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Stove hearth design: heat storage vs. heat shielding

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by precaud, Jun 30, 2006.

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  1. precaud

    precaud Minister of Fire

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    As the demolition/cleanup phase of my fireplace-to-stove-platform conversion comes to a close, I'm having serious doubts about the whole concept of using a massive brick hearth behind a freestanding stove for heat storage.

    Since it is all coupled into an even-more-massive outside brick wall, it looks to me that it would mostly be dumping the heat into a cold wall and dissipating there. So I wonder... wouldn't it be better to make it reflective rather than absorptive? Sure, all that metal would make the esthetics a real challenge... but it's worth considering.

    I'm curious what others have thought and/or done about this.

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  2. smirnov3

    smirnov3 Feeling the Heat

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    is there any way you could make a 'faux-hearth', that has a layer of insulation between it and the outside wall?
  3. Todd

    Todd Minister of Fire

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    Iv'e podered that same thought for a couple years now. I had a brick hearth installed with a 1" air space between it and the outside concrete wall. I do have a rear and bottom heat shield but the brick still heats up. I think the house takes a little longer to heat up while the brick absorbs heat, but since I burn 24/7 it's pretty much up to temp most of the time. (kind of a mosonary heater effect)I'm sure some heat probably escapes outside, but hopefully the air space acts as an insulator and keeps too much heat from escaping outside. I can't be losing too much heat since my highest gas bill was $30 last winter, and that was mostly for hot water!
  4. precaud

    precaud Minister of Fire

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    Thanks for your input, guys.
    Gideon, I agree that a well-insulated metallic barrier would be ideal. But how to make it look good - that's a hurdle.
    So I may end up just stuffing as much insulation as I can behind the brick. I have some really dense styrene board removed from a local school's roof when they resurfaced it, that should work fine. (Had I had my thinking cap on, I should have gone back and taken it all when the opportunity was there... sigh.)
  5. elkimmeg

    elkimmeg Guest

    Not to bust up euphoria. but what are the burn properties of that poly insulation? If the bricks are going to store heat, the insulation behind it should not be combustiable or have a low melting point? Normal fiberglass might work better providing the heat does not reach glass melting point.
  6. precaud

    precaud Minister of Fire

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    Good question, Elk. Here's what I found:

    EPS Flammability Characteristics

    Recommended Maximum Use Temperature: 165ºF (75ºC)
    Melting Point: As a thermoplastic, polystyrene does not exhibit a true melting point. It will begin to soften at about 212ºF (100ºC) and, as more heat is applied, melting occurs.
    Flash Ignition Temperature*: The lowest initial temperature of air passing around a molded sample of EPS at which a sufficient amount of combustible gas is evolved to be ignited (ASTM D 1929). 698ºF (370ºC)
    Self Ignition Temperature*: The lowest initial temperature of air passing around the specimen at which, in the absence of an ignition source, the self-heating properties of the EPS lead to ignition or ignition occurs of itself. (ASTM D1929) 752ºF (400ºC)

    This is for the base Expanded PolyStyrene (EPS) material. They go on to say:
    The modified grades of expandable polystyrene contain flame-retardants designed to decrease flammability due to accidental ignition from a small flame source. The expanded polystyrene manufactured from these products have been tested in numerous small scale fire tests and meet the requirements of the nation’s building codes and applicable industrial, federal, and state requirements.
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