ceramic spacers for wood stove heat shield

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dmt50000

New Member
Jan 5, 2021
5
Amherst, MA
I am interested in using Perma Base cement board to construct a heat shield with 1" air space in back and stone veneer facing on the front attached to the wall (drywall, 2x6 wood studs) behind my wood stove. Is there any information available on the issue of heat transfer through the heat shield fasteners (screws) into the wood wall studs? Most applications I have seen use ceramic spacers to create the air space, but you still have the screws connecting through. In my situation, I would expect the stone veneer (1.5"+- thick) would provide additional protection from the stove heat. Thanks for any info you can provide.

Dave Thompson
Amherst, MA
 

begreen

Mooderator
Staff member
Nov 18, 2005
89,580
South Puget Sound, WA
Yes, your assumption is correct. The stone would spread the heat over a much larger area than the screwhead. So would tile.

What is the stove being installed and why the wall shield? Is it necessary? If it is, there may be a stronger way to support the wall with the heavy stone than on ceramic spacers.
 

dmt50000

New Member
Jan 5, 2021
5
Amherst, MA
Yes, your assumption is correct. The stone would spread the heat over a much larger area than the screwhead. So would tile.

What is the stove being installed and why the wall shield? Is it necessary? If it is, there may be a stronger way to support the wall with the heavy stone than on ceramic spacers.
Thanks for your speedy reply. It is a Vermont Castings NC 1450, installed about 13 years ago, using the 6" chimney of a Majestic manufactured fireplace, which we lined. The stove is currently about 3' out into the room, and we would like to move the stove back closer to the wall. This year we removed the Majestic fireplace and chimney and installed a new 8" Selkirk chimney, which improved the operation of the stove quite a bit.

I would be interested to hear other ways that you think the wall might be constructed.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
25,492
central pa
Thanks for your speedy reply. It is a Vermont Castings NC 1450, installed about 13 years ago, using the 6" chimney of a Majestic manufactured fireplace, which we lined. The stove is currently about 3' out into the room, and we would like to move the stove back closer to the wall. This year we removed the Majestic fireplace and chimney and installed a new 8" Selkirk chimney, which improved the operation of the stove quite a bit.

I would be interested to hear other ways that you think the wall might be constructed.
What does vermont castings require for sheilding?
 

dmt50000

New Member
Jan 5, 2021
5
Amherst, MA
What does vermont castings require for sheilding?
The stove came equipped with rear and bottom steel heat shields. For the stove setbacks, the manual references a vertical stove collar heat shield, which I don't have but am looking into getting. With all the various heat shields installed, the manual indicates a 17" setback from unprotected surfaces, but with regard to protected surfaces the owners manual says: "Clearances with double-wall connectors and protected surfaces have not been tested for the Encore NC." This stove installation is about 13 years old, so I am going to have to find out if the table has been updated.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
25,492
central pa
The stove came equipped with rear and bottom steel heat shields. For the stove setbacks, the manual references a vertical stove collar heat shield, which I don't have but am looking into getting. With all the various heat shields installed, the manual indicates a 17" setback from unprotected surfaces, but with regard to protected surfaces the owners manual says: "Clearances with double-wall connectors and protected surfaces have not been tested for the Encore NC." This stove installation is about 13 years old, so I am going to have to find out if the table has been updated.
The table cannot be updated the clearances are as tested. Do they describe what a protected surface is in the manual?
 

dmt50000

New Member
Jan 5, 2021
5
Amherst, MA
They give a description and diagram of a wall shield on spacers providing a 1" airspace between the shield and the combustible wall. They also reference NFPA 211 Standard for Chimneys, Fireplaces, Vents and Solid Fuel Burning Appliances and Equipment, which includes tables of clearances from protected and unprotected walls, and different methods of wall protection. I did just find a statement in the stove manual that says "Top exit installations that do not include wall protection must include a vertical flue collar heat shield", which implies that if wall protection is provided, that this shield is not necessary.
 

dmt50000

New Member
Jan 5, 2021
5
Amherst, MA
A continuation of the above discussion: All of the ceramic spacer kits I have scene employ a long screw passing through the cement board, through the ceramic spacer, through the existing drywall and into the existing wall studs, which allows for some possibility of heat transfer through the screws and into the stud wall. Some of the how-to's on constructing this type of wall specify not having fasteners directly behind the stove, e. g., the heat shield instructions by Permabase state "Do not install any nails or screws into the wall area directly behind the proposed location of the appliance". Has anyone ever seen an insulator where the insulator body is cast onto the head of a screw which could be driven into the existing wall, and the opposite end of the insulator is threaded to receive a machine screw to attach the cement board. This would provide a gap in the conductive metal portion on the fastening. Just a thought. Maybe I am overthinking it.
 

MongoMongoson

Member
Feb 6, 2021
162
Wisconsin
I had some of the same concerns when I installed a heat shield for my old stove, and it sounds like we both suffer from overthinking things.
NFPA 211 states the same thing as the Permabase instructions. You are not supposed to have any fasteners behind the stove. In my case, I was doing a corner install so I took that to mean not behind or to either side of the stove.

There is not enough heat absorbed by the screw to transmit it into the combustibles and cause an issue. The spacers don't need to be ceramic. They are supposed to be "non-combustible" spacers, which means you could use copper if you want. The air gap behind the shield acts like a chimney. If your shield gets warm, you'll get a draw of air up behind the shield which ventilates everything.

My heat shield is zinc plated steel, and I've got zinc plated fasteners with aluminum spacers. Nothing ever got warm, at all. Not even when the stove was cranking away with the sides, back and top at over 600 degrees. I could touch the heat shield and it felt room temperature. With your masonry shield that might not be true. It won't reflect as much as galvanized steel, but the air gap behind it will keep the spacers/fasteners cooled.

I had considered using 80/20 extrusions, screwing those to the wall, then attaching the heat shield to the 80/20 extrusion with a screw and T-nut in the slot on the 80/20. I figured that way, there would be no direct heat transfer through the screw into the wood of the wall (overthinking things...). In the end i used 80/20 spacers because that material is readily available to me as scrap from where I work, but I just ran the screws through the shield, spacer, and into the wood of the wall.

You also need to have a gap between the bottom of your heat shield and the floor, or at least both sides of the heat shield and the top open. See attached images.
 

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