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Snow Melt??

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MikeJ
MikeJ Member Posts: 103
After the concrete is pour for the snow melt, is there a amount of time to let the concrete cure before utilizing the system? any concern of using too soon?

If you have the temperate of the slab, thickness, square footage. What is the best way to figure what kind of warm up time is needed?

What supply temperatures are most commonly use? I have seen 90-95 degrees?
Using higher temperatures will this do harm to the slab?
Does the temperature of the slab determine the temp to use?

I have done several radiant systems, but never snow melt. Have bid several snow melts, no one has pulled the trigger on them.

Needed a new drive at my place, no better way to learn all the ins and outs for snow melting :)
Thanks







Comments

  • Leon82
    Leon82 Member Posts: 684
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    I can tell you it takes 28 days to cure. I would ask the concrete supplier for a timeline.
    MikeJ
  • HVACNUT
    HVACNUT Member Posts: 5,863
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    Not positive, but once it's cold enough for snow, or if it's in the forecast, you turn it on and leave it on, otherwise it won't do its job.
    I do know that I'm waaay jealous.
    MikeJ
  • JUGHNE
    JUGHNE Member Posts: 11,075
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    Is your gas meter large enough for all this heat?? >:)
    kcoppMikeJ
  • Zman
    Zman Member Posts: 7,573
    edited November 2017
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    I don't think you will get a straight answer on the cure time. 28 days is definitely safe. Depending on the mix and temps during cure, you may be able to heat it sooner. I would be comfortable with low water temps after a week.

    The required temps will vary a bit depending on the installation. Lower temps will give you more even melting, too low and it won't work. I like 90-95 for starters, don't go higher than 130, yes you can damage the concrete.

    The slab sensor location is critically important. Find a spot between the tubes and out in the middle. You want a slab melt temp in the middle to upper 30's. If you have good downside insulation, you shouldn't need to "idle" the slab before a storm unless you are doing an airport.

    For residential applications, 100-125 Btu's per square foot is a good design guideline.

    You could probably calculate the actual time needed to heat up the concrete based on it's weight. A 4" thick slab will weigh about 50 lbs per square foot. At a specific heat of .18, that would be 9 btu's per square foot per degree. If the slab is 20 degrees before the storm, it would take 135 BTU/foot to bring it up to a 35 degree melting temp. Theoretically that would be a little over an hour. I reality, it would take several hours because of the heat being given off to the air. If you have inadequate insulation and you are wasting energy melting the ground under the slab, all bets are off. A pound of frozen water takes 144 btu's to melt from solid 32 to liquid 32.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    HenryMikeJ
  • Jamie Hall
    Jamie Hall Member Posts: 23,433
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    On the cure. Concrete is weird stuff. It will reach it's specified strength in seven days, but it will continue to cure although at a much slower rate for the 28 days mentioned -- and, in fact, for long after that. I don't see rebar in your photos. You absolutely need it, and in a pour that size you also need deliberately created expansion joints. There is a rebar detail for that -- the rebar continues across into the adjacent slab typically 6 inches, but is lubricated so that it will no bond to the adjacent slab so that they can expand and contract, but not move vertically or left and right. Goes both ways across.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    MikeJ
  • Zman
    Zman Member Posts: 7,573
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    Be careful tying slabs with snow melt to elements that do not. I see lots of slabs and sidewalks where one has lifted because of frost. It may not be a big deal in your climate. Just something to be aware of.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 22,255
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    You can heat it right after the pour, I doubt you will get it warmer than a sunny day, with 95F supply temperature in the winter.

    We poured plenty of slabs in the winter, with the tubing installed and running, covered them with concrete blankets. The concrete installers prefer warm slabs to frozen ones?

    28 days is often the suggestion before you drive on a slab. I question that also since it is a compression product, rock and portland?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    MikeJ
  • Jamie Hall
    Jamie Hall Member Posts: 23,433
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    hot rod said:

    You can heat it right after the pour, I doubt you will get it warmer than a sunny day, with 95F supply temperature in the winter.

    We poured plenty of slabs in the winter, with the tubing installed and running, covered them with concrete blankets. The concrete installers prefer warm slabs to frozen ones?

    28 days is often the suggestion before you drive on a slab. I question that also since it is a compression product, rock and portland?

    Indeed. In fact, freezing new concrete is absolutely the worst thing you can do to it -- short of driving through it. It will ruin it.

    Where did the 28 days come from? When one tests concrete for structural work, it's the seven days cylinders one is really looking for, and they should test at the specified strength. Can't see why one would need to wait longer.

    The other bad thing you can do to concrete (well, another bad thing!) is to let it dry out. It doesn't like that much, either -- concrete cures, not dries, and requires moisture to do it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Henry
  • MikeJ
    MikeJ Member Posts: 103
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    Thanks Guys

    I probably will not have the boiler going in 28 days, "the cobbler's children have no shoes" as the saying goes so i'm safe their.

    @Jughne you may be on to something about the size of the gas meter, It is a AC-250 meter, I do have 2lbs coming into the house so if i'm reading this chart right its good for 600 SCFH, if i'm reading it wrong please let me know.
    chart is here https://www.screencast.com/t/ux1Z9OwUBY

    @Zman thanks for the information helps me a lot. The drive is about 2000 sq ft and a 6" pour. My truck is about 4 tons.
    Plan on going with a 265,000 Lochinvar boiler. Whats nice is the drive faces south, garage helps block the North wind and the west wall should also help with blowing snow. I have sensors in two different locations.

    @Jamie Hall I do have rebar, you can see some in the 2nd photo, they were not done installing. Drive is not being tie in to any other concrete, have expansion materiel around the entire perimeter, and yes they did pull the rebar up during the pour

    @hot rod good point about the sunny day

    Pics of the big pour









    EzzyT
  • lchmb
    lchmb Member Posts: 2,997
    edited November 2017
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    on a side note..depending on your sensor, they are very sensitive on install and not cheap to replace if they fail..https://www.rehau.com/download/874866/snow-ice-sensor-090-094-data-sheet.pdf
    MikeJ
  • JUGHNE
    JUGHNE Member Posts: 11,075
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    Looking at that AC 250 info, it is confusing. Hopefully there is someone here to educate us.
    The "Features" side indicates 5 PSI MAOP and 250 CFH at 1/2" differential.
    The other chart indicates at 2 PSI with 2" drop you could flow 600 SCFM. ( I guess you could sacrifice 2" drop if running 2 PSI)

    However the right hand chart shows operation up to 10 PSI while the features indicate 5 PSI to be max operation pressure??



    MikeJ
  • MikeJ
    MikeJ Member Posts: 103
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    not going with the snow/ice sensor, they are a lot of money replaced a couple for a collage in town that had them go bad.

    I have couple of tubes in the concrete that I can slide a temperature sensor into , I use pex tubing for the sensors wells, plan to go with a 12hr spring timer then use the sensors for a safety shut down. Then to turn the boiler on and off to maintain slab temperature, if it ever shuts off once on. No idea, its going be a learning experience