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slab start up

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Greg_40
Greg_40 Member Posts: 43
Design loss was around 190KBtu (don't have file here). They used a M-199 Munchkin. Insulation is the vinyl bagged type R-13, yes it is squeezed down at purlins & girters.

I didn't do a blower door on this building, but it may come to that to check leakage. Some metal building builders seem to know how to seal theirs up much better than others. I've tested some and was very surprized how tight they were and others, couldn't get a 50 Pascal reading to save my soul.

Doors are open periodically, no straight answer here- one guy said 'open a lot', another said not much during the cold spell in question. At 2050 Therms, the Munchie 199 would have to run too much to really add up in a 31 day billing cycle. But this isn't counting for the Hotsy running all. At 365KBtu, it could suck a lot of gas alone. I wish there was an hour meter on the Hotsy. We haven't offered to install a separate meter to check consumption. Except for the coming little cold spell over the next few days, we should be pretty much out of heating season (hopefully). Does the Vision track run times? Not being the installer, and not a Munchie guy, I chose to not touch anything to keep my exposures minimized. This one does not have a Vision controller, I was just asking for info sake.

For you Munchie guys, do you find them pretty much in tune right out of the box? Were at 2700 ft. above sea level give or take. As I mentioned, no analyzer was used on this one at start up. I may put mine on it when I get back over there. Still, only so many Btu's will flow through an orifice!

The building gas meter was removed and tested - OK.

Thanks, all!

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  • Greg_40
    Greg_40 Member Posts: 43
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    I was not the installer on this project, but called in afterward for high energy consumption review. It was a shop with 60' x 150' radiant slab, R-10 beneath the slab. R-4 was spec'd as perimeter insulation, but I can't tell if any perimeter insulation was installed. There are no obvious insulation breaks at the overhead door approach/slab joints and the building perimeter has been skinned with metal siding which hides the perimeter from examination. The building was completed in January, gas meter set, Munchkin was NOT set with a combustion anlyzer. The slab was very cold, as we have experienced HDD about 10-15% greater than 10 year average for this last winter.

    Without knowing the initial start up slab temp, but can only assume it was 'cold' as the building wasn't completed until January during this past very cold winter, can one estimate the Btu's needed to bring the slab to temp?

    2051 Therms were used this first month, 287 for February's usage and the next billing seems in line, but will be less than the 287. Utility co. checked for leaks, verified readings- all OK. I am looking for plausible terms I can use to help educate the owner that it legimately took almost an entire winter's worth of fuel to bring the slab up to temp. Employees are more free with information than the owner and I prefer to interview them because of that fact. They claimed the building was low-mid 70's and it was T-shirt weather inside while in the teens outside and near or below zero at night often that first month of operation. The installer told him his first month's usage would only be "a little higher than normal" due to start up.

    I can't get a M-199 to run that much Nat. Gas through in a typical 30 day billing period. They have a Hotsy steam cleaner at 365KBtu rating, but no one can or will give a straight answer how much this machine runs. Their business is oil field tank cleaning and I know the Hotsy runs a lot, but no exact idea how much is 'a lot'.

    Any advice to help re-educate the owner? If I had start up temps, I could maybe calc Btu's to bring slab up to temp. along with our local HDD numbers. I think playing catch up with 2-1/2 to 3 months of cold weather on an exposed slab caused most of the consumption.
    Thanks! Greg
  • hr
    hr Member Posts: 6,106
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    Do you have the load calcs

    that the system was designed to?

    70 is pretty warm for a working shop. 62- 65 is plenty if bodies are in motion.

    Somehow you will need to separate the loads and meter the boiler by itself. Gas meters are fairly inexpensive, google them :)

    Overhead doors open much? The infiltration number is real hard to nail down on shops like that.

    Exhaust fans?

    Ceiling insulation in metal buildings is always suspect. Typically thin and squeezed down between the girts and purlins. Nowhere near what they claim.

    Do some more snooping, and good call on talking to the workers in the shop. Stories may vary :)

    hot rod

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  • GMcD
    GMcD Member Posts: 477
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    Heat loss of the building

    If the building is a cheap pre-engineered tin shed with minimal insulation, a lot of radiant heat is radiating through the light skin of the building to that cold night sky. Slab, or wall- fin, or unit heaters, heat loss is heat loss and energy in = energy lost through the skin of the building.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
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    Late January/early February weather was quite brutal in much of the US this year.

    As Hotrod wisely mentioned 70+F temps in a working space heated radiantly is VERY high. Workers may sincerely have left doors open just to stay reasonably comfortable.
  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
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    Ramp up times Just for fun before bed time

    This graph comes out of one the radiant heat courses I took. It shows the number of hours to raise the temperature over a specified time in hours for various slab thicknesses. Of course a sufficient gpm and flow rate would be necessary also.

    Basically, this chart would indicate that you should have been able to bring the slab up to temperature within 24 - 48 hours - depending on the heat load for the building at start up and adequate btu input and flow rates.

    It sounds like the slab was insulated adequately, but as HR was alluding to, there are lots of other key factors - namely the heat load of the buidling and the heat output that this slab was designed to produce.

    T-Shirt weather inside implies it was overly warm inside - maybe too warm and as HR hinted, maybe some doors were left open a bit. I think asking lots of questions is a very good idea.

    Just for some bed time excercise, I decided to try and calcuate the heat energy needed as you requested.

    The specific heat of concrete and masonry can generally be assumed to be 0.2Btu/lb·°F. (ASHRAE Handbook of Fundamentals, 2005) Using this, you could calculate the energy needed to raise the slab from one temp to the other. Of course we have to make a few assumptions.

    The building is 60 x 150 so thats 9,000 sqft. At 6" deep, that would be approximately 4500 cu ft of concrete. Concrete, Limestone w/Portland has a weight of 148 pounds/cubic foot (according to Reade.com, so the weight of the entire slab would be 4500cuft x 148lbs/cuft = 666,000 lbs. Using the specific heat from the ashrae guide of 0.2Btu/lb·°F we can now calculate the btu's of heat necessary to raise tis slab from say 30F to 70F - a 40F differential.

    The forumula I believe would be Q=MC(Ts-Tf) where Q is quantity of heat, C is the specific heat of the substance, M is the mass, Ts is the starting temp and Tf is the final temp. So heres the math I think the Heat energy = 666,000lbs x .2 btu's/lbs F x 40F =5,328,00 btu's.

    If we assume there was no heat lost to the surrounding building(which we know wasn't true during the winter) and the boiler produced 200,000 btu/hr, it would have taken approx 26 hours (666,000 btu's/ 200,000 btu's/hr) of run time to heat the slab. Wow it makes sense so I may have gotten this right.

    The problem here is, you need to know the heat load of the building during the start up period, add that 666,000btu's to increase the slab temp and divide by the net output of the boiler. Sound right ??
  • jp_2
    jp_2 Member Posts: 1,935
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    specific heat of concrete

    you need to find the specific heat of concrete, multiple that by the assumpted delta, T running temp - cold start temp, then multiple by weight of all the concrete, don't forget the footing.

    btu= specific heat x delta T x slab weight.

    google specific heat concrete" to find your answer.

    you'll most likely find where all those missing btu's went.
  • wondering if

    Wondering if you checked the perimeter of the building for lack of snow or early flowers since it'll proves alittle or lacks of insulations at the footings... Another thing, is there ground water in contact with footing? That'll suck the heat away from the building...


  • Heat cap of concrete: 29.4 BTU/CU FT/DT

    9000 sq ft of concrete x 6 inch depth (assumed)= 4500 cu ft

    4500 x 29.4 = 132,300 BTUs/DT.

    Call it a therm and a half per degree rise. Maybe 60 therms, maybe if you start it up really really really really cold as much as 120 therms.

    That doesn't account for any initial moisture that might need "powering out", I've seen some variability in slab starts that I can only assume are related to moisture (perhaps concrete content, don't know for sure).

    Certainly could contribute, but it doesn't tell the whole story here.
  • Greg_40
    Greg_40 Member Posts: 43
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    Early spring (late winter) flowers is one indicator I saw in some early 1950's slab heated homes. No flowers here, mud from recent rains, alphalt millings, concrete walkway & a lot of concrete approaches are what surrounds this building. It appears that there is nothing in the way of thermal breaks from the slab to the concrete approaches. The GC said his crew installed the sub slab concrete, but he couldn't recall any perimeter insulation and he was positive there was no perimeter insul. at the seven 16' overhead door approaches. There is no high water table, but a higher than normal rainfall lately probably has pulled some heat from the perimeter. I have a thermal imager that I will take next time to try to check for perimeter insulation and breaks at the doors.

    I thank you one and all for the help with calculations and advice on figuring start up slab Btu needs. Couple that with a heatloss of near and several nights exceeding design temps for most of January and we see that the Munchie probably ran with it's tongue hanging out all month long. No one will give me an accurate run time of the Hotsy, but since that's their business- cleaning tanks, I can only assume it is on lots. Last month's consumption is in the high 200 Therm area, which I would say is compatible post start-up. Our HDD for this area were over 36% higher than last year and around 12% higher than the last ten years' average for Jan. & Feb.

    Now, if I can finalize some calculations and put this into English that an oil man will understand and accept, I've got it made. If you've never dealth with oil field people, whisky used to be a necessary sales tool in the old days. Maybe I need to bring a bottle, sit the guy down and get to the bottom of it! Thanks!
  • Tim Doran_4
    Tim Doran_4 Member Posts: 138
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    Doesn't Seem possible

    2051 therms divided by thirty days is 68.36 therms per day or 2.85 therms per hour which is 285mbtu per hour. I don't think that the Munchkin could have possibly been that over fired. 199mbtu input is basically 2 therms give or take. It seems to me that the gas had to go somewhere else.

    Tim D.
  • Weezbo
    Weezbo Member Posts: 6,232
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    Well, just how many of these tanks ....

    did they run in and out of the shop that month? every time one is brought into the building it eats heat, every time the doors are left open for hours on end navigating things in and out eats up heat, the equipment to move them is what i would call Unknown appetites of existing heat...all these things could draw heat right straight out of the slab...that is a heck of a Lot of therms to burn though...if you have trucks that have materials brought in one day and rolled out the next...i have to think it could absorb all the heat out put and a Healthy chunk of capacitance within the slab...then every day the door opens and the semis roll out...they are off to parts unknown ,re distributing BTU's :)

    Observation of the operation might be in order.....

    the equipment (Boiler) has fairly set amount of maximum useage...unless there is something else packing out the nat gas....
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