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help sizing cast iron rads (leo g)

to all of you who took the time. i got back there today, and unharrnessed the rest of the rads (17 in all) so the painters can get behind them. i am now trying to talk the builder into getting them sent out and sand-blasted, then recoated to their original bronze look. should find out by weeks end!

there are so many different styles and sizes in this house, just crazy!!!

am now trying to locate about 3 more for the "new york loft" so when i do, i'll give you guys more info, to see if we have "enough" coverage. til then, i guess i should order dan's book eh?

leo g


  • leo g_54
    leo g_54 Member Posts: 4
    big room, big heatloss

    hey, can anyone help me with sizing for this "New York Loft" that a client wants to put on the top floor of their home?

    I have done a heatloss, and come up with 46,000 btu's. My problem is that they want to stay with the "old-fashion" hot water cast iron rads, and I have no idea how to tranfer btu's to radiation!!!


    leo g
  • Mad Dog
    Mad Dog Member Posts: 2,595
    Hey LEO is my middle name next to Mad Dog

    That heatloss is huge.....is there no insulation? or is it that big. NY loft....het it could be an old loft in the meatpacking district, right? It's very simple, give us a size, type, or brand of cast Iron radiation , and we can help you. There are so many different sizes and shapes of CI rads. Mad Dog

    To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"
  • EDR to BTUH

    Rads might be rated in Sq Ft EDR, which is 240 BTUHs per sq ft as steam, and lower as hot water.

    190° F water is 190 BTUH per

    180° F is 170 BTUH

    170° F is 150 BTUH

    and on and on.


  • This is NOT the formula that you will find in many U.S. texts regarding sizing for iron radiation. Most of those tables will show that a 90° radiator in a 70° room has ZERO output. I can ABSOLUTELY AND FOR CERTAIN say that this is wrong!

    While I don't belive the output of standing iron radiation to be perfectly linear, I do believe it is VERY close to linear in the range of say 85° - 150°; many/most iron systems will operate in this range MOST OF THE TIME.

    (Avg. rad surface temp - room temp) * 1.5 * EDR = BTU/hr

    You can VERY safely assume that the average radiator surface temperature will be about 10° below the supply temperature AT OUTSIDE DESIGN TEMP.

    Size your radiation based on your desired average radiator surface temperature at OUTSIDE DESIGN TEMP. Remember that supply temp will be about 10° ABOVE this temperature!!!!

    All below are assuming 46,000 btu/hr in a 70° space:

    180° = 279 square feet EDR

    170° = 307 square feet EDR

    160° = 341 square feet EDR

    150° = 383 square feet EDR

    140° = 438 square feet EDR

    130° = 511 square feet EDR

    120° = 613 square feet EDR

    FOR REFERENCE: A thin-fin-tube radiator of approximately 100 square feet EDR occupies a space about 45" wide x 38" high x 10" deep. Plan for the radiator to be 2½" from the FINISHED surface of the wall behind.

    Choose the appropriate radiator size/temperature based on the type of boiler and/or control system you are using.

    This is HIGHLY IMPORTANT and VERIFIED by "standard" formulas for computing radiation: IF an object is MAINTAINING the temperature in a space the LOWER ITS TEMPERATURE THE HIGHER ITS PROPORTION OF RADIATION!!!!!!!!!!!!!!!!!!!!!!!

    In a space of the proportions I am imagining that would have that sort of heat loss I would sincerely suggest using a 3-4 HUGE radiators in the 125 - 150 square foot EDR range. This would likely be VERY similar to "original" sizing in similar spaces in NYC. Of course actual layout if spaces that are not completely "open" like bathrooms. If at all possible consider radiant floors in the baths.

    Radiators make a design statement. Smaller rads in such a large space would somehow seem "inadequate".

    If at all possible use TRVs or some other method of PROPORTIONAL control with iron radiators.

    This is a rather new and certainly controversial opinion, but when I looked it up it appears to be gaining ground in Europe, where iron rads are still common and TRVs are common as well.

    IF you are using proportional control AND the windows are BOTH reasonably low in infiltration AND reasonable in size, place the radiation IN VIEW OF THE OUTSIDE WALLS/WINDOWS, not underneath windows/on outside walls as suggested by the Dead Men.

    I would not do this if the space has huge non-thermal-break metal sash, poor infiltration characteristics or is single pane, clear glass without storm windows.

    Why this placement? The radiator will "see" the coldest portions of the space--the outside walls/windows. This MAXIMIZES radiation and MINIMIZES convection.
  • leo g_54
    leo g_54 Member Posts: 4
    here ya go mad

    dog, this is typical of the style throughout.

    thanx! leo
  • leo g_54
    leo g_54 Member Posts: 4
    thanx Noel

    one thing, to measure the square footage, does that mean inside , sides, and back of every section?


  • leo g_54
    leo g_54 Member Posts: 4
    i'll tell ya mike

    thanx , this one's going to the archives in the hope that one day i will really understand what you meant!

    leo g
  • Leo, yes on that.

    Each section will have a value, (1.5; or 2 1/3; etc.) published, and you would just multiply that by the number of sections.

    Keep in mind that these numbers are in square feet and the BTUH number is a variable number related to water temperature. That's why they are rated in EDR instead of BTUs.

    Like Mike said.

  • How tall and how wide is that?

    I can count the sections and see that the section is divided up the center, and only once.

    Some called that a single column, some called it 2-column. The numbers will enable us to help you rate it.

    And I left my copy of EDR at work....


  • Boilerpro_3
    Boilerpro_3 Member Posts: 1,231
    Mike, I use that formula too...

    I think it was from Audel's. It applies to radiators only, not convection. I hae other charts that have seperate columns for the heat output of cast iron radiation and convection based on the EDR. They are different. Not all EDR's are created equal.

    Everything you say agrees with all that I remember from my chemistry and physics courses. I think the reason rads were placed under windows in the old days was because of the large infiltration. However in new, well built homes, (not necessary the standard "high efficiency" custom homes typically being built, infiltration is low and radiant losses are much bigger.

    Your comments about higher proportion of radiation at lower temps also make perfect sense, and are well supported by ASHRAE heat loss calculation methods, if you look closely. Convection works by temperature difference between the air and heated object. The hotter the object, the faster the air movement, and air movement is not linear to object temp. If you look at the infiltration factors used for heat loss calculations, the mulitplier grows as the temperature difference increases. Looking at the average construction column, it goes from 0.9 air changes per hour at a 20F outdoor temp to 1.3 air changes per hour at a -30F outdoor temp. Thats an acceleration of about 60%, without accounting for the greater temperature difference between indoors and out. Example:

    Let's say you have a home with 10,000 cu ft of air at 70F interior temp. AT 20F outdoors (50 F delta tee)you have a heat loss due to infiltration of 10,000 cu ft x .9 x 50F x.018 infiltration factor = 8,100 btu/hr. Now if you double the delta tee to 100F, -30F outdoors, you have 10,000 cu ft x 1.3 x 100 x .018 = 23,400 btu/hr. See....Only double the delta tee, but almost triple the heat loss, or transfer.

    Mike, you ain't crazy, at least not about this.

  • Probably an American Rococo

    (A couple others are similar but rad sizing was quite standard between brands.)

    If doorknob is at "standard" height, that rad is 38" high and each section is 8 sq. ft. EDR so the entire rad is 64 sq. ft. EDR.

    If doorknob is a bit higher than normal rad is 44" high and each section is 10 sq. ft. EDR so the entire rad is 80 sq. ft. EDR.
  • leo g_55
    leo g_55 Member Posts: 1
    i am

    going back to the site today, i will endeavor to get more specifics.


    leo g
This discussion has been closed.