with no addressing the particular situation.Going back to post #9 in this thread, suppose the gas inside the chimney pipe is 400 F, and the room outside the chase is at 70 F. The total temperature difference is 330 degrees. Further, suppose the 1" of insulation between the pipe walls gives R4.If the walls of the chase are not insulated (typical, I would think), call it R1 worth of insulating value. At steady state, the drop over the pipe insulation will be 4/5 of 330, or 264 degrees, and the drop over the XPS will be the remaining 1/5 of 330 or 66 degrees. The surface of the XPS inside the chase would get to 400-264 or 136 F, which is no problem at all.Now add just 2" of XPS foam and sheetrock to the chase walls, call it R10 total. At steady state, the drop over the pipe insulation will be 4/14 of 330, or 94 degrees, and the drop over the XPS will be the remaining 10/14 of 330 or 236 degrees. The surface of the XPS inside the chase would then get to 400-94 or 306 F.If I added even more insulation to the chase, say to R15, then the chase inside temperature calculates out to 330 degrees F.Clearly, I wouldn't want to use a plastic foam insulation under such circumstances. Arguably a different insulation, such as Roxul, would work, but I still wouldn't feel very secure about the air inside the chase getting to 300+ degrees.
That's good to know. One other point would be how many btu/hr. would transfer if the chase interior is, say, 70F. If you can figure out how much convection (in terms of temperature rise times volume of air per hour) you need to dissipate that, then you can see if the gap at the top is sufficient to allow it.
I think I said a while back that I believe you are overthinking this whole thing.Simpson told you the same.and you are still thinking and thinking, but basing it all on your assumptrions.The Simpson product is a class A triple wall pipe that is cool enough to lay you hand on when the flue is running at over 400°F.
The only reason for the 2" space cushion is in case you have a chimney fire.
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The transitions through floors and ceilings are usually specified carefully in the installation manual. I haven't done an actual chase installation, but looking at things like the Simpson firestop radiation shield gives me the impression that a fully sealed chase would be impossible due to an air gap in the part. The roof flashing is similar. My guess is that the the system is arranged to keep a 1" air gap through all transitions, except at the bottom where the ceiling support box is.
For Simpson, the chase itself is supposed to be 1-hour fire rated. The installation manual is on their website if needed -- unfortunately it's not clear on questions of ventilation or insulation.
As far as running combustion air down the chase, I can see that it might have some advantages (for instance when wind creates differential pressures vs. a ground-level air intake), but make sure you don't have any buoyancy issues when the stove door is opened, or when starting a fire. If the air in the chase is warmer than the chimney (eg. during startup) the draft could be reversed.
What Piffin said.
Beer was created so carpenters wouldn't rule the world.