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Thread: 3x Oversized ModCon with Concrete Slab, New Home

  1. #1
    DIY Member Buffalobillpatrick's Avatar
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    Default 3x Oversized ModCon with Concrete Slab, New Home

    Hello, this is my 1st post here. Great Site BTW.

    I'm designing my new ICF home. 2900 ft2 slab heat 3-zones with master/slave zone synchronization.

    I did a ResCheck DOE heat loss estimate with my proposed plans & got UA = 400
    Location is Cripple Creek Colorado so my design Temp is about -30*F, thus at 70* indoor my Delta T is about 100* 400 x 100 = 40,000 BTU/hr.

    I have a new in box Buderus GB142-45.

    Looks like this boiler is about 3x oversized after being derated for 10,000' altitude.

    I did the math on my slab, 40yds concrete x 3,800#/yd (with rebar) = 152,000#
    With a Specific heat of .2 it will take 30,400 btus to raise the slab 1*f

    So at -30* my boiler duty cycle will be about 33%, at freezing (this is a guess) possibly a 10-15% duty cycle?

    So my question is: Should I sell the Buderus & buy smaller ModCon OR will slab act like a large buffer tank & Buderus will perform fine with low duty cycles?

    Thanks BBP

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The boiler will work, but it won't reach its rated efficiency. I think you'd be better off with the smallest version they sell. If I ever build a new house, I really like the concept of ICF construction...hope yours turns out great. Some of the other people may have some more specific help. Max comfort levels are reached by longer, lower temp burns. The outdoor reset will help, but that one just can't modulate down low enough.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Member Buffalobillpatrick's Avatar
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    Default More Info

    jadnashua, I think that this Buderus can modulate low enough as the slab can absorb a large amount heat quickly from PEX.

    My 1st source was wrong on weight of reinforced concrete, S/B 4,050# / yd3 = 162,000# x .2 = 32,400 btus to raise 1* F
    Additional 10,000# doesn't change my question, just an even larger buffer effect.

    Plans are to use SPF 2# foam at R30 under floating slab with extra perimiter foam on outside of ICFs, below grade. Have bought 4,000' O2 barrier PEX, each of the 20 loops will be 200'

    Am using a constant circulation Grundfos 15-58 Brute pump in each of the 3 zones.
    Miniature 15 watt zone injection pumps 3gpm each. Slab overshoot protection sensor (Fenwal) & an air thermostat in series in each zone.

    Great-room / kitchen is largest zone with the most windows, this will be the master zone in my zone synchronization. Each bedroom zone will be slave, & can only call for heat if the master is also calling for heat. Through a latching relay design, a bedroom zone can continue to recieve heat after the master is satisfied.

    If I sell the Buderus, I will loose about $300 due to shipping.

    So what's wrong with a low duty cycle?

    BBP
    Last edited by Buffalobillpatrick; 09-26-2011 at 11:31 PM.

  4. #4
    DIY Member Buffalobillpatrick's Avatar
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    I was thinking along the lines of 20 minutes on & 40 minutes off at design temp -30*
    This would be what I would call a 33% duty cycle.

    Depending on how I setup the ODR, I think I could have pretty long burns with this much mass absorbing the BTUs.

    Planning for a 10* boiler Delta-T

    Thanks, BBP

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    Each time the boiler shuts off, it wastes heat and it takes it a bit to reestablish high effiency to it, too. Ideally, if the boiler could exactly match the need, it would run constantly. On/off cycles hurt both efficiency and life. Thus, the desire to get a unit properly sized for the load to minimize those cycles. In slab temps generally need to be fairly low (as opposed to baseboard) or you can damage things (crack your slab, for example), so proper mixing and supply temps are critical.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    In the trades Dana's Avatar
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    The thermal mass of the slab is PLENTY of buffer to keep minimum-burns sufficiently long- it won't be short-cycling itself to either low-efficiency or an early grave. If the smallest zone was a low-mass staple-up radiant or fin-tube you could potentially run into problems, but not with concrete. The hysteresis of the boiler's controls is enough to keep the minimum burn times long, even if you don't end up with continuous modulated burns the way it would if the boiler were right-sized for the load. Ideally the min-mod would be less than half your design-condition load- yours will cycle more often than a right-sized boiler, but not 10x more often.

    Just for yuks, what's the thermal mass of the section of slab comprising your smallest zone, and what is your required water temp at design condition?

    The non-derated minimum modulation of the GB142/45 with 122F water is ~48K, (42K with 176F water) so with derating for altitude it IS about "right sized" as a non-modulating boiler if you set the ODR curves so that it rarely leaves min-mod, and with high-mass radiation it'll do just fine.
    Last edited by Dana; 09-27-2011 at 12:12 PM.

  7. #7
    DIY Member Buffalobillpatrick's Avatar
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    Each of the 2 bedroom zones are about 50,000# of concrete, with great-room kitchen zone at about 62,000#

    Note: that with my zone syncronization design, a bedroom zone will not call for heat without the Great-room zone also calling for heat.

    with a 40,000 btu design load and 2900ft2, and each degree that the floor is above my stat. setpoint of 70* provides 2 btus/ft2
    (40 000 / 2 900) / 2 = 6.9, 70 + 6.9 = 76.9 average water temp in loops, & i'm planning on about a 10* boiler delta-T , So about 82* water injected from my Hydraulic Seperator / Low Loss Header.

    Thanks Dana, I have read many of your informative posts.

    BTW, on another site where I have this discussion going, they have me talked into selling the Buderus & getting a much smaller boiler. I really will not take a beating on the Buderus as they have gone up in price since I bought it.

    The TT Solo 60 that I am fighting with in my current house would be about the right size, if I could just get it to work at high altitude. BTW after repeated clocking of how long it taked to heat up my Lochinivar 80gal DHW tank, it seems to only put out about 35Kbtus on high fire.

    I also like what I've read about the Knight Lochnivar WHN085 with the Fire Tube heat exchanger, I'm waiting for a quote on one with the High Altitude version controler.

    BBP
    Last edited by Buffalobillpatrick; 09-27-2011 at 01:59 PM.

  8. #8
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Thermal mass is not considered in any proper heat load analysis, which is where every professional starts before sizing a boiler. This is where every savvy homeowner starts as well.

    You can do the math but you can't determine the design temperature without a accurate heat load.

    Your killing me here.

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