Boiler for Radiant Slab Pole Barn

Discussion in 'Boiler Forum' started by Reicherb, Oct 16, 2020.

  1. Reicherb

    Reicherb Member

    Joined:
    Nov 7, 2013
    Location:
    Linden, MI
    I have a new 30x40x14 barn that has lean tos that could be enclosed some day bringing the square footage to 50x40. It's got 2" foam under 5" concrete. The ceiling will be blown in insulations. The walls will be spray foam. The overall 50x40 space has nine 1/2" loops for a total pex length of 2000' ft. I'm in central Michigan and am looking for a natural gas boiler to heat the building. I may eventually have domestic hot water as well.

    Can you please help me select the correct boiler or perhaps an on demand water heater if that is a viable option.

    Thanks,

    Brad
     
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    It's probably not a bad idea to pay an engineer to calculate your actual heat load. Most heating contractors will way oversize the boiler which can lead to less comfort, reliability, and higher equipment costs. They will take into account the exposure, windows, doors, insulation levels and your zone.

    Is this all going to be one zone? Some suggest a maximum length loop of 200', so you're a bit higher than that. One zone, versus more will dictate the pump(s) needed. Don't oversize it for the use of domestic hot water unless you expect to be needing huge quantities for extended time frames. A bit for a shower or home kitchen an indirect on a priority zone is nearly always sufficient since the slab will have a lot of mass, diverting heat to reheating water in the indirect is not an issue.
     
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  4. Reicherb

    Reicherb Member

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    Nov 7, 2013
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    Linden, MI
    The main building has 5 of the 9 loops. It will all be one large open space. Some day there may be an open loft in part of it. The other 4 loops will eventually be enclosed into 4 10x20 rooms. 1 or 2 soon and the other two, years down the road. There are 4 3x3 windows and two 12x14 insulated overhead doors.

    This will not be living space. It's a shop. I intend on generally heat it to 40-60 degrees (maybe 70 on a rare occasion) depending on how I'm using the space. The domestic hot water is for a half bath in the building as well as a utility tub. I don't expect frequent use so having some form of domestic hot water is great but not worth adding much to the cost.

    In the future, I could turn one of the 10x20 rooms into a man cave living space.

    I'm already above my budget on the project so paying an engineer seems like a last resort. I was able to spec and install a boiler for my home using help from this forum a year ago and I'm very happy with the way it turned out. My home is more comfortable and my utility bills went down.
     
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    You need to know the ceiling area, the wall area, the exposure, the infiltration rate, the R-factor of your insulation and the area and U-factor for the windows and doors...there are calculators that will let you input that sort of information that are free, but to get a better result, you need to often pay for a better answer. Then, you need to know your local degree-day heating requirements and your design temperature. Keep in mind that a slab like that will have a huge thermal mass, so any changes you want to make will require quite a long time, often days to make a radical change...it's not like bumping the thermostat up on a forced air system...or even radiators, you are trying to change the temperature of the entire slab. Generally, people will be more comfortable at a lower temperature with a radiant floor than other types of heating. MI can get frigid.

    You will also need to know the spacing and length of the loops to determine how much energy transfer is even possible. You don't want it to require a super hot floor, as that's not comfortable nor is it very efficient. The slab will have issues if you try to get it too hot, and the pex won't like it either as it will be expanding and contracting more if the delta-T is high. Generally, the actual delta-T isn't very big in a slab, so that isn't an issue at all.

    You need to look at what the minimum return water delta is allowed on whatever boiler you choose, as that can be radical, especially on startup from cold soaking. A condensing boiler can generally handle fairly cold return water, but you do need to keep that in mind.
     
  6. Reicherb

    Reicherb Member

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    Nov 7, 2013
    Location:
    Linden, MI
    This is a pole barn. I'm not looking for residential comfort. There must be a typical situation that would apply here.
     
  7. Reicherb

    Reicherb Member

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    Location:
    Linden, MI
  8. Dana

    Dana In the trades

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    Jan 14, 2009
    Location:
    01609
    Not paying for a competent pro to design and fully specify the system can end up costing more in the end.

    But at the very least, run a LoadCalc or CoolCalc load calculation on this project using aggressive (rather than conservative) assumptions about air tightness, R-values, etc or the tools will end up oversizing by a large factor (even by 2x or more). With appropriately aggressive assumptions it will still overshoot reality by a bit, but only rarely more than 1.5x compared to what a competent engineer would get using more flexible professional tools.

    That said, the AB-80C is almost certainly enough boiler for your heating load, but on the skimpy side if you need to run a shower in the dead of winter, and would be a bit tedious for drawing 20 gallon batches of at a time. It's water tube heat exchanger won't tolerate the flow rates your radiation will need, so it needs to be plumbed primary/secondary with a properly sized pump for both the primary (boiler loop) and another properly sized for the secondary (radiation loop), with an adequate hydraulic separator where the loops meet. (Closely spaced tees using fat enough pipe for low turbulence at the target combined flow rates will do.) Read and understand section 17 in the manual, beginning on p.35 if designing and installing the system completely DIY.
     
  9. Reicherb

    Reicherb Member

    Joined:
    Nov 7, 2013
    Location:
    Linden, MI
    Dana, I'm thrilled that you joined the discussion. You helped me spec a boiler for our house. It's working very well.

    I'm not sure how well those tools are going to work. This is a 2000sqft open pole barn. It's got 14' walls with 2" of spray foam. The ceiling isn't yet insulated but will be with blown in insulation. I'm not yet ure of the R value. There are 2 12x14 insulated overhead doors, a fiberglass entry door with no windows and 5 3x3 dual pane windows. The floor is 5" concrete with 2" rigid foam under and around it.

    The domestic hot water is simply for a sink in a half bath.

    Thanks.
     
  10. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609

    The laws of physics work the same in all buildings. You'd have to estimate the U-factor of the pole barn's wall base on construction and select a wall type of similar U-factor in those freebie tools.

    If the 2" of foam is R13-R14 closed cell and CONTINUOUS, covering all of the structural posts/beams etc it would be comparable to a 2x6/R19 framed wall, or a 2x4/R11 wall with a half-inch to inch of exterior foam. If it's not continuous be prepared to do a bit more arithmetic.

    Manufacturers of insulated doors usually specify a U-factor.
     
  11. Reicherb

    Reicherb Member

    Joined:
    Nov 7, 2013
    Location:
    Linden, MI
    Apparently I'm using the tool way wrong as I put in some rough numbers to get in the ballpark and it came up with 86,000 BTUs of heat load. Can that be right? The building is 40x50x14. I used 14' ceilings even though it's only 14' at the center. It's 10' on the edges.

    upload_2020-10-31_19-50-9.png
     
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