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Fill the remnants of a chimney for thermal mass

Austin G | Posted in General Questions on

A couple weeks ago, I demo’d a chimney that protruded into the unconditioned attic down to the floor joists.

I was thinking about the “fireplace” that’s still there and the two stories of brick that are inside of the envelope.

Would there be anything wrong with filling the chimney with sand to add thermal mass?  Would there be a better material?

I know they’re not really viable in modern builds, but I’ll never forget this giant masonry heater I saw as a child.  The fire had burned out and it was still so warm and nice.

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Replies

  1. DCContrarian | | #1

    In a word, no. A lot of the benefit that people associate with"thermal mass" is really high conductivity of heat allowing a point source to spread heat more broadly. That masonry heater was comfortable not because it was massive, but because it was conductive. An aluminum heater would have had the same effect.

    1. Austin G | | #4

      My understanding was similar to likening a wood stove with a masonry heater. When a wood stove goes out (forgetting about soapstone for a moment) it stays hot for a short period of time, but the room tends to cool of relatively quickly. Air doesn’t hold a ton of heat, so depending on the heat loss of the room through the walls, windows, floor, etc. But the increased mass stores the heat and allows it to dissipate over time. I know, I know, no such thing as a free lunch - but a wood stove will sweat you out of a room in a hurry, it’s more just a method of buffering that heat over a period of time.

  2. Charlie Sullivan | | #2

    If you want thermal mass for some reason, considerations include:

    1. Water is better, and cheaper than sand, if you can find a way to contain it.

    2. The extra volume of the interior of the chimney is probably small compared to the mass you already have, so likely not that big a help.

    But what do you hope for the benefit of the thermal mass to be? Are you in a climate that often swings to uncomfortably high temperatures during the day and uncomfortably low temperatures at night, such that you need to run A/C and heat in the same 24 hour period? If not, it's hard to know how you would get a benefit from it.

  3. Austin G | | #3

    I should have clarified in the original post - it’s not so much a question of the purpose that thermal mass plays as it is whether or not sand is the best substrate and if there are any reasons I’m not aware of that make this a bad idea.

    As far as the reason for wanting more thermal mass, we do regularly get swings in temperate this time of year as much as 40-50 degrees between day and night. My mini split system makes changing between cool and heat kind of a pain. Secondly, we lose power constantly. I have a generator - but normally I don’t bother if I know the power will be back on in an hour or two. Might as well “charge the battery” of thermal mass for when this happens.

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