Making a sheetmetal mantel shield

[Planeweird]

So after my install in front of my existing fireplace, I'll only have 25.5" between the stove top and the underside of the wood mantel. I believe Jotul calls for 32" for the F3CB.

Soooo.......I'm going to fabricate a nice sheet metal shield to hang underneath the mantel during burning season. My question is, do you think 1" of dead air space between the wood and the metal will be sufficient or should I make it 2" or more. I plan on having it angle in a couple of places to move the heat out and in front of the mantle, but wonder about the heat transfer. That's obviously gonna be one of the hottest areas.

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___________I This is a rough side view. Mantle with sheet metal "floating" underneath
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Thoughts?

 

[MountainStoveGuy]

you should get a 2/3 reduction in clearance with a 1" airspace. Any more then that is just overkill. make sure the shield has room in the back so that air can circulate between the shield and the mantel.

 

[Planeweird]

MSG,

Thanks for that. The only part of the shield that will be in contact with anything will be the back of it which I will fit up against the stone on the fireplace face. I'm doing that so the least amount of hot air can creep up behind it. The front and sides will all be open.

 

[Dune]

MSG,

Thanks for that. The only part of the shield that will be in contact with anything will be the back of it which I will fit up against the stone on the fireplace face. I'm doing that so the least amount of hot air can creep up behind it. The front and sides will all be open.

I could be wrong (often am), but it seems as though you misunderstood. In order for the one inch airspace to be effective, air must be able to freely flow behind the sheild.

 

[Hogwildz]

Mantle heat shields from manufacturers I have seen mount against the mantle or insert surround etc., to keep the hot/warm air from rising directly to the wood mantle.
The 1" air space I believe is needed for a wall shield, not a mantle shield. Seems to me it would be pointless to let hot air rise behind it and have a direct path to the mantle.

 

[MountainStoveGuy]

Correct dune, Plane, go back to my post, it talks about leaving a airspace between the shield and the wall to allow air to flow freely.

 

[Planeweird]

MSG,

Thanks for that. The only part of the shield that will be in contact with anything will be the back of it which I will fit up against the stone on the fireplace face. I'm doing that so the least amount of hot air can creep up behind it. The front and sides will all be open.

I could be wrong (often am), but it seems as though you misunderstood. In order for the one inch airspace to be effective, air must be able to freely flow behind the sheild.

I realize that air needs to circulate, but I was thinking of allowing that flow from side to side passage and not from the back out the front. This way, the air circulation would be from cooler ambient air and not the super heated air rising off the stove.

I too, think it makes little snese to have hot air flow up from the back even for circulation. That seems to basically nullify the point of the shield. Am I missing something?

 

[Dune]

Yeah. Your missing that the purpose of the sheild is to deflect radiant heat, not hot air.

 

[afblue]

For what is worth, on page 9 figure 9.1 shows what Quadrafires 5100i deflector looks like. The purpose of the mantel shield is to reflect the radiant heat away from the fireplace at its hottest point, Directly above the surface at a 45 degree angle.

(broken link removed to http://www.quadrafire.com/downloads/installManuals/man_5100iact.pdf)

I am only going to call this on my basic thermodynamic knowledge that 1"or 2" airflow behind or not is not going to make a difference, because they are both wrong. The shield needs to be as close to the radiant source as possible to reflect the radiant waves heat out when they are strongest. Think of it more like a heat mirror, and not a heat barrier.

 

[branchburner]

Maybe something like Micore “floating” underneath mantle instead of sheet metal. Don't know about code, though...

 

[MountainStoveGuy]

I would treat it like a wall shield to be on the save side. For you people that like a real bore, here is a cut and paste below.

Table 12.6.2.1 Reduction of Appliance Clearance with Specified Forms of Protection
Figure 12.6.2.1(a) Clearance reduction system — fastener location. [Exist- ing Figure 9.6.2.1(a)]
Figure 12.6.2.1(b) Distance to combustible wall/floor. [Existing Figure 9.6.2.1(b)]
Figure 12.6.2.1(c) Masonry clearance reduction system. [Existing Figure 9.6.2.1(c]
Figure 12.6.2.1(d) Fastener detail. [Existing Figure 9.6.2.1(d)]
Figure 12.6.2.1(e) Wall protection using materials in Table 12.6.2.1. [Exist- ing Figure 9.6.2.1(e)]
Figure 12.6.2.1(f) Ceiling protection using materials in Table 12.6.2.1. [Ex- isting Figure 9.6.2.1(f)]
12.6.2.2 Clearances from solid fuel-burning appliances to combustible mate- rial shall be permitted to be reduced, provided the combustible material is protected by an engineered protection system acceptable to the authority having jurisdiction.
12.6.2.2.1 Engineered systems installed for the protection of combustible material shall reduce the temperature of such materials to 90oF (50oC) rise above ambient.
12.6.2.2.2System design shall be based on applicable heat transfer principles, taking into account
(1) the geometry of the system, (2)the heat loss characteristics of the structure behind the combustible material, and (3)the possible abnormal operating conditions of the heat-producing
sources.
12.6.2.3 Clearances from solid fuel-burning appliances to combustible mate- rial shall be permitted to be reduced by the use of materials or products listed for protection purposes.
12.6.2.3.1 Materials and products listed for the purpose of reducing clear- ance to combustibles shall be installed in accordance with the conditions of the listing and the manufacturer’s instructions.
12.6.2.4 For clearance reduction systems using an air space between the combustible wall and the wall protector, adequate air circulation shall be pro- vided by one of the following methods as shown in Figure 12.6.2.4.
Figure 12.6.2.4 Air circulation methods.
12.6.2.4.1 Adequate air circulation shall be permitted to be provided by leav- ing all edges of the wall protector open with at least a 1 in. (25.4 mm) air gap.
12.6.2.4.2 If the wall protector is mounted on a single flat wall away from corners, adequate air circulation shall be permitted to be provided by leaving only the bottom and top edges or only the side and top edges open with at least a 1 in. (25.4 mm) air gap.
12.6.2.4.3 Wall protectors that cover two walls in a corner shall be open at the bottom and top edges with at least a 1 in. (25.4 mm) air gap.
12.6.2.5 All clearances shall be measured from the outer surface of the com- bustible material to the nearest point on the surface of the solid fuel-burning appliance, disregarding any intervening protection applied to the combustible material.
12.6.2.6 All clearances provided between solid fuel-burning appliances and combustible materials shall be large enough to maintain sufficient clearances between chimney connectors and combustible material as required in Section 69.5.
12.7 Accessories. Factory-built accessories for solid fuel-burning appliances such as heat exchangers, stove mats, floor pads, and protection shields shall be listed and shall be installed in accordance with the terms of their listing. Exception:
12.7.1 Unlisted accessories that are acceptable to the authority having juris- diction shall be permitted to be installed in accordance with the approval of the authority having jurisdiction and the appliance and accessory manufac- turers’ installation instructions.

 

[Hogwildz]

Per the manual for your stove. The minimum clearance to mantel from top of stove is 34".
They have a short leg kit that lowers the stove 2-1/4".
Unless I am misunderstanding, the fireplace/wall facing itself is not combustible, but the actual mantle is, and you want to install a heat shield.
Not sure for stoves, but for inserts, most insert manuals I have read, the mantle heat shield mounts directly & tightly to the insert or surround and has no gap, usually in place of where the top . a gap will merely let radiant heat pass through it.
As said by the other gentleman, your basically making a radiant heat deflector. Some photos would be helpful.
It depends on many factors as to what you need. Like how far out of the fireplace your stove sits. How far out the mantle protrudes, etc.

Some photos would be helpful.

 

[branchburner]

Stoves won't take the same shield as inserts, it needs to hanging up below the mantle or extending from the masonry right above the fireplace opening. But like H says, a gap in back will let radiant heat straight up to the mantle. What Planeweird has in mind looks exactly like HomeSaver Mantel Shield, which appears to have 1" ceramic spacers down from mantle. Nothing about rear airspace. Directions in fact say make sure shield extends back so no mantle is exposed (which sounds like no rear airspace).

 

[MountainStoveGuy]

Yep, the instructions completely contradict what i had stated earlier. Here are the instructions.
(broken link removed to http://www.northlineexpress.com/Images/Pdf/StovePipeShieldInstallInstruct.pdf#page=2)

 

[Planeweird]

Branchburner and MSG,

That's it. That's exactly what I have in mind Thanks for posting that link. I haven't seen one before, and am glad to know that I was on the right track.

I love it when a plan comes together.

 

[Dune]

Looks like Dune is wrong again, for those keeping track.

 

[webbie]

If a mantel shield is not addressed directly in the specs for the stove, I usually use a 50% reduction as opposed to the usual NFPA ones (66%).......that has worked well for me over the years.

And, yes, I usually have put the rear edge in a mortar joint or against the masonry - no space.

An airspace is needed under the mantel, but my "rules of thumb" for that are not fixed at a certain measurement. Just keeping it down a bit from the mantel is the idea. I don't think it would make it dangerous to be 3/4" in one area, etc......probably more important is the exact stove, radiant surface on the top of it, the slope of the shield (a slope makes it more self-cooling), etc.

Speaking of rules of thumb, the big idea is to monitor the mantle when the stove is at a very hot temp - for a long burn. Being able to easily touch the wood behind the shied is a good sign.

You have to watch for some mantels which are buried back into the masonry fireplace face. These are likely to heat up even with a shield, as heat passes right through the masonry above the stove. Again, this depends on the particular model. Some stoves, examples being the Resolute Acclaim and N/C Defiant, put tremendous radiant energy out the top rear of the stove...in those cases, follow ONLY the manufacturers instructions as they have tested them that way.

 

[Wood Heat Stoves]

you should get a 2/3 reduction in clearance with a 1" airspace. Any more then that is just overkill. make sure the shield has room in the back so that air can circulate between the shield and the mantel.

depending on the inspector,

2/3 reduction for non listed appliances depending on how you interp nfpa211. cannot reduce stove clearances beyond that for listed appliance w/o makers aok and specific listing!, also inspectors interp of the situation might not match yours.

 

[Wood Heat Stoves]

MSG,

Thanks for that. The only part of the shield that will be in contact with anything will be the back of it which I will fit up against the stone on the fireplace face. I'm doing that so the least amount of hot air can creep up behind it. The front and sides will all be open.

I could be wrong (often am), but it seems as though you misunderstood. In order for the one inch airspace to be effective, air must be able to freely flow behind the sheild.

quite true! airflow and specifics are very much spelled out in nfpa211

 

[Dune]

MSG,

Thanks for that. The only part of the shield that will be in contact with anything will be the back of it which I will fit up against the stone on the fireplace face. I'm doing that so the least amount of hot air can creep up behind it. The front and sides will all be open.

I could be wrong (often am), but it seems as though you misunderstood. In order for the one inch airspace to be effective, air must be able to freely flow behind the sheild.

quite true! airflow and specifics are very much spelled out in nfpa211

Wait!, it's rolling back toward the green! Maybe Dune is right after all.

 

[Cluttermagnet]

...I'm going to fabricate a nice sheet metal shield to hang underneath the mantel during burning season. My question is, do you think 1" of dead air space between the wood and the metal will be sufficient or should I make it 2" or more. I plan on having it angle in a couple of places to move the heat out and in front of the mantle, but wonder about the heat transfer. That's obviously gonna be one of the hottest areas.

___________
___________I This is a rough side view. Mantle with sheet metal "floating" underneath
____________/
/

Thoughts?

I made and installed a mantle heat shield like you have in mind:
https://www.hearth.com/talk/threads/29864/

I have less clearance than you do- to 6in single wall pipe. The sheet metal is touching the bricks in back, under the mantle. Any gap at that point would not be a good idea. 1in clearance between the shield and the mantle at all points, however. It works great and produces a very noticeable temperature decrease. I don't think 2in clearance would make it any better than 1in. But it probably wouldn't hurt, either.

I don't know if the angle below would help further. It might, by decreasing air turbulence. Seems intuitively that the angle makes the underside more resemble an airfoil (airplane wing).

I do believe that bringing the front of the shield up to the top of the mantle does help some. Otherwise you have bare wood with very hot air passing right near it. Any turbulence at that point and you get more heat transfer. But yeah, it's uglier that way. If you must make it shorter, at least bring it out a good bit further. Personally, I'll gladly take the ugliness hit and live with it. ;-)

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BTW I believe that a heat shield at this location is probably decreasing both radiant and convection heating of the mantle. And I suspect it is more effective than a stove pipe heat shield, as well- by dealing with that convection heating. But I'll leave that to the real experts on the Forum. Just a hunch.

Back off man- I'm a scientist!

(Bill Murray- "Ghost Busters")

She blinded me with science!

(David Bowie?)

 

[johnnywarm]

do you atach the shield to the mantel or the mortor???

 

[SHH_150]

I made my heat shields out of brass plate. for the horizontal shield I tapped into the metal plate that supports the bricks above the stove.

 

[Cluttermagnet]

do you atach the shield to the mantel or the mortor???

Mine goes across a course of brick. I have it touching the brick, as much as possible. There are some tiny gaps, 1/16in or less, and a few vertical mortar lines. I doubt there is much heat getting by, because of turbulence effects. Most of the convection draft probably rolls under there and then spills up past the front face. I suppose I could have used hi temp gasket cement, but I didn't bother. The mantle is barely warm to the touch with a good fire going.

Hmmm- rereading your question, I think that what you are asking about is the attachment method? Mine is bolted into the wood on the underside of the mantle using 1-5/8in drywall screws and flat washers. There are 1in copper spacers over each screw that make the sheet metal hang 1in below the wood.

 

[johnnywarm]

do you atach the shield to the mantel or the mortor???

Mine goes across a course of brick. I have it touching the brick, as much as possible. There are some tiny gaps, 1/16in or less, and a few vertical mortar lines. I doubt there is much heat getting by, because of turbulence effects. Most of the convection draft probably rolls under there and then spills up past the front face. I suppose I could have used hi temp gasket cement, but I didn't bother. The mantle is barely warm to the touch with a good fire going.

Hmmm- rereading your question, I think that what you are asking about is the attachment method? Mine is bolted into the wood on the underside of the mantle using 1-5/8in drywall screws and flat washers. There are 1in copper spacers over each screw that make the sheet metal hang 1in below the wood.

tHANK YOU! cAN YOU POST A PICTURE OF YOUR SHIELD??