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Do you trust your numbers?

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Boiler replacement job; 1,800 square foot slab-on-grade 1940's house. My heat calculations come out to 77,000 BTU's which is 43 BTU's per square foot (no underslab insulation, old wall and ceiling insulation, single glazed windows). All is good, no?
The boiler that died is a Laars JVS-160H which has a "heating capacity" of 136,000 BTU. Everything I've learned is to size to the heatloss of the house, but the existing boiler size is bugging me. Would you size the new boiler to 77,000 or 136,000 BTU? That's quite a difference.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab

Comments

  • hot_rod
    hot_rod Member Posts: 22,144
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    I doubt that you could ever transfer 43 btu/ sq ft?

    Rule of thumb 2 btu/ per sq ft for degree difference for a slab.
    92 slab surface -70 ambient room =22 X 2= 44 btu/sq. ft.

    You would need to run a 92 surface temperature! I doubt you could stand on that slab comfortable, granted this is at design.

    Some say 30 btu is possible on a slab so 85 surface-70= 15 X 2 = 30 btu, 85 is still a hot surface temperature.

    I'd size it for 1800 x 30 btu/ ft = 54,000 btu/hr input. Either a 75K conventional, maybe a 50K mod con.

    Still in the bay area I assume :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    kcopprick in Alaska
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,296
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    Hello, Wouldn't it make sense to, at a minimum, air seal and insulate the ceiling and then put in some better windows before sizing the heating system? $ saved on the smaller system could help offset the shell tightening costs and comfort would be easier to achieve. B)

    Yours, Larry
  • Zman
    Zman Member Posts: 7,569
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    If you want a bit of reassurance, you could clock the meter during a cold snap. Just stop by and take a picture of the meter a couple times and do the math.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    kcopp
  • GW
    GW Member Posts: 4,693
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    Your heat loss seems realistic. I get all happy when I see a big boy I’m about to remove. Friday we installed a 40,000 furnace in a ranch home (downflow, supply ducts in the slab). We pulled out an oil furnace, probably double or more the size of the new furnace. I didn’t look at the nozzle or technician service cards.

    I didn’t do a heat loss. After doing hundreds of heat loss calculations, I know an older ranch is 20 or 25k, just guessing. A two stage 40,000 input—-perfect
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
  • Alan (California Radiant) Forbes
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    Thanks guys for the support and thank you HR for the math! Good stuff! Yes, still in Berkeley.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • ChrisJ
    ChrisJ Member Posts: 15,687
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    > @GW said:
    > Your heat loss seems realistic. I get all happy when I see a big boy I’m about to remove. Friday we installed a 40,000 furnace in a ranch home (downflow, supply ducts in the slab). We pulled out an oil furnace, probably double or more the size of the new furnace. I didn’t look at the nozzle or technician service cards.
    >
    > I didn’t do a heat loss. After doing hundreds of heat loss calculations, I know an older ranch is 20 or 25k, just guessing. A two stage 40,000 input—-perfect

    wouldn't the size of the ranch be important? I've seen everything from 800sqft to 2000sqft
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • GW
    GW Member Posts: 4,693
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    Chris yes I wasn’t very clear, you’re right.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
  • ChrisJ
    ChrisJ Member Posts: 15,687
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    > @GW said:
    > Chris yes I wasn’t very clear, you’re right.

    Curious which size you were working on. I assume smaller?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • GW
    GW Member Posts: 4,693
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    Yes about as small as they come. Maybe 38 x 26 just guessing
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
  • GW
    GW Member Posts: 4,693
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    Years ago our zero% loan program (Mass Save) required a heat loss so I did many. I’ve also done classes with Wrightsoft. Also the Loan program needed a heat gain for AC installations.

    My service tech called me the other day, temp rise too high. Anyone care to guess what a contractor installed in this home 7 years ago?
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    FWIW, even on jobs that I personally performed loss and gain calculations for and did the actual installation, I have yet to see any of them running at 100% of capacity even at design conditions. Best I've seen is a 50% duty cycle.

    Everyone assumed you're doing a RFH system, except me :smile: Regardless of the delivery system, go with your gut and your loss calculation, but make certain you have adequate output capacity, or your system too might short cycle itself to death.

    If everything goes to hell in a hand basket, and you are asked to show proof of due diligence, you've got it covered.

    Say hi to the wifey. And come to Colorado this year why dontcha.

    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • GW
    GW Member Posts: 4,693
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    Right on! And who’s wife are you saying hi to hahaha
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
  • Alan (California Radiant) Forbes
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    Thanks, ME. She says she wants to do a road trip this year. Utah, New Mexico; maybe we'll swing by or rendezvous somewhere.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    Mark Eatherton