Slab as a heat storage

Woodfarmer1 Posted By Woodfarmer1, Jan 5, 2018 at 9:10 PM

  1. Woodfarmer1

    Woodfarmer1
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    Not....
    I have the Garn jr. 2000sf of 3” concrete on top of 4” Dow sm shiplap with pex.
    It’s been -20’s C or -10 F here for more than a week, had the slab warmed up yesterday I thought.
    I have a mixing valve on that zone, usually I fire at 10pm so I have around 160* in the secondary and tempered down for the slab. At 8am the secondary is usually no lower than 120*.
    Well woke up this morning house was low 60’s, rads and in floor pep were cool, secondary at 80*.
    What the heck!!
    I thought with the warm slab it would cruise through the night, nope it apparently sucked ALL the heat from the system.
     
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  2. peakbagger

    peakbagger
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    Unless the slab is insulated underneath and on the sides its going to match the temp on the exposed edges and the ground temp.
     
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  3. maple1

    maple1
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    Yes, but if the heat from the system went into the floor, it would then go into the house. It wouldn't just disappear, as long as the slab is well insulated. Then from the house, it would go to the outdoors via heat loss. So the slab didn't suck the heat out, it was ultimately the outside coldness.

    The extreme cold across pretty well the whole continent is pushing heating systems to their limit right now. And would also reveal any design shortcomings, if there are any.

    Are you actually measuring the actual temp of the slab itself? It might not be getting as warm as you are thinking.
     
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  4. tom in maine

    tom in maine
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    The slab is just a delivery system. It will store a significant amount of energy, but if your heat load is too high, it will just cool off.
    2000 sq. ft. of floor can deliver about 60,000 btus an hour. Heat is stored in the slab was you increase the floor temp, which increases its heat output.
    This weather is a challenge, so it the slab edges are not insulated well, and the house heat load is higher, it is pretty simple to pull all the heat out of the entire system.
    A thermal imaging camera is very helpful in this weather in sorting out thermal compromises in a building envelope and slab.
     
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  5. Fred61

    Fred61
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    With water having the highest specific heat at "1" and all other materials except for eutectic salts are lower on the chart. I can't recall where concrete lies. Anyhow, concrete doesn't hold much heat and what heat it has stored is given up too slowly to keep up with the heat loss.
     
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  6. salecker

    salecker
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    Slab as heat storage is the wrong title.
    Slab as heat sink is the proper title.
    There is no way to get a return on your BTU's using a concrete slab.You may get a small return but it will cost you a lot of BTU's get a few back.Very inefficient.Water is the best.
     
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  7. Woodfarmer1

    Woodfarmer1
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    That was originally what I named the thread, then changed it lol.
    The slab edges are surrounded by a 10” icf foundation.
    I just couldn’t believe it sucked that many btu’s out of the system.
     
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  8. __dan

    __dan
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    The slab is a load with a very slow response time between taking in the heat and giving it off. It's not clear from your post if you wanted heat out as hot water for the rest of the system, back out of the warmed slab, or heat out as radiant transfer to the rest of the building. If you want hot water back out, no the slab will not do that. If you want the heat back out at a later time with a substantial delay in the range of 12 to 24 hours, yes the slab is doing that for you (because it is working as storage).

    I have a non standard system using only the basement slab as storage, as a constant load slaved to the Froling when the Froling is firing (mixed down to 130 F with a Tekmar controller and pri - sec injection pumping). I am tapped for tank storage but the system is working perfectly as is. If it is 45 deg F outside, the slab seems to match the demand perfectly and I only have to fire one day out of two or three to keep the house perfectly even at 70.0. I never intend to burn overnight.

    My storage is truly in the unburned cordwood and the timing of the fuel feeding. I never load more than half the firebox and I usually only put two to three splits on a coal bed if the boiler is hot and firing but burned down. I fire to make DHW and the basement slab is slaved to the firing cycle. If the house is cold, I make a fire in the Froling and if the house is over 72 or 73, I stop putting wood into the firebox. Slab runs at a surface temp in the winter of 85 to 95 deg F with no stat, solely slaved to the Froling circ and the Tekmar OAT reset controller. I could not ask for more, or be happier with the Froling, which was essential to making my system work.

    At -10, you have a demand for heat that exceeds what the slab can deliver from (it's time delayed heat input). Also, the heat input will not penetrate the slab fully in one day of firing. My slab is much more heat penetrated in January than it is in November. My slab is never off, it's always on. It's a constant load if the boiler is firing and a slow heat source when the boiler is off.

    I don't want to get in trouble by promoting a nonstandard arrangement. The slab is not storage in the way, the manufacturer requires, as a defense against the owners who cannot time their fuel loading to avoid overfiring, overtemp and slumber. The slabs as a heat bank are awesome.
     
  9. tom in maine

    tom in maine
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    Depending on the size of the slab, it is equivalent to at least a couple hundred gallons of water.
    The specific heat of concrete is about 0.2. Water is 1. The weight of concrete is quite a lot and a delta T of, say, 5-10F will hold a lot of heat. It will not stay there long, depending on the thermal integrity of the building on top of it and the insulation around the slab, but it will store some heat. For some time. (sounds like a politician talking!--I am not one!!)
     
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