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Hearth thickness?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by agz124, Jul 22, 2007.

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  1. agz124

    agz124 New Member

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    Ok guys, advice needed.

    I am replacing my coal stove with a Quad 4300. My old stove has been there for since I bought the house (6 years) and according to the original blueprints for the house (1951) the ceramic tile hearth has been there since day one. The top of the tile are flush with the top of my hardwood floor. The million dollar question is: What is under the tile and is it combustible? There is no basement at that part of the house so looking at it from below is not an option. I have operated the coal stove there for 6 years without a problem as did the original owner for 50+ years and the new wood stove has 10" legs on it. I can't imagine that the bottom of the stove would get that hot that it would ignite plywood that is below at least 3/4" of ceramic tile and mortar. Who knows?....maybe they put cement board under the tile which make it upto code, right? Any input on this situation is welcomed. Thanks

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  2. nshif

    nshif New Member

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    Im about to build the hearth for the same stove in new construction. Not quite sure what I need yet as Quad is rather vague in their terms of floor protection. From their installation manual
    "A. Hearth Protection Requirements
    FLOOR PROTECTION: Floor protector must be non-combustible
    material, extending beneath heater and to the front,
    sides and rear as indicated. The fl oor must be non-combustible
    or otherwise adequately protected from radiant heat
    given off by the unit and from sparks and falling embers. A
    layer of thin brick or ceramic tile over a combustible fl oor is
    not suffi cient.
    For US installations, it is necessary to install a fl oor protector
    of a minimum of 3/8 inch (9.5mm) thick metal clad millboard
    or equivalent a minimum of 16 inches (406mm) in front of
    glass and 8 inches (203mm) to both sides of the fuel loading
    door. Open the door and measure 8 inches (203mm) from
    the side edge of the opening in the face of the appliance."
    No mention of R value just metal clad millboard. Which I have no idea what that is. I had planed on using Duroc but not sure now.
    As far as your situation I doubt that as far back 1950 there is anything fire resistant under your tile and it would not be up to current code. And the 16" in front now needs to be 18.
    There are a few good topics on hearth building latley check those out for additional info.
  3. begreen

    begreen Mooderator Staff Member

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  4. agz124

    agz124 New Member

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    This leaves me in quite a dilemma. I agree that the manual specs are not very clear but I know for sure that tile on plywood is not good, I just wish I knew that that was the case for all I know there may be a concrete pad under the tile. I think the hearth was rebuilt in the 70s or 80s, I wonder if there is a record of what was done? The owners have since died. I believe that because I am replacing a stove and not installing one I do not a permit/code inspection but I still want it to be safe.
  5. begreen

    begreen Mooderator Staff Member

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    Would it be possible to very carefully do some exploratory surgery in one tile square? If you do, choose one in either a discrete location or front and center. That way you can replace it with a decorative tile of the same dimensions.
  6. nshif

    nshif New Member

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    Can you get below the floor where the hearth is? If so and it is just wood odds are, if the tile is at finshed floor height there is nothing in between but thin set motar.
  7. nshif

    nshif New Member

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    And I think technicly you do need a permit but Ill defer to Elk on that. but safe is better...just because you havent burned the place down yet doesnt mean you wont.
  8. webbie

    webbie Seasoned Moderator Staff Member

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    Is the house on a cement slab or on a crawlspace? If on a crawl, you might be able to get under just to see whether the area was"dropped" down to allow for a couple inches of wet bed (cement).

    My guess is that it is built well enough to support the situation you are using for the stove. Back at that time, they generally used a minimum of a deep wet bed (cement layer) to set the tile in. In fact, if it was a semi-custom house (as it appears to be since you have blueprints), it might be built even better than that. Asbestos sheets were a standard building materials back then, and so was vermiculite and cement. If that tile was simply glued down I doubt it would have lasted 60 years!

    With a tall leg like that I don' think you will have any problem with excess downward heat radiation. If you want to be sure, you can use the old rule of thumb method - that is, install the stove and monitor the temperature of the hearth by holding your hand on it when the fire is really cooking....and has been for hours and hours. If the hearth is not too hot to hold your hand on for a couple seconds, it is well within the safety zone. If it gets too hot to touch, then you might consider hanging a heat shield an inch under the stove - or under the existing heat shield if one exists on the stove. That should cut most downward heat to an acceptable level.

    Burning a house down from downward radiation with such as stove (10" legs, etc.) would be virtually impossible.....the standard allows wood to hit a temperature of about 160 degrees. Just for comparison, the hot water pipes from a typical hot water boiler can easily hit 190 degrees.

    To put all this in perspective, there are quite a few studies and even myths about the ignition temperature of wood. Forestry labs have exposed samples of various woods to temps from 230-300 degrees for THOUSANDS of hours straight without ignition. That does not mean you should do this, of course! The current science shows that the actual agreed upon temperature at which wood would ignite is about 480 degree F.

    BUT, the standard is set at about 160 degrees for a number of reasons, including that the wood will dry out, shrink, discolor and even turn to charcoal at certain lower temperatures. However, this allows us to see the inherent safety in the "rule of thumb" as well as the UL standard.....the chances of any stove owner keeping their unit at max for thousands of hours is rare.

    The info above was presented just to give a foundation to the actual standards, and I would not advocate using it to exceed or fudge with the standards. If a hearth under a stove gets too hot to touch on a regular basis, you should make certain of a hearth with a decent R-Value.
  9. agz124

    agz124 New Member

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    Thanks for your info. I should add that the stove has the optional ash pan installed so I would think that would help deflect some heat from the box. As for the tile, they look perfect, I mean not a crack in the mortar or the tile anywhere (that is another reason I really don't want to cut one out just see what's under it). I have done a few tile jobs myself and I have learned (thankfully from books and friends) that tile on plywood will almost always crack especially with a 300 pound stove that is heating the area (expansion and contraction) resting on the center of it for 30-50 years! I think the chances are really good that there is cement or the like under the tile. Thanks for all the words of wisdom, I will be sure to update you all with my progress.
  10. Gooserider

    Gooserider Mod Emeritus

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    What about the possiblity of doing a "biopsy sample"? Use a long thin drill bit to drill down at an angle from the edge of the floor into the hearth, or possibly a skinny masonry bit going down between two tiles - Look at the chips coming out of the hole, if you go down a couple of inches and still bring up cement or other non-combustibles you should be OK, if you hit wood then there may be a problem. If you use a small enough drill bit, it should then be possible to plug the hole back up so as to be almost invisible...

    At any rate, thanks for those millboard references BG, I may try contacting the mfgrs and see if I can get more data about the exact R-values and suitability for use as hearth material, then add it to the Wiki article. I would love dearly to have "official" sources for all the R-values in that table (I.E. something that would make an inspector like Elk happy) and to list as many appropriate materials as possible. Unless there is a reason those millboards wouldn't work, they look pretty ideal - High temp resistance, light weight, stiff, might even be a better "super insulator" than Micore if you don't have to worry about it being squishy...

    Gooserider
  11. begreen

    begreen Mooderator Staff Member

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    Yes, I hope this turns out to be an easily purchasable item. It looks tailor made for the job of hearth building, claiming greater structural rigidity and compression resistance. Keep us posted.
  12. agz124

    agz124 New Member

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    Thanks for the biopsy idea! Last night as I was tossing and turning thinking about the arrival of my new stove and the excitement that fall will bring I thought, "Why don't I just drill a small hole between the tile under the stove and look at the chips?"
  13. Gooserider

    Gooserider Mod Emeritus

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    Will keep folks posted on the millboard question,

    Glad the "biopsy" idea was useful - I'm in sort of the same situation, which is why I thought of it...

    Gooserider
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