wood stove heat shields

[Charles2]

I'm interested in knowing more about ways to reduce clearances as discussed on this page: https://www.hearth.com/econtent/index.php/articles/stove_wall_clear

Specifically, materials, methods and possible designs for such shields.

I'm wondering whether a double shield would be possible. I mean, if a 1" air gap is good, why not have 2, 1" gaps?

And finally, is it allowed to fill the 1" gap with noncombustible insulation? If not, why not?

 

[Heatsource]

a double shield wont give you any reduced clearance, but sure its possible if overkill
You can not fill the gaps with any material, the air is the insulator and the air must be able to flow thru the 1" gap at the top and bottom of the wall shield for it to have the desired effect.

 

[begreen]

If you search this forum, titles only, for "wall shield" you will get dozens of installation threads showing all sorts of ideas. As long as the wall shield conforms to NFPA 211 guidelines it is ok to use most any non-combustible material within practical limits. But it must be ventilated meaning open at least on the top and bottom per regs. I had a cement board wall shield for our 602 at one point and measured the temp. 175 F on the surface and 82F on the wall behind the shield.

What stove is this for and what is the clearance distance you hope to achieve?

 

[Charles2]

a double shield wont give you any reduced clearance, but sure its possible if overkill
You can not fill the gaps with any material, the air is the insulator and the air must be able to flow thru the 1" gap at the top and bottom of the wall shield for it to have the desired effect.

I don't understand why a double shield wouldn't allow a reduced clearance. Clearly the more air spaces you have, the better the insulating value. Moreover, insulation has to be a better insulator than air - if it were not, no one would buy insulation for any purpose.

If you search this forum, titles only, for "wall shield" you will get dozens of installation threads showing all sorts of ideas. As long as the wall shield conforms to NFPA 211 guidelines it is ok to use most any non-combustible material within practical limits. But it must be ventilated meaning open at least on the top and bottom per regs. I had a cement board wall shield for our 602 at one point and measured the temp. 175 F on the surface and 82F on the wall behind the shield.

What stove is this for and what is the clearance distance you hope to achieve?

Where can I see NFPA 211? I have not decided on a stove, but am trying to achieve the closest possible distance so as not to sacrifice floor space in a narrow room. I wish there were stoves that could be hung on a wall.

A related question: Do there exist any stoves with soapstone on the back that allows the same small clearance as a masonry heater?

 

[Heatsource]

I don't understand why a double shield wouldn't allow a reduced clearance. Clearly the more air spaces you have, the better the insulating value. Moreover, insulation has to be a better insulator than air - if it were not, no one would buy insulation for any purpose.

Where can I see NFPA 211?

questions for the guys who write the code, i'm just letting you know a double shield wont get you a reduced clearance anyone will sign off on.
why an air space vs insulation? also a question for my betters, just telling you the facts of current code

here is an old copy i uploaded years ago, im not sure of any online versions that might be newer
(broken link removed to https://woodheatstoves.com/free/NFPA211_old.pdf)

you could make the wall out of steel studs, and masonry and have a "zero clearance" wood stove

 

[bholler]

I don't understand why a double shield wouldn't allow a reduced clearance. Clearly the more air spaces you have, the better the insulating value. Moreover, insulation has to be a better insulator than air - if it were not, no one would buy insulation for any purpose.

I am sure that double shielding would reduce heat transfer but it has not been tested so it is not acceptable the same goes for insulation in that airspace. There are stoves with soapstone backs yes but a soapstone stove and a masonry heater are totally different things they work very differently in a masonry heater there is a massive amount of stone or masonry surrounding the firebox which allows for less clearance. You need to start looking at stoves many have pretty close clearances now

 

[Charles2]

here is an old copy i uploaded years ago, im not sure of any online versions that might be newer
(broken link removed to https://woodheatstoves.com/free/NFPA211_old.pdf)

Their drawings and descriptions seem awfully vague. Says to leave the bottom and top open for airflow, but doesn't say how close to the floor or how high the shield runs. Says not to put spacers directly behind the stove, but doesn't say how "directly behind" is defined, nor how wide the shield is, nor the vertical spacing of the spacers. Surely there is a better source for construction details?

 

[bholler]

That is what you have to go by Height and width requirements will depend on the size of the stove so they cant tell you. Spacing of the spacers depends on the material used and the construction of the wall behind.

 

[Charles2]

That is what you have to go by Height and width requirements will depend on the size of the stove so they cant tell you. Spacing of the spacers depends on the material used and the construction of the wall behind.

In what way does it depend? What are the exact formulas and details? Does one have to ask the stove's manufacturer?

 

[bholler]

In what way does it depend? What are the exact formulas and details? Does one have to ask the stove's manufacturer?

you need a wall shield anywhere you want to reduce clearance That will depend entirely upon the size of the stove. How could they give you exact requirements with out knowing what size stove you have and as far as the spacers if you are doing backer board and stone over that you are obviously going to need more support than if it is just sheet metal. There is no way for them to spell out every possible scenario if they tried the book would be massive and you would never be able to find anything

 

[Charles2]

If no one can spell out every possible scenario, how do you know you are doing it right for your particular scenario? Does one have to hire an engineer to run calculations?

Regarding sheet metal vs. stone, sheet metal expands and contracts much more than masonry in response to temperature changes. So how do you keep metal panels from coming apart from all the expansion and contraction?

 

[bholler]

You need to go by their requirements and figure out what to do in your situation. You just over lap your joints enough that it isn't a problem.

 

[Charles2]

You need to go by their requirements and figure out what to do in your situation. You just over lap your joints enough that it isn't a problem.

Who are "they"? NFPA or somebody else?

Another problem with sheet metal is that it is flimsy compared to masonry. How do you prevent the metal from being dented, especially in the area directly behind the stove where spacers are not allowed and the metal has no support?

Also, is the metal or masonry allowed to be painted?

 

[bholler]

yes it can be painted and as far as the rest code are writen as a basic guide you still need to have an understanding of construction techniques

 

[Charles2]

...you still need to have an understanding of construction techniques

The fact that I have an understanding of construction techniques is precisely why I'm so shocked that such a critical assembly is not clearly spelled out somewhere!

 

[bholler]

it is spelled out everyone else figures it out fine i am sorry you cant good luck

 

[begreen]

Their drawings and descriptions seem awfully vague. Says to leave the bottom and top open for airflow, but doesn't say how close to the floor or how high the shield runs. Says not to put spacers directly behind the stove, but doesn't say how "directly behind" is defined, nor how wide the shield is, nor the vertical spacing of the spacers. Surely there is a better source for construction details?

The definitions for an NFPA wall shield clearance reduction are specific to stoves that are not UL tested. If the stove is more modern and has been UL tested, then the clearances are determined by those tests and published in the stove manual. At that point there are no exceptions granted by code. If the manufacturer of the stove has tested with an NFPA wall shield and certified those clearance reduction results in the manual, then they are acceptable. No stove company has tested with a double wall shielding and published those results in their manual.

It sounds like what you are after is a close clearance stove. There are several of them. Look for stoves that are convective and have good rear shielding plus an outer cladding of metal that reduces the side radiance of the stove.

Your best bet would be to start looking at stoves that are a good fit for the space and decor and have close clearances without any wall shielding.

 

[NVHunter]

The Ideal Steel Hybrid by Woodstock allows a 6" rear clearance with the use of their rear metal heat shield. It is 1" off of the rear of the stove, and the 6" is measured from the shield, so technically it's 7" from the rear of the stove.

 

[begreen]

Yes, there are stoves that allow as low as 2-3" clearance with their rear heat shield.

 

[Charles2]

It sounds like what you are after is a close clearance stove. There are several of them. Look for stoves that are convective and have good rear shielding plus an outer cladding of metal that reduces the side radiance of the stove.

Your best bet would be to start looking at stoves that are a good fit for the space and decor and have close clearances without any wall shielding.

Well, the close clearance is not the only goal. The other goals I listed in another thread:

1. Small and low output, but should still accommodate standard 16" firewood.

2. Safe, so intended to be hooked to 6" diameter class A UL 103HT double-wall stainless chimney.

3. Efficient, around 80% AFUE.

4. Clean, meets Washington State emission requirements. Would be great if it could also meet the proposed EPA requirement of 1.3 g/hr.

5. Durable/reliable, so a non-catalytic make/model with a long, unblemished reputation.

P.S. How do you like your Jotul 602? Does the lack of an ash pan bother you?

 

[begreen]

The Jotul 602 is retired to greenhouse duty. It only gets fired up during very cold spells. It's a willing little heater, but not a very clean burning stove.
No, I don't particularly like ashpans and don't use the Alderlea's ash pan at all.

 

[Charles2]

It's a willing little heater, but not a very clean burning stove.

At 3.76 gram/hr?

 

[begreen]

My 602 is from 1984, not the CB model. Folks down the road have the F602CB and like it, but definitely not a 24/7 burner nor a stove with close clearances.

 

[s atreas]

Charles, just in case you are still wondering, that 1" gap is not for insulation, it is like a little chimney that moves the heat away, that's why it has to be open top and bottom. air is only insulation if it is not moving and there just isn't any insulation that would work in a situation like we are discussing. Insulation does not stop heat it only slows it down and in a stove installation that combustible material will eventually get just as hot as it would without any insulation, it will dry out and might spontaneously combust.