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Makeup for "sand cushion" under slab

GroundUpGroundUp Member Posts: 372
I'm having a hard time figuring this out. I met with a customer the other day with a new 32x48 pole shed, wanting to install a boiler to heat it. He laid the tubing himself, stapling 6 loops of 1/2" (250ft) down to 2" XPS, but for some reason dumped 9" of coarse sand on top of the foam and then 5" of concrete over that. There were a few builders around here back in the 90's that pushed this as a "heat sink", but they also ran 180* SWT to everything. I did a heat loss on this building and came up with just over 30k, about 20 BTU/ sq ft. It's well insulated, including 2" XPS around the perimeter but has tall sidewalls and big overhead doors. My original thought was a UFT-80W would serve it nicely, but I'm having trouble negotiating the sand. My math says the thermal resistance of 9" of sand and 5" of concrete ends up equivalent to an R value of roughly 6.5. Seeing as there is R10 below and R6.5 above, would it be accurate to pump up the BTU needs by 155% to overcome the downward loss? I feel like I should know this already but am drawing a blank; I know one of you fine gentlemen know the answer though, anyone willing to lend some info please?

Comments

  • kcoppkcopp Member Posts: 3,236
    The sand was a foolish move. I would let him know that you cant guarantee results. CYA.
    I cant figure out why the sand in the radiant floor heat continues to be thing... Its an old wives tale.
    The sand will not be a heat sink... it is an insulator. I have even seen some (silly) people put the idea forward here on the wall over the years to use sand as a sandwich between floors as a "heat sink"
    I have even heard of high water temps in tubing embedded in sand causing the sand to stick to some pex tubing. Thus further insulating the heat transfer.
  • ZmanZman Member Posts: 4,571
    I think that the genius that came up with the detail needs to size the boiler. If they can tell you what kind of sand and how wet they plan to keep it, I think you would be able to do the math.



    At this point, I would suggest they hang a unit heater from the ceiling and be done with it.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot rod_7hot rod_7 Member Posts: 9,105
    That sand bed "storage" is still activity promoted by Bob ram low up in Wisconsin. They do a seminar every year at MREA on sand bed, year around, storage radiant slabs.

    The concept is to load the slab and sanded with solar thermal for free, or low cost heating.

    https://www.builditsolar.com/Projects/SolarHomes/SandBed/RamlowSandbed.htm

    I could never get the numbers to work out when comparing to sand, to lets say water as a seasonal storage? But it is an integral building thermal storage system.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GroundUpGroundUp Member Posts: 372
    It's dry coarse sand. Vapor barrier and foam at grade and 9" + 5" on top of that. It's bone dry and will stay that way. I recommended the unit heater twice and he's not having it. I told him it's going to work like crap and that I'm not putting my name on the system, he still wants it connected. I'm doing the job, I'm just hoping to have a little more understanding of what sort of input I'm looking at before I dive in
  • neilcneilc Member Posts: 589
    put in your small boiler first ?
    with contractural disclaimer that a second one may be needed? TBD?
    room for 2?
  • GroundUpGroundUp Member Posts: 372
    neilc said:

    put in your small boiler first ?
    with contractural disclaimer that a second one may be needed? TBD?
    room for 2?

    Before I do that I'll just install a single 120k or 140k unit as they have a 10:1 TDR, but I'd like to size it properly instead of guessing like everyone else does around here
  • neilcneilc Member Posts: 589
    I wazza guessing , , ,
  • ZmanZman Member Posts: 4,571
    As a heat dump for a Hydronic Solar setup, there may be some merit. If you are going to throw the heat away or put it under the slab, why not put it under the slab?
    To take heat that you have to pay for and dump it there does not make much sense, with only R10, much will be lost both down and out.
    I think your thought process will give you a reasonable SWAG. The UFT80 or UFT100 should get the job done. I would hang the final decision on the owner. You could site the lack of available industry data on their slab design as the reason.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • GordyGordy Member Posts: 8,654
    Quite a difference between thermal storage, and a direct heating plan. Thermal storage is designed to have mass for solar energy, or wood so it can soak up all the uncontrolled production of btus for latter use.

    This design is using paid energy. I think he will be spending a lot of money to heat verses a straight forward insulated slab detail.

    You have an r 3.5 difference in your favor to drive heat to the working surface to heat the building.

    In the end I think it will just take much longer to reach thermal equilibrium, but it will work. Be hard to control.
  • DZoroDZoro Member Posts: 565
    Agree with @hot rod_7 I have been to one of Bobs seminars. This idea came from solar storage ideas. The thought behind this theory is to store free solar heat in the fall months. This heat is stored in the sand and slab. During warm months, you open windows to get rid of the excess heat ;). Then come actual winter you have a large storage mass beneath you. Unfortunately you have no control over the heat that you get.
    In your case it will be similar too that. My suggestion is, very low water temp, with outdoor reset control, floor sensors. Forget changing the temperatures you set the stat at. Almost like a constant circ. system. But what you get for heat is just that, what you get, maybe hot (sunny days, open window), comfortable on the steady temp days.
    Low and slow.
    D
  • LeonardLeonard Member Posts: 505
    edited January 1
    I only see a heat sink that huge as a cheap way to store large amount of FREE solar heat for warmth at night. My house has 1 ft tall cast iron FHW baseboards along walls and that's PLENTY of heat sink to keep room temp CONSTANT.

    To size heat sink right would want solar dayly input to be something like min of ~ 4+ times daily losses, so warm heat sink can carry building during night.

    Sand heat sink above the tubing is just dumb, even if heat is free Solar. You'll have to wait for it to heat up before room air ( and you) starts to warm up. This implies for comfort you want a continuous heat source, not intermittent solar ( think coming into a cold shop in the morning).

    But I'ld hang box heaters ( Moden type or fueled) for when want shop warm right away on cold mornings ( don't want to wait till 1pm to be warm enough to work). You run the numbers. If solar... sometimes in Feburary haven't seen sun for a month....cloudy everyday

    Sand heat sink below heating coils is not much better. It's just a cheap to install storage method , but will slow down heating of the building air.

    Side note..... I remember in heat transfer class in school, takes something like 30 ft of rock or concrete to have same insulation as ~ 8 inches of pink fiberglass.
  • GroundUpGroundUp Member Posts: 372
    Yeah, in the free solar storage idea it makes some sense despite regulation difficulty but as others have mentioned it's pretty foolish to buy BTU simply to dump it in the ground. It's done and that's that. I did suggest a hydronic unit heater to the fella and he's still pondering on that but I am doing the boiler and radiant regardless. The more I think about it, the more I think maybe a UFT-100W and leave tees for a future UH zone once he sees how responsive it's not. Thinking pipe the 75k UH in as an "indirect water heater" zone with 150* SWT and priority so he can bring it up to temp quicker when needed and potentially abandon the floor once he gets a few LP bills. He's an old retired union electrician and will be out there on a daily basis, hopefully I can sell him on the UH plan. What do the masses think about that?
  • Larry WeingartenLarry Weingarten Member Posts: 1,385
    Hello, If there is a way to put tubing on top of the existing slab, and have that heated by a boiler and then hook up a solar system to the tubing underneath; that might work nicely and make some use of the lower tubing. ;)

    Yours, Larry
  • GordyGordy Member Posts: 8,654
    I would give him what he wants. You have already expressed your concerns, rightfully so.

    For some seeing is believing. I think he will see then believe. If even only in his own mind, or after he sees his wallet for fuel.
  • DZoroDZoro Member Posts: 565
    Unfortunately that is what he has to work with. Any change of temperature will cost him $. Need to keep the slab/sand mix as constant of a temperature as possible. Any variation of slab/sand temp will cost him even more$. A good slab sensor buried in the bottom of the crete would be a good thing. Don't use a air sensing thermostat in this case.
    D
  • GroundUpGroundUp Member Posts: 372
    DZoro said:

    Unfortunately that is what he has to work with. Any change of temperature will cost him $. Need to keep the slab/sand mix as constant of a temperature as possible. Any variation of slab/sand temp will cost him even more$. A good slab sensor buried in the bottom of the crete would be a good thing. Don't use a air sensing thermostat in this case.
    D

    How would you suggest drilling a hole for a slab sensor? There is no way to know where the tubing is, and I'm not going to be the guy to hit one. Short of grouting one into the surface, I'm feeling pretty limited to an air stat here. Electrical is done by the homeowner in this case, so that's not up to me. A constant slab temperature never works for s### in a building like this anyway from what I've been a part of. Always too hot or too cold without a secondary heat source. The 519 Tekmar with both has been serving me well, but I'm having reservations about that here because of the lack of sensor well
  • ratioratio Member Posts: 1,774
    Wouldn't you want a temp close to the surface of the slab? I'd drill a c. ¼" hole, big enough for a bullet probe, just an inch or so down. Maybe half-way through the slab. If they're really at the bottom of the mix you'd still be over a foot away from them!
  • DZoroDZoro Member Posts: 565
    Understand it's not by any means ideal, but it's what you have. Like ratio said, tubes "should be out of the way".... And yes the water temp is going to be low. You maybe surprised how low, and still get a comfortable heat from it. Last thing you want with this is too hot of water temps. I'm guessing but 95* or so will be close to max temp. There will be no way he will be able to make any adjustments, or set backs to this system.
    Your right a hydronic unit heater for quick recovery when he opens up the doors, and yes it may become his primary heat.....

    D
  • GordyGordy Member Posts: 8,654
    I'd deffinetly do a slab sensor, just to say I did it. However I don't think it will help a heck of a lot. There is way to much mass to control, and the fly wheel will always be there. It will raise its ugly head everytime the OAT swings drastically.

    My view would be keep the slab temp as low as possible, and use a unit heater to take the edge off for short cold snaps, or to bridge longer duration cold OAT until the slab can catch up. Coming out of a cold snap into milder OAT you're screwed open the doors if it gets to hot.
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