Safe construction of wood framed chimney chase

[Eiffel93]

Just installed a new RSF Opel 3C fireplace in an addition to an old house. Chimney chase was constructed on outside wall of the addition, with 2x4 wood stud construction, Roxul mineral wool insulation in stud bays, Zip sheathing at the exterior, Thermoply at the interior for vapor barrier, and 5/8" fire-rated GWB over that. House is 2 storeys and at each floor line the carpenters extended the 3/4" Advantech floor sheathing across the chimney chase. (Draft block I assume?) There is a radiation shield at each floor line where the chimney pipe passes through the Advantech sheathing. The top of the chimney chase was capped with OSB, which is a concern to me. The dealer and installer did not comment about it when they were at the jobsite, but the installation instructions for the ICC chimney pipe states that the material must be non-combustible. They also did not include any radiation shield for this area and it seems like there should be something there, just like at the other two locations where the pipe passes through sheathing. The top of the chase will be covered with a fabricated SS chimney cover. Instructions for fabrication state that the opening should be 13" dia. or 2" on all sides of the pipe. This is for proper clearances and I assume this is also to vent the chimney chase, but what is to keep the cold air from dropping down into the chase and the rest of the house?

The manufacturers installation instructions are vague and do not provide adequate information for this type of chimney install. The dealer & installer didn't comment about the wood construction materials, but they also don't seem to know much about wood burning fireplaces. Gas is the big seller in our area,... for all of the lazy people who want a remote-controlled fire.

We had a wood burning stove in our first house and loved it, but it was free-standing in the middle of the room with no chase whatsoever. Now that we have this RSF fireplace installed, I'm concerned about the safety of how everything was constructed and installed. The interior walls are not closed in yet, and I want to make sure this is safe before doing so.

Any feedback or suggestions would be appreciated!

 

[rwhite]

Pics would be helpful.

 

[Eiffel93]

I have lots of pics! (First two are 1st floor, next two are from 2nd floor) In these photos you can see that currently the rad shields are installed incorrectly. The mfr instructions state that the shields should be installed from the underside of the framing, or in our case the sheathing. This will be corrected when they come back to install the cover at the top of the chase and also the last section of pipe, rain cap, etc. The installers are young and they are trying to get this right, but I just don't think they have any experience with this type of fireplace. And apparently they didn't bother to read the mfr instructions.

At the first floor, the out-to-out dimensions of the chase framing are 24" D x 64"W, which gave us room for the Heat Dump to the basement. At the second floor the chase narrows to 24"D x 42"W which leaves the interior dimensions at about 19" x 33-1/2". And at the attic it's slightly less at about 17-3/4" x 33-1/2". This seemed like plenty of clearance space until I came across a thread on this site that was discussing concerns with overheating in the chase. The entire chase is lined with 5/8" fire-rated GWB, but the Advantech sheathing at each floor/ceiling transition is exposed, as well as the OSB at the top of the chase. Should these be covered with a layer of GWB on the underside where the heat rising in the chase will most likely be the highest temp? And the OSB at the top....should a layer (or two) of non-combustible material be added to the underside to protect the OSB, or should it really be replaced? I hate to tear into this since it is already weather-sealed nicely and it's NOT easy to access.

I have lots of other questions, but safety is the first issue that I want to address. Our primary goal/purpose for this fireplace is for supplemental heat when the weather is cold, or when the power goes out, but we do not plan to heat the entire house or have a fire burning at all times. I could never leave my house with an active fire burning, nor could I get any sleep until I know the fire is dying down for the night. So there won't be any roaring fires built in this fireplace. Having experienced a chimney fire in the past, we tend to err on the safe side and start with a small fire that we can build on slowly. So with that said, overheating may or may not be an issue for us, but I would like get the opinion of others who have much more experience than we do. All of the pretty pictures in the manufacturers brochure show the fireplace and chimney enclosed, but their installation instructions show primarily an open and exposed chimney pipe. Completely contradictory.....

I don't want to flood this with irrelevant information, so if you need more, let me know.

 

[begreen]

That looks like a much better install than average so far.

 

[kennyp2339]

So there won't be any roaring fires built in this fireplace

Install looks great so far, very impressed, but I want to touch on the human behavior of this statement, please remember that a low fire in this type of unit with low stove temps and chimney temps can do more harm then good, run the stove between 450 and 600 deg f to achieve the cleanest of burns so your not creating a dirty chimney which can lead to a chimney fire.

 

[webby3650]

There’s no reason to change your draft stops or add anything to them. I can’t see overheating being an issue at all, the pipe barely even gets warm when in use. You’ve already exceeded clearances by a good ways it sounds like. There’s no reason to make sure the fire is out before you leave it unattended either. Many of us here have a fire 24/7 and its very safe to do so.
Did you install a stainless steel chase cover?

 

[Eiffel93]

Glad to hear that so far everything looks good. Our framing contractor had zero experience with wood burning fireplaces, so after a LOT of research, we finished the interior of the chase ourselves. I was amazed at how little information there is in the IBC/IRC regarding chase construction and fireplaces in general. The primary purpose of the code is for safety right?

We have met or exceeded all of the clearance requirements from the manufacturer, but after reading several threads on this site about chase temps getting too high, I'm now concerned about future problems in our chase. It is wide open now, but will be closed off once I feel confident that it is safe. I am planning to frame it in with steel studs, mineral wool insulation and drywall/plaster veneer on the finished side. I would like to get type X GWB on the interior side of that framing, but haven't had that discussion with the inspector yet. (Frame the wall slightly smaller than the opening, attach the GWB to the interior side, slide it into the opening, shim & fasten the wall in place and then fire-foam the perimeter?)

We plan to install a 22 ga. stainless steel chase cover that will be fabricated by a local metal shop. I've worked with them in the past so I know they will produce a quality product. My question still stands about the OSB that will be underneath this cover. We have 2 layers of 3/4" already in place, sealed with Carlyle HT ice & water roofing underlayment. It is ready for the cover, but does the OSB need something non-combustible applied to the underside? The ICC instructions say that the material under the cover flashing should be non-combustible. It would be difficult to remove & replace the OSB now, but we could add a non-combustible layer if needed. Or would a rad shield provide more protection? The dealer did not order one for this location, but isn't it similar to a roof penetration?

The dealer also did not order a vented flashing for the top, but I found a diagram in the ICC instructions that shows an enclosed chase with vented flashing at the cover. Seems like vented flashing would allow heat rising in the chase to escape and keep the temp from building up to high. But vented flashing would also allow cold air to drop down into the chase which could cause a whole different issue.
RSF recommends NOT putting insulation in the interior wall in front of the fireplace, but if cold air seeps down that chase and creates a cold wall in my otherwise well insulated & sealed home, I would not be too happy. After numerous rolls of zip tape, Roxul batt insulation in the stud bays and Roxul rigid CI at the exterior of the sheathing, I don't want to compromise the building envelop now by leaving the interior wall to the chase uninsulated. Am I correct that the pipe cannot be fire-sealed at the rad shields?... so any cold air that enters at the top will drop down into the chase whenever a fire is not burning? I guess that just boosts the case for keeping a fire going all the time, but until I retire and can be at home to keep an eye on it, that's just not going to happen.

Once we start using this fireplace and build some confidence and trust that everything is going to work safely, then yes we will use it more, and yes we will try to keep it operating at best efficiency, but at this point there are still so many unknowns.....

 

[webby3650]

I have never seen anyone overthink a chase build so throughly! Lol

 

[Eiffel93]

Well, my life could depend on it...literally. My bedroom will be on the 2nd floor right next to this chase. Fire is a living, breathing entity with a mind of its own, and if you don't respect that, you could very well regret it. I don't want any of those regrets.

I have spent the last 7 years of my life working on this house project. The last thing I want is to see it go up in flames. This is the family homestead that was built for my great-great grandfather. If I fail to do my homework now, and it gets destroyed later, I would be haunted by that for the rest of my life. Don't want to go down that road.....

Plus, my profession is drafting, so thinking through the details is what I do. I'm also a perfectionist so.....that can be good or it can be bad.... All depends on how you want to look at it.

 

[rwhite]

Well, my life could depend on it...literally. My bedroom will be on the 2nd floor right next to this chase. Fire is a living, breathing entity with a mind of its own, and if you don't respect that, you could very well regret it. I don't want any of those regrets.

I have spent the last 7 years of my life working on this house project. The last thing I want is to see it go up in flames. This is the family homestead that was built for my great-great grandfather. If I fail to do my homework now, and it gets destroyed later, I would be haunted by that for the rest of my life. Don't want to go down that road.....

Plus, my profession is drafting, so thinking through the details is what I do. I'm also a perfectionist so.....that can be good or it can be bad.... All depends on how you want to look at it.

I appreciate your attention to detail and overthinking. You would do well as a general contractor to make sure everyone does their job right!

 

[Eiffel93]

Funny you should say that.....as my boss handed me my bonus check at the company Christmas party, he thanked me for keeping an eye on them to make sure things were done right. The two of us have debated many issues other the years, but in the end, if I was right, he was appreciative.....even if begrudgingly so. I check and then re-check everything I do. Some would call me anal...Lol!....I call it being proactive rather than reactive. Take the time to think everything through. Measure twice, cut once!

 

[kennyp2339]

I thin your worrying much about nothing, the threads with "help my chimney is over heating" or "my wall /ceiling is to hot" are usually from those that have single wall pipe, your using class a insulated chimney pipe that's rated for chimney fires that have temps upwards of 2200 deg f, as long as you clean your systems once a year or twice if burning more then a few cords or sub par wood you'll be fine with preventing a chimney fire.
Stove top temps can reach and should reach an average of 650 deg f, by manufacture spec to achieve the clean advertised burn, as long as the chase is build to with the clearance specs in the manual you will not have any issues,
The roxal does not burn, and it provides a r rating, if anything the added insulation is only helping with keeping the flue warmer during the coldest of cold snaps so you have less of a chance of flue gases condensing (<250 deg f)

 

[begreen]

Well, my life could depend on it...literally. My bedroom will be on the 2nd floor right next to this chase. Fire is a living, breathing entity with a mind of its own, and if you don't respect that, you could very well regret it. I don't want any of those regrets.

I have spent the last 7 years of my life working on this house project. The last thing I want is to see it go up in flames. This is the family homestead that was built for my great-great grandfather. If I fail to do my homework now, and it gets destroyed later, I would be haunted by that for the rest of my life. Don't want to go down that road.....

Plus, my profession is drafting, so thinking through the details is what I do. I'm also a perfectionist so.....that can be good or it can be bad.... All depends on how you want to look at it.

If you have put this level of detail into the sealing, windows and insulation of this house it should be a pleasure for a long time and not need much energy to keep it warm.

 

[Eiffel93]

Yes, that is the ultimate goal. Would love to be net-zero, but don't think we could get there without some PV or wind power. Two possible options for the future.....

 

[mcdougy]

I understand your serious thoughts on protecting the wood from direct exposure....I say cover it as best you can because it's now or never basically. In my zc chase I also added a couple vents near the ceiling height. As I too debated how warm it would get in there.

 

[Eiffel93]

Venting the chase would definitely help with any heat build-up, but it is also a source for cold air infiltration, and that kind of defeats the purpose of all the insulation and sealing that we have done. I considered leaving the chase open to the attic space, but the attic is semi-conditioned because we have HVAC in there for the second floor. The attic is non-vented and the rafters are insulated, so while the extra heat from the chimney might be beneficial, any cold air coming in, would not be. Adding vents to the exterior side of the chase would have been an option a few months ago, but the exterior stone is now in place so I'm not messing with that.

Based on the clearances in the RSF installation instructions I never gave the chase temperature any thought. We were well over their minimum dimensions. And then I ran across a couple threads on this site about high temps in a chase that was similar to mine. Panic and paranoia took over at that point.... (And a lot of choice words were expressed as well.)

I am waiting for RSF technical support to respond to my inquiry about all this. I would really like to see their test reports and see how they compare to my particular situation. Tried to find them via Google, but didn't have any luck. I'm not concerned about the pipe or the fireplace, I'm concerned about the framing around it. I have seen multiple references to pyrolysis on this site.....don't wanna go there.

I tried to compare all this to a kitchen oven, which can also get upward of 500°, but the oven is always located in an open space and the heat can dissipate. The cabinets next to the oven never seem to be affected. But then think about how hot the kitchen always gets during the holidays with the oven running so much....now cram that into a small closet where the heat can't get away....

Will let you know what I hear back from RSF.

 

[begreen]

It is not an issue, but nothing will probably convince you of this. Vent it with a closeable vent. Put a remote probe in there for peace of mind.

 

[mcdougy]

Why do you call it a unconditioned space? Is the rear wall of the chase not insulated, like the rest of exterior walls? I guess I'm assuming your doing a 24/7 ish burn which would be the heat source. If not a vent that can close would work when the rsf is not in use.

 

[Eiffel93]

Vent it where? 1st floor, 2nd floor, attic? Guess I should talk to my HVAC guy. He does primarily commercial work, so he may have an idea for some type of motorized vent that would seal well.

The remote probe idea is also intriguing. If I could monitor the space, it would help alleviate some of the concern. And at least I would know whether or not there is even a problem. Can you provide more specifics, such as brand, model, etc. (Another question for the HVAC guy maybe?)

The chase is currently insulated on 3 sides, and once the interior partition wall goes in, we plan to insulate that as well. I would prefer to have a backup plan (vent?) in place prior to closing everything up.

I did get a reply from RSF, but their recommendations just complicate the situation more. They recommended that we use a vented flashing at the top of the chase, and make the cover collar 6" larger than the diameter of the pipe. They also recommended creating an air gap under the cover, by adding 1/2" shims underneath and on all sides so it would allow the heat to escape that way as well. These are both viable options in Canada, where there is probably less concern with insects and small flying creatures that like to nest in small spaces. However, in the Midwest these are potential problems that would require some serious screen or other material to keep them out. I'm sure the bees, wasps, hornets, bats, etc. would love to take up residence in my chase during the summer when the fireplace is not in use. And since it's about 30 feet off the ground, we can't readily keep an eye on it. I will have to ask RSF about screening the vented flashing. I don't think the air gap under the cover would provide much benefit.

 

[begreen]

Just put a closable register on the face feeding into the first floor. It certainly doesn't need to be motorized unless the budget is unlimited. This isn't necessary, but if it relieves personal anxiety, then go for it.

 

[dadama]

Chiming in late, you could have used fire-rated OSB or plywood. I am considering using these to reline a portion of my chimney chase.