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Heat loss formulas and questions

Pulse
Pulse Member Posts: 78
edited December 2022 in Radiant Heating
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U VALUE = 1 / R VALUE(0.2 * INCHES(SLAB THICKNESS))
DELTA T = INDOOR TEMP - GROUND TEMP
HEATLOSS = U VALUE * SQ FT * DELTA T
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Is the above formula correct to calculate the heat loss through a slab? I have a 4" uninsulated basement slab. For just under a 400sqft basement, I have a BTU loss of ~9500, in the concrete slab alone. That seems extremely excessive?

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Air infiltration using area
CFM50 = BLOWER DOOR TEST
CFH = CFM50 / 20 * 60
DELTA T = INDOOR TEMP - DESIGN/OUTDOOR TEMP
HEATLOSS = (CFH * (ROOM EXTERIOR SQ FT / HOME EXTERIOR SQ FT)) * DELTA T * 0.018
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Air infiltration using volume
CFM50 = BLOWER DOOR TEST
ACH50 = CFM50 / TOTAL HOME VOLUME * 60
ACH(NATURAL) = ACH50 / LBL FACTOR
*OR*
ACH(NATURAL) = ((CFM50 / 20) * 60) / TOTAL HOME VOLUME
HEATLOSS = ROOM VOLUME * DELTA T * ACH * 0.018
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Which air infiltration formula should I be using or is there a better one? I am relying on a CFM50 value from a blower door test done 10 years ago. Is it possible to calculate without the blower door test?

Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 16,495
    What temperature difference did you use for the basement floor? Are you heating the inside to 70 deg. The floor temp won't be 0 it will probably be in the 40s or 50s. That will affect your calculations so your TD will change
  • Hot_water_fan
    Hot_water_fan Member Posts: 2,040
    If you have the fuel usage, you can calculate heat loss that way too - you’ll be looking at what actually occurred that way 
  • Pulse
    Pulse Member Posts: 78
    edited December 2022
    @EBEBRATT-Ed
    @Hot_water_fan

    1.25(U) * 380(SQFT) * 20(70 indoor - 50 ground temp ) = 9500 BTU

    Right now there are 3x 4' electric baseboards down there. They do not run constantly/excessively. I just want to make sure I do not create a sauna down there when I size my panel rads.

    The boiler will also be down in that area, should I subtract a certain amount of BTUs for it?
  • Hot_water_fan
    Hot_water_fan Member Posts: 2,040
    What are the baseboards rated? Turn two off and see how it does. 
  • mattmia2
    mattmia2 Member Posts: 10,955
    The 10 year old blower door test is probably a better number for infiltration than you would get trying to guess.
  • Pulse
    Pulse Member Posts: 78
    edited December 2022
    Hot_water_fan said:

    What are the baseboards rated? Turn two off and see how it does. 

    240/1000, so i guess ~3400 BTUea, so maybe i was overthinking this

    My main concern is just the air infiltration then, if I am calculating that properly
  • Hot_water_fan
    Hot_water_fan Member Posts: 2,040
    What’s the fuel usage? The odds of a heat loss calculation of the manual J variety coming in undersized are slim. And the smallest boilers are usually much bigger than the typical load. 
  • Pulse
    Pulse Member Posts: 78
    Hot_water_fan said:

    What’s the fuel usage? The odds of a heat loss calculation of the manual J variety coming in undersized are slim. And the smallest boilers are usually much bigger than the typical load. 

    the load @ 72 inside temp:
    ~32K BTU @ 20 degrees outside
    ~37.K BTU @ 7 degrees outside(design)

    Boiler will be an 85k(79k actual, 8500 min fire)

    I know you shouldnt size for the indirect, but i was thinking about a 110k boiler. It brings my GPM recovery up to 2.25(101K) vs 1.75(79K). The min fire would be 11K vs 8.5K

  • hot_rod
    hot_rod Member Posts: 23,405
    Actually, you should always size to the largest load. If you want high performance indirect capacity, the boiler needs to cover that.
    In todays well insulated, efficient homes, the DHW load is often the largest load.

    The mod cons are much better suited to zoned systems compared to fixed output boilers in most all cases.

    It's rare to see a standard 110K cast boiler turn down to 11K :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Ironman
    Ironman Member Posts: 7,555
    What’s being overlooked in calculating the slab is that up to 60% of the heat loss is at the edge and perimeter.
    With a 70* delta T, you should use 56.7 btus per lineal foot with no edge insulation or 14.7 btus per lineal foot with 2” of edge insulation.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    STEVEusaPA
  • Pulse
    Pulse Member Posts: 78
    edited December 2022
    hot_rod said:

    Actually, you should always size to the largest load. If you want high performance indirect capacity, the boiler needs to cover that.
    In todays well insulated, efficient homes, the DHW load is often the largest load.

    oh, good to know, but I would be buying a mod con.
    Ironman said:

    What’s being overlooked in calculating the slab is that up to 60% of the heat loss is at the edge and perimeter.
    With a 70* delta T, you should use 56.7 btus per lineal foot with no edge insulation or 14.7 btus per lineal foot with 2” of edge insulation.

    For me that would be ~59' * 56.7 = 3345.3 BTU

    I am now at:
    9500 BTU - slab
    3300 BTU - slab edge
    2600 BTU - windows/exterior walls/air

    15400 BTU loss for a 380 sqft finished basement, that equals the heat loss of my other 1600 sq ft of living space..lol

    seems incorrect..

  • Hot_water_fan
    Hot_water_fan Member Posts: 2,040
    15400 BTU loss for a 380 sqft finished basement, that equals the heat loss of my other 1600 sq ft of living space..lol

    seems incorrect..
    For sure ! Just use your fuel usage, these calcs are not getting you anywhere. 
  • Pulse
    Pulse Member Posts: 78
    edited December 2022
    Hot_water_fan said:

    15400 BTU loss for a 380 sqft finished basement, that equals the heat loss of my other 1600 sq ft of living space..lol

    seems incorrect..
    For sure ! Just use your fuel usage, these calcs are not getting you anywhere. 
    I am sorry, but what do you mean fuel usage?

    I have electric heat currently
  • Ironman
    Ironman Member Posts: 7,555
    Pulse said:
    Actually, you should always size to the largest load. If you want high performance indirect capacity, the boiler needs to cover that. In todays well insulated, efficient homes, the DHW load is often the largest load.
    oh, good to know, but I would be buying a mod con.
    What’s being overlooked in calculating the slab is that up to 60% of the heat loss is at the edge and perimeter.
    With a 70* delta T, you should use 56.7 btus per lineal foot with no edge insulation or 14.7 btus per lineal foot with 2” of edge insulation.
    For me that would be ~59' * 56.7 = 3345.3 BTU I am now at: 9500 BTU - slab 3300 BTU - slab edge 2600 BTU - windows/exterior walls/air 15400 BTU loss for a 380 sqft finished basement, that equals the heat loss of my other 1600 sq ft of living space..lol seems incorrect..

    No, just use the edge calculation for the slab.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Pulse
    Pulse Member Posts: 78
    edited December 2022
    @Ironman

    Ah, alright only use edge

    So where did you get your figures. The only formula I can find is:

    F-factor * Perimeter * Delta T

    For F factor, I found two charts, one said .73 for uninsulated, the other went by climate zones, which for my zone 5 is: .54

    .73 * 59' * (72-20) = 2239 BTU
    .54 * 59' * (72-20) = 1656 BTU

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