Heat shield, can it reflect heat.

[guest5234]

I am putting my Jotul f250 into an inglenook against an 12 inch brick platered wall, I am thinking of putting some sort of heat shield against the wall to stop the plaster cracking from the heat....would it be best to use unpainted metal "aliminium or steel" to reflect the heat back into the room or would it be OK to paint it the same colour of the walls..
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Thanks Folks.

 

[Ken45]

If I understand you right, you are going to be close to the wall? How close? You need good clearance (or an adequate heat shield) or the wall will transmit the heat to the studs behind it and could cause a house fire! Perhaps you already know this and are taking proper precautions.

Although bare metal will probably do a bit better, a painted surface will still work pretty good, IMO, especially if there is air space above it for the heat to escape.

Ken

 

[guest5234]

If I understand you right, you are going to be close to the wall? How close? You need good clearance (or an adequate heat shield) or the wall will transmit the heat to the studs behind it and could cause a house fire! Perhaps you already know this and are taking proper precautions.

Although bare metal will probably do a bit better, a painted surface will still work pretty good, IMO, especially if there is air space above it for the heat to escape.

Ken

Thanks for reply Ken, in the UK to a none combustable wall the clearance must be no less than 50 mm or 2 inches, the wall had a big hot Rayburn next to it for 50 odd years so I am not worried about the safety of it, I took off the old tiles that were behind the rayburn and will put up a metal heat sheid to protect the plaster which may crack with the heat.

 

[Apprentice_GM]

If you can, do 2 layers of shield. 2 x 3mm shields 25mm apart will shield better than a 6mm shield 50mm from fireplace. It create convection "chambers" which carry heat into room and cool the shields - end result, a warmer room and cooler wall.

 

[fossil]

I know of an Aussie who would probably install an array of about 7 heat shields, arranged so that he could adjust the spaces between and/or remove one or more shielding elements, all the while taking careful measurements of all the pertinent parameters and then he'd reduce his data down into a Rule of Thumb. :lol: Rick

 

[begreen]

We've seen a couple installations that used stainless steel for a rear heat shield. They looked quite nice. If you mount the shield on ceramic spacers or even metal washers to hold it off the wall, it will be more effective. Don't take it all the way to the floor, leave a gap at the bottom and leave it open at the top. That will promote airflow behind the shield and should keep the plaster behind it much cooler.

 

[guest5234]

We've seen a couple installations that used stainless steel for a rear heat shield. They looked quite nice. If you mount the shield on ceramic spacers or even metal washers to hold it off the wall, it will be more effective. Don't take it all the way to the floor, leave a gap at the bottom and leave it open at the top. That will promote airflow behind the shield and should keep the plaster behind it much cooler.

Brilliant. thanks guys I would never have thought to leave a gap between shield and wall to aid convection.

 

[fossil]

We've seen a couple installations that used stainless steel for a rear heat shield. They looked quite nice. If you mount the shield on ceramic spacers or even metal washers to hold it off the wall, it will be more effective. Don't take it all the way to the floor, leave a gap at the bottom and leave it open at the top. That will promote airflow behind the shield and should keep the plaster behind it much cooler.

Brilliant. thanks guys I would never have thought to leave a gap between shield and wall to aid convection.

That's really the way it does its job, Neil...convection. Reflection, not so much. BTW, I think an unpainted stainless steel heat shield mounted behind that Jotul might be stunning. Good luck with it, and be sure to show us the progress & finished product...especially with a nice burn going. Rick

 

[guest5234]

We've seen a couple installations that used stainless steel for a rear heat shield. They looked quite nice. If you mount the shield on ceramic spacers or even metal washers to hold it off the wall, it will be more effective. Don't take it all the way to the floor, leave a gap at the bottom and leave it open at the top. That will promote airflow behind the shield and should keep the plaster behind it much cooler.

Brilliant. thanks guys I would never have thought to leave a gap between shield and wall to aid convection.

That's really the way it does its job, Neil...convection. Reflection, not so much. BTW, I think an unpainted stainless steel heat shield mounted behind that Jotul might be stunning. Good luck with it, and be sure to show us the progress & finished product...especially with a nice burn going. Rick

Thanks Fossil, how big do i need the stainless steel shield ? big enough to fill the back wall except for gap top and bottom or just to overlap the width of the stove.

 

[fossil]

Sawdust has a good point...except changing your mind later and moving the stove to put up a shield would be something of a bother. Your installation may not even require a shield (sounds like it doesn't), but if you want one, then it becomes more of a design element than a required safety element, which gives you the freedom to size it pretty much any way it pleases you. Rick

 

[guest5234]

Sawdust has a good point...except changing your mind later and moving the stove to put up a shield would be something of a bother. Your installation may not even require a shield (sounds like it doesn't), but if you want one, then it becomes more of a design element than a required safety element, which gives you the freedom to size it pretty much any way it pleases you. Rick

Thanks Guys, will try it without when it arrives hopefully this week and post some photos.

 

[myzamboni]

We've seen a couple installations that used stainless steel for a rear heat shield. They looked quite nice. If you mount the shield on ceramic spacers or even metal washers to hold it off the wall, it will be more effective. Don't take it all the way to the floor, leave a gap at the bottom and leave it open at the top. That will promote airflow behind the shield and should keep the plaster behind it much cooler.

 

[fossil]

That's the one I remember having seen. Stunning. Rick

 

[begreen]

Yep, that's it! Thanks for the reposting. It really looks sharp.

 

[Apprentice_GM]

I know of an Aussie who would probably install an array of about 7 heat shields, arranged so that he could adjust the spaces between and/or remove one or more shielding elements, all the while taking careful measurements of all the pertinent parameters and then he'd reduce his data down into a Rule of Thumb. :lol: Rick

Yeah, yeah

Actually what I'd be more interested in from a performance viewpoint - of keeping the plaster wall cool - would be different materials and colours. Black absorbs more heat but radiates more too. From a performance viewpoint, black on the fireplace side of the wall sheet would probably be better, and white or silver on the back or wallside. But aesthetically I agree the pic above looks great.

From what I've read here it seems US building code for fireplaces / woodstoves is more relaxed that Down Under? We have to use 6" stainless steel flu with a 7" steel and an 8" steel 50% perforated / 50% solid (perforation to roomside, solid to wall) plus a 9" diamter half shield on the wall side. So 3 x shields between the active flu and the plaster wall. There are convection holes to allow airflow through the cavities. It means the flu is pleasantly warm to touch and there is only the slightest noticeable difference between the plaster wall behind the flu and elsewhere in the same room. Once we are in the roof cavity space we revert to a 6" active S/S and an 8" shielding pipe. Minimum 12" to cumbustibles.

Australian installations often have a heat shield hanging off the wall side of the woodstove, and then a wall mounted heat shield offset from the wall as described above to provide a cavity for convection. The heat shields are usually thin ~3mm black powder coated mild steel.

 

[fossil]

I think in the case of these heat shields we're talking about, convection trumps any other mechanism. The materials are so thin that the effects of absorption and re-radiation are likely negligible, and reflection's probably not an issue, unless there's some sort of coating that reflects IR...dunno. Rick

 

[KeithO]

fossil, you have the right answer for regular operation, but if there is a chimney fire and the temperature gets significantly over 600 C then radiation becomes the dominant heat transfer mode and it bridges those air gaps as if there were not even there. It is not uncommon in very temperate countries (like australia) that the market may not support an industry making Class A insulated pipe, and furthermore, fires are not burnt for extended periods because of the short winter. Thus it is common to see the air gap insulated chimney pipe, sometimes made of mild steel !

 

[Apprentice_GM]

<snip> the temperature gets significantly over 600 C then radiation becomes the dominant heat transfer mode and it bridges those air gaps as if there were not even there.

Fascinating! Does the transfer method incrementally switch from convection to radiation as temp increases?

 

[fossil]

I should have clarified the limitations of my comments. I wasn't talking about chimney pipe, I was talking about the heat shields associated with the stove installation at "ground level", so to speak...wall shields in particular. Rick

 

[KeithO]

Radiation heat transfer is a cube law effect (the old exponential curve), so at low temperature it has almost no effect until finally at the top end it completely dominates. The sun is a good example, heating all of us by radiation through the vacuum of space where obviously no convection is possible.

 

[fossil]

Well, not to quibble, but the amount of radiant energy to which an object is exposed is inversely proportional to the square of the distance of the object from the source of the radiation. There is no cubic factor in the phenomenon. Rick

 

[Apprentice_GM]

Radiation heat transfer is a cube law effect (the old exponential curve), so at low temperature it has almost no effect until finally at the top end it completely dominates.

Well, not to quibble, but the amount of radiant energy to which an object is exposed is inversely proportional to the square of the distance of the object from the source of the radiation. There is no cubic factor in the phenomenon. Rick

Both can be true - the first is an expression of heat transfer from source vs other heat loss eg convection from source, the second is an expression of the spread of that heat transfer eg through air and space from that heat source. I wonder how either was determined or proven without measurement though? :lol:

 

[KeithO]

Rick: 2 different topics. The inverse square law relates to the distance between source and target. The cube law rule relates to the radiant energy transfer as a function of the Temperature of the source (in my example, the inner wall of a double wall pipe).

Well, not to quibble, but the amount of radiant energy to which an object is exposed is inversely proportional to the square of the distance of the object from the source of the radiation. There is no cubic factor in the phenomenon. Rick

 

[fossil]

Rick: 2 different topics. The inverse square law relates to the distance between source and target. The cube law rule relates to the radiant energy transfer as a function of the Temperature of the source (in my example, the inner wall of a double wall pipe).

Well, not to quibble, but the amount of radiant energy to which an object is exposed is inversely proportional to the square of the distance of the object from the source of the radiation. There is no cubic factor in the phenomenon. Rick

I'm with ya, Keith, sorry for my misunderstanding...I was stuck down by the stove, and hadn't gotten my head up into the chimney. Rick

 

[RedRanger]

Apparently, so can a mantle sheild, if you don`t have the proper clearance.