The folly of using air spaces for hearth construction.....

[webbie]

You are absolutely correct in that VC may have just thrown their hands up and refused to test with an R-Rated hearth assembly. It doesn't make any sense at all- but it is possible. In other words, why limit the options of your customers...or, worse yet, have a fire caused because you didn't include the heat shield standard and almost no existing floor would do without it. And, keep in mind that VC is the only company in the entire industry that has this "bare earth" thing. If nothing else, we may get to the bottom of that.

In the case of HearthStone, it makes sense to assume they actually did the testing....and the requirement of 20 layers of Durarock makes it clear in my mind that the floor temps hit MANY hundreds of degrees (on top of hearth)....in order to still hit 160 under all those layers.

I'm getting a lot of info, but the final piece of the puzzle would be an actual test report on one of these stoves.....exactly what the thermocouples under the stoves were reading. I'm sure anyone with decent heat transfer knowledge could work it out backwards....like this.....

Assume wood floor was at 150 degrees (passing temps)...now assume R 6.6 layer - what would the temp have to be on the top of the layer to have 150 on the bottom? This should be simply algebra on the R-Values, but I dropped out of that class! In simpler terms, what is the temp % reduction achieved by R 6.6 ?

Anyone?

 

[begreen]

It seems a bit odd that the Lopi Leyden, purported in earlier threads to be a carbon copy of the VC everburner, has quite minimal hearth requirements. All it asks for is a sheet of non-combustible metal.

 

[Gooserider]

Well the Cat Encore manuals I've looked at, both the 0028 original PDF, and the latest model paper copy I got at VC require the OEM heat shield on anything but the baire concrete install. With the heat shield they require a 1/4 "non-asbestos mineral board" or 24g sheet metal. The Everburn Encore also requires the heat shield, but has a hearth R-value requirement in addition of 1/2" approved pad, or R-0.59

The Defiants that we saw being packaged had the bottom heat shield included in the box, presumably included in the price, as part of the accessory kit. I also don't see why they wouldn't be used as the shield is not particularly noticeable.

One clue that might explain why the Hearthstone has such a high requirement might be the statement in the VC manuals that the heat shield takes care of the RADIANT energy while the hearth pad takes care of spark, ember and thermal protection. This is consistent with my understanding that radiant energy can't pass through solid objects.

Does the Hearthstone have a heatshield with the short legs? What about with the long ones?

Gooserider

 

[webbie]

Goose, three are three types of heat transfer:
Conduction, Convection, and Radiation

Radiation is the one that, in general, passes through things with less dissipation. It is not even calculated in most R-Values (to my understanding). Note this quote (can't confirm the source"
"R-values describe the thermal resistance of materials to conductive heat-loss"

Convection is the movement of heated air currents, etc......while Conduction is the one that we speak about in most hearth protections...the heat "soaking" into mass and then exciting the molecules as it goes.

Main point here is....that these two stoves and probably others do require some heavy duty protection...which I cannot imagine HearthStone having pulled out of a hat. I would LOVE to see the actual test on that stove. In the VC situation, it is a big question mark as to exactly why the bare earth thing is specified.

Another thing we can gleam from the VC information is that a lot of downward heat is by Radiation, because the bottom heat shields...which are provided NOT painted (so they have reflection) do the job that NO amount of mass hearth will.

 

[elkimmeg]

Goose there are some other factors working here the Encore has a separate ashpan compartment that in itself acts as heat shield

My encore I can place my hand on the hearth under the stove on the stones and hold it there as long as I want without burning skin

or even getting uncomfortable. The other factor with the encore the heat is directed up with the draft threw the cat combustor chamber
there is more need for sides and rear protection than downward needs Infornt of the stove radiant heat will be felt threw the glass doors. It is warmer infront of the stove on the hearth than directly under it I do not have a heat shield but the hearth is built on the concrete floor slab raised 6" of granite field stone

I mean I have experience these conditions burning 24/7 at 600 degrees griddle temps. The stones behind the stove get hot I mean above 120 degrees
this is good in my case it store the heat. The wall inf ront of the flue is 16" thick of granite stones and masonry then the 8" flue and double bricks 32 " of masonry
before it gets near wood.

 

[elkimmeg]

Goose there are some other factors working here the Encore has a separate ashpan compartment that in itself acts as heat shield

My encore I can place my hand on the hearth under the stove on the stones and hold it there as long as I want without burning skin

or even getting uncomfortable. The other factor with the encore the heat is directed up with the draft threw the cat combustor chamber
there is more need for sides and rear protection than downward needs Infornt of the stove radiant heat will be felt threw the glass doors. It is warmer infront of the stove on the hearth than directly under it I do not have a heat shield but the hearth is built on the concrete floor slab raised 6" of granite field stone

I mean I have experience these conditions burning 24/7 at 600 degrees griddle temps. The stones behind the stove get hot I mean above 120 degrees
this is good in my case it store the heat. The wall inf ront of the flue is 16" thick of granite stones and masonry then the 8" flue and double bricks 32 " of masonry
before it gets near wood.

Might I note dead air space is not the same as free flowing as for considering R values In wondow construction the dead air space R value is usually effected by argon gas and Low E coating on the glass normal aira space between glass panes is .97 per inch dead sir space that is with Low E or argon. Again as Web said, is tested for normal temps in housing construction and not at 500 degrees. At this point ,I do not know if the rvalue of dead air space holds outside that tested range. I do know and goose and I witnessed the testing to combustiables where the stove can be rolled in and out while thermo couplers monitor wall floor tenps. I do know code spells out requirement for floor protection given the height of the legs. That can be found in NFPA 211

 

[companyman]

I've read the inputs on air spaces for hearth construction and find some basic flaws with
the inputs generated so far.
1) Hot air rises. I've stood on my hearth with bare feet on cold days and yes, it is warm
enough that I don't want to stay there, but it's not hot enough to burn my feet. So the premise temperature range of 500 to 550 degress F, is definitley out of line
2) Based on #1, the heat on the hearth wants to rise, not penetrate down.
3) One response was to raise the stove with blocks of wood. In most cases, putting taller
legs on a stove will reduce the R-value for the hearth. But this is not "sealed air space." Most
insulation contractors that I've dealt with, will advise that sprayed in place foam against the
outside wall and dead air space against the interior wall is a very effective combination. Almost all the contractors were using this combination with 2" x 6" wall cavities. But again, the
reference is to a "dead air space", not blocks placed under the legs of a stove to create more air space.
4) The information in an Owner's manual has to be approved by the independent testing laboratory that did the safety testing on a product. Every page that the manufacturer has for reference, has the testing lab's stamp on it. If the information in the manual changes, the testing lab has to approve it, since they are the ones who have the responsibility if the product fails in the field and it's been properly installed and manufactured.

I do agree that if I had my choice, I would use Micore topped with cement board and some sort of a decorative non-combustible. Just like wall protection, the connectors supporting the material are not to be behind the appliance; so I have questions on how to support the non-combustible above the "dead air space" without transmitting heat down. However, the "sealed air space" value for hearth protection is valid per the independent testing laboratory.

company man

 

[Gooserider]

So Elk, does NFPA 211 account for the high hearth requriement on the 4" Leg Hearthstone Homestead? FWIW, I did just go through their manual, and found that the Homestead manual also lists the 1/8" dead airspace as acceptable, and this time it DID include the stacking prohibition. They even specifcally listed both the use of airspace, or the list of rock-wool in their examples of how to make a pad. It also (on Page 9) included a statement that a concrete slab was acceptable for mounting WITHOUT listing any R-value requirement, just like the VC stoves OK to put the stove on a concrete slab w/o the heat shield.

Elk - Does NFPA have anything at all to say about requiring clearances to NON-combustibles? My understanding is that it doesn't, that if you had a structure made of non-combustible materials, you could in theory put the stove right up against the surfaces in all directions, even casting it in concrete... (Don't know why you'd WANT to, but just saying you COULD)

If so, then it seems entirely possible to me that it would be possible that the stove makers said you can put the stove on a concrete slab w/o NEEDING to do any testing, or listing clearances since there would be no combustible materials involved. (IIRC the Duchwest stoves even allow you to put the stove on a slab w/o using ANY legs at all, I suspect the other stoves only require the legs on a slab for reasons of stability or structural support of non-load bearing things like sheet metal ashpans)

However getting back to the 4" legs - I noticed the R-6.6 requirement only applied to the area directly under the stove, and possibly a short distance in front of it (depending on the hearth height) The balance of the protector only needed to be R-2.5, or even just non-combustible, again depending on hearth height. (Possibly interesting in light of our discussions of the R-value for "split-level" hearths) The 6" leg stove required R-2.5. This seems to me consistent with my assertion that you have an inverse square rule in play in determining the heat at the hearth pad. In effect you are getting R-4.1 worth of reduction from the extra 2" on the legs, or about R-2 per inch of open space. (not quite as good as Micore, but close)

Gooserider

 

[elkimmeg]

Goose you are correct that a stove can be placed anywhere in relationship to noncombustiable construction ,But one should use common sense.
Common sense would be to alow atleast 6" air space or more around the stove,, so that air space will prevent it from overheating. Airspace will allow transfere and natural convection air currents. For the same reason legs can't be cut. besides being tested for clearance, they need that air space to promote natural convection. and heat dissipation from the stove

Another point not mentioned, 5/8" fire code sheet rock is designed to protect the combustiable wall it covers, for one hour. More an effort of containment then fire protection
The paper facing on sheet rock drywall is considered combustiable.. Taping should be fire tape and not the common paper tape.

 

[Gooserider]

Goose you are correct that a stove can be placed anywhere in relationship to noncombustiable construction ,But one should use common sense.
Common sense would be to alow atleast 6" air space or more around the stove,, so that air space will prevent it from overheating. Airspace will allow transfere and natural convection air currents. For the same reason legs can't be cut. besides being tested for clearance, they need that air space to promote natural convection. and heat dissipation from the stove

Not arguing that providing some space is a good idea - note that I said I didn't know why anyone would want to do zero clearance to non-combustibles, just wanting to be sure that they COULD... However given that, it would seem logical to me that a manufacturer would be able to say that no floor protection is required when putting the unit on a slab, without needing to do any testing since they know it's automatically OK because the slab is a non-combustible...

If that's the case, I don't see that the fact that a stove requires protection on a combustible floor, but not on a slab really proves anything.

I would maintain that the Hearthstone Homestead requiring R-6.6 with the short legs and only R-2.5 with the regular legs is simply a reflection of the fact that radiant heat follows the usual inverse square law, and the extra convection cooling space. BTW, looking at my list of R-values, the ONLY way I can see to get R-4 of added insulation needed in less than 2" of space (if you can't do it in less than 2" you are better off with the longer legs) would be to have a series of 1/8" air spaces, separated by non-combustible (even if it has no R-value) sheet metal - Given that one only needs the R-6.6 part to be as wide as the stove and 4" in front of it, I think this would work, be buildable, and meet code requirements - Even if Craig doesn't like it :cheese:

[/quote]Another point not mentioned, 5/8" fire code sheet rock is designed to protect the combustiable wall it covers, for one hour. More an effort of containment then fire protection
The paper facing on sheet rock drywall is considered combustiable.. Taping should be fire tape and not the common paper tape.[/quote]

Well, I made the gross assumption that sheet rock was what they were refering to in my chart on the line " Gypsum or Plaster Board, 1/2”, R- 0.45" (I'm surprised by the R-value, BTW, as the other cement type boards were only R-0.26) - and so I added the note to that (and also the fiberglass listing) that paper backing must be removed to be non combustible. Not sure how easy that would be to do, but... Actually would it be better to put a better note saying what material is meant, and / or that paper backed sheet rock is NOT acceptable?

Gooserider