prepping an alcove with convection wall in earthquake country

[robman]

Hi
I am building an alcove where we removed a 1910 fireplace (new foundation and excavation) for a seismic upgrade. We are less than a mile from the Hayward Fault. The alcove is 57" wide by 31" deep and 9 feet to the ceiling. I have a Lopi Patriot but may upgrade to a larger and better stove at some point in the future and thus want to have convection walls on the side. I need one in the rear regardless, I believe. We will use double wall pipe (selkirk metalbestos) and I will reframe the front wall in steel and add a large grate at the top for air to escape. I want to line the alcove with ceramic tile which will also require backer board (durock). I plan on going up 7 or 8 feet with the tiled convection wall.

I have three sets of questions. First, I plan on attaching the outer layer of durock to steel framing and am wondering what I should use to close the seams to keep heated air from moving behind the outer wall and to wood framing (behind the steel framing)? I am thinking something like thin set but do not know if it is fireproof enough.

Second, I need to attach the 1" standoffs to the outer layer of durock. I am thinning of using 1" aluminum or 1 5/8 steel framing. I see problems with both. If I need to leave a 1" gap at the bottom (in other words, if the aluminum tubing cannot rest on the floor with the duck and tile starting an inch up), there will be a lot of weight pulling down and/or in on the tubing. Can the tubing rest on the floor or at least can I remove an inch from the outer face and have it rest on the floor? If one of these is possible, aside from attaching it with countersunk screws, can or should I use a mastic to improve its adhesion to the durock? The other option I see, and I believe it would do a better job of carrying the load, is to use 1 5/8" structural steel framing for the wall. I could have the framing run from the durock on the hearth to the ceiling so there would be plenty of gap above the tiles. The problem is the track (about an inch and a quarter high.) Can I attach a strip of durock and tile to the track and then have an inch or inch and a half air gap right above the track?

Third, I would like to have a wooden mantel at about 6' height. I am thinking that when I rebuild the front wall in steel, I could have a piece of 1.5 or 1.375" thick by 6" wide C channel with flanges on the ends that could be screwed to the wood framing (maybe with a piece of durock as a buffer) and some threaded posts above and below for attaching the tracks. I would then want stronger threaded posts coming out the front to attach the mantel piece. I am worried about heat transfer here though. Any recommendations? I will durock but not tile the interior of the front wall and on the exterior I plan on using plaster above (over eurock) and 6x6 ceramic tile up to the mantel.

Thanks

Rob

 

[begreen]

There are some misconceptions here. The alcove may or may not need a ventilated NFPA wall shield. That is entirely dependent on the clearance requirements for the present or future stove. Some new stoves have quite close clearances and don't require shielding if the alcove is wide and deep enough. Some of the structures described are overkill.

The cement backer board seams gets taped with a fiberglass tape, usually made by the board mfg. Yes the tubing can rest on the floor. Another simpler way of creating the spacers is to make 3" wide board length strips of durock, double them up to make 1" thick furring strips. Don't obsess over the weight. The shear strength of a screw is impressive and you will have a screw every 8".

The wood mantel could pose clearance issues. Verify that the clearances are proper from the stove pipe and stove top.

 

[robman]

thanks Begreen
That is great news that the tubes can rest on the floor (I've already bought the aluminum). Does the fiberglass tape get covered with some type of cement? (Like mud in sheet rocking?) Also, if the mantel is outside of the firebox and has two or three feet of wall below (and is 4' above the top of the stove) plus a steel wall (which could also have a convection wall on it) would that not be OK? The big question here, I believe, is how to keep any heat from transferring to the mantel from the steel header. Or how to attach it to the wall that would not have too much heat transfer to it. I could probably make some type of a noncombustible box that would be attached to the wall and then have a 3/4" box of wood around it. Also any special type of thin set to deal with high heat?

Thanks

Rob

 

[begreen]

Cover seams with fiberglass seam tape. Fill in and smooth with thinset mortar. Let dry.

Also, if the mantel is outside of the firebox and has two or three feet of wall below (and is 4' above the top of the stove) plus a steel wall (which could also have a convection wall on it) would that not be OK?

Explain that. Better yet, can you include a sketch of the proposed design?

 

[robman]

Hi Begreen
I have drawn up some sketches. There are two things I am worried about. First is if I can have a wood mantel or wood clad mantel, how to attach it to the wall. What I am thinking is to have a piece of 55" steel run horizontally at the height of the mantel. It will be attached to the wood framing on the outside of the firebox (I'll weld some tabs running vertically and use simpson structural screws to attach it to the wood. I am thinking I may need a layer of durock as a buffer for heat transfer between the steel and the wood). The steel will probably be 5 or 6" tall by an inch and a half deep. It will have holes on top and bottom to attach rails for steel framing (15/8", heavy gauge), which will be the front wall. The inside will either have one layer of durock or one layer and then a convection wall if that will help with the mantel. The outside will also have a layer of durock, which below the mantel will have a tile face and above will be plastered to match the rest of the room. The structural steel will need to also take the load of the mantel which will project. The easiest way to do this is with a cleat which the mantel will fit onto and then be screwed to, but I don't know if this will work with the possible heat transfer. I could have an air gap behind the cleat and have strategic holes in the mantel to allow air to circulate behind it. I am attaching three drawings. Thanks again

Rob

 

[begreen]

Great. Thanks for posting the pictures. The first one shows serious overkill on the protection for the walls in a standard alcove install. There is little to be gained past a certain point, but I can see you may be going by the Lopi manual guidance which has an additional layer of protection as compared to most stove alcove requirements.

The big issue is with the fireplace like enclosure that will trap a lot of heat, even with a top vent. This is not a standard alcove enclosure and not what the manufacturer tests for. The Patriot alcove design is for an 84" min. open front alcove. For certain the face wall would need to be 100% non-combustible construction, ceiling too probably.

 

[robman]

Thanks for responding so quickly. So any ideas on what would pass inspection and be kosher with Lopi with less overkill? I am going to reframe the front wall in steel. Do I need the convection wall on the inside of that too? And what do I need to do to the ceiling? Will a layer of steel framing under the wood framing and then a layer of durock work there or must I remove all the wood and replace with metal? And any thoughts on the wood mantel?

Thanks again

Rob

 

[begreen]

If the face wall is totally non-combustible I don't think it needs a convection wall. As noted, this is not in the manual so you will need to get the design approved first. Before starting work, pass drawings by the inspector. If the stove is entirely recessed, then the mantel may be ok, but that is just my opinion and not necessarily that of the inspector who has the final say. I'm pretty sure the stove will need a blower. Even with the vent grille this design will be a heat trap. Maybe consider a full width grille placed at the top to help better vent the heat.

 

[robman]

If the face wall is totally non-combustible I don't think it needs a convection wall. As noted, this is not in the manual so you will need to get the design approved first. Before starting work, pass drawings by the inspector. If the stove is entirely recessed, then the mantel may be ok, but that is just my opinion and not necessarily that of the inspector who has the final say. I'm pretty sure the stove will need a blower. Even with the vent grille this design will be a heat trap. Maybe consider a full width grille placed at the top to help better vent the heat.

Thanks again Begreen
We do not have a blower for the stove and I don't know if they make a specific one that I can retrofit. Is there any aftermarket blower that will work? I do have one of those convection fans that goes on top of the stove but it never seemed to do too much. Any further ideas on the ceiling of the alcove? It is 9 feet above the floor. The stove will be entirely recessed.

Thanks

Rob

 

[robman]

Another question: I think I can simplify the left wall by having a layer of durock against the original wood framing and then have a steel framed wall act as the convection wall and holding the tile. That wall would have normal steel tracks and studs and the tracks are about an inch tall. Can I have the 1" gap start right above the track?

Thanks

Rob

 

[robman]

Good morning
I have been pondering on the heat trap question. Our house is not very big and the wood stove is in the middle. We also don't leave fires when we leave the house. We are going to reframe the ceiling of the alcove in steel so I think that should help too. That said, I am thinking that if the heat trap issue is a worry, I can put some type of thermostat alarm towards the top if such a thing exists. Does someone make a settable or preset thermometer with an alarm that we would be able to install towards the top of the alcove? We would be able to hear an alarm throughout the house and cool down or put put the fire.

Thanks again for all

Rob

 

[robman]

Hi again
I have one more idea that may be crazy but who knows. If there is some type of thermometer that would let me know if the alcove is getting too warm, I could build a damper into the ceiling of the alcove and have the heated air go to the attic. The ceiling of the attic is another 6 feet up and the attic is huge. This would do away with the heat trap problem. Is this feasible?

Thanks again

Rob

 

[begreen]

That would violate the fire stop at the ceiling. Try not to overthink the problem. I'd just have a full-width grille on the face at ceiling height. Before going too far down this route visit the inspector and see if the basic plan is ok with them.

 

[robman]

Thanks Begreen
I have spoken with the inspectors a few times and they, unfortunately, are not too knowledgeable in this and will go with what Lopi says. Does it make sense to ask Lopi and will they be helpful? I think the Lopi manual just says nothing combustible up to 84" so in many ways I am going way above the manual specs. So I will talk with the building department next week but in presenting them with plans, is it OK to have the gap in a convection wall starting at 1.25" above the floor as long as it is a 1" air gap? (and actually I have clearance on this stove to only need the convection wall on the back, I believe.) This will allow me to use metal framing for the convection wall to the left of the stove. Secondly does some type of thermometer/alarm exist? (tl let me know if this is acting as too much of a heat trap)

Thanks again for all of your insight and help

Rob

 

[begreen]

I think putting a layer of cement board on the studs, then a convection wall for the sides and top should be sufficient. Starting the wall at 1.5" is fine. Yes there are all sorts of temp alarms, but if the design is proper then this should not be necessary.

 

[robman]

Thanks Begreen
I am hoping to get my buddy the welder over here this weekend to make up the piece that will hold the mantel. I can send photos when it is in progress if you'd like.

Thanks

Rob

 

[robman]

Hi again
I now have all the framing materials. I'll be framing the ceiling of the alcove with 14 gauge 2x6 track and 16 gauge studs. An engineer said that would be fine for anybody walking on the subfloor above. The track is attached to 4x6s (screwed to walls below) with simpson strong screws with a layer of durock between them. My fustian deals with how to run the double wall metalbestos through the ceiling. The framing is not combustible but the attic will have plywood as a floor. I just need to know side of opening of whatever the piece will be to frame the 2x6s.

Thanks

Rob