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What's the sq.ft. EDR for modern rads?

I’ve just installed a mod/con boiler and am playing with the controls to achieve the most efficient settings.  I would eventually like to remove the room thermostat and let the thing run on the outdoor reset curve alone.  I have lots of radiation and a boiler that modulates from 15-150 MBH. 


Because I have lots of radiation (it’s an old house), rather then just picking a number, I am trying to calculate the Design Supply Temp. at the Design Outdoor temperature of –5F for my area.  I figure (or hope) it may be about 110F.  To calculate my Design Supply Temp., I will divide my Heat Loss figure by the sq.ft. EDR of installed radiation in the house, and match the result to a table to get my Design Supply Temp.


Now, I can look up and calculate my sq.ft. EDR of my cast iron rads, but I have a combination of cast iron, Stelrads and Mysons, as well as a small radian floor zone.  But I don’t have the sq.ft. EDR for Stelrads and Mysons.  Technical Brochures for the Stelrad list Surface area in sq. meters, but I don’t think it’s as easy as doing a metric to imp. conversion, because the figures I get don’t check out.


I tried to back into a sq.ft. EDR for the Stelrads and Mysons by taking a rating in Btu/hr and dividing by a known output factor. For example, a Stelrad is rated at 4100Btu/hr @ 140F supply.  So I take my rating of 4100Btu/hr @ 140F supply and divide by the radiation factor of 125 Btu/sq.ft. @140F and get 32.8 Sq.Ft. EDR for this Stelrad.  Am I doing this correctly?


Any other suggestions?


  • CMadatMeCMadatMe Member Posts: 3,066
    Best I can do for you

    Is a correction chart but its for another companies panel rads that I deal with. Should get you in the ballpark.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Vlad_ENGVlad_ENG Member Posts: 3
    Thanks, good info, but it does not state sq.ft. EDR heating surface

    Thanks for the info. Those are pretty nice looking rads, with lots of choices for style, and hookup.  The prices are decent as well for us here in Canada. 

    The correction factor sheet is good info too, but I was actually looking for something to compare to the old style collumn cast iron rads, being sq.ft.EDR. I think I may be overcomplicating it, but I need a common factor to compare. 

    Like I said, it may be as simple as taking the known radiation rate of 125Btu/[email protected] 140F supply.
  • Charlie from wmassCharlie from wmass Member Posts: 4,158
    edited December 2009
    Maybe go the other way?

    You can easily get the BTU output for the old rads for a given temp. then compare them to the new ones that way. This would get you to a comparison you could use.E.D.R. is more of a steam figure from what I have seen and since they do not use the new rads for steam they do not have those numbers. At least we do not use them for steam I am sure someone will.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • NRT_RobNRT_Rob Member Posts: 1,013
    as charlie says

    I would use the EDR conversion to figure the output of the existing rads, and use the MFG data/correction factors directly to figure output of the new rads.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • CMadatMeCMadatMe Member Posts: 3,066
    Try this

    Don't know if this may help you a little more.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Vlad_ENGVlad_ENG Member Posts: 3
    edited December 2009
    I went the other way

    Thank you gentlemen, and thanks for the extra info radiantwizard.   I went the other way and backed into the sq.ft. EDR for the modern rads by dividing the figure 125Btu/sq.ft. @ 140F into the adjusted BTU/hr ratings for those rads at 140F. 

    So I came up with an approximate value of 928 Sq.ft. of radiation for the house (radiators only, did not count exposed pipe in basement). My heat loss for this house is 118,000Btu/hr if I use the option "Loose House" for how well the house is sealed (the house is 95 years old and insulation is sparse, although basement and a few rooms on main floor are well insulated). 

    If I use the option "tightly sealed" the heat loss comes down to 78,000Btu/hr.  So dividing my 928 into 118,000 gives me a required 127 Btu/sq.ft. of radiation.  If I use the tight heat loss I get 84 Btu/sq.ft. of radiation.  Now using the "Btu load on radiation" table I get a Design Supply Temperature of 155F for the "loose house" or 136F for the "tight house".   (These are my temperature settings for my reset curve, for the design outdoor temperature of -5F for the Toronto area.)

    But I must have gone wrong somewhere.  On the coldest day of the year, the supply temperature is no where near that, I would say its about 120-125F. 

    The outdoor temperature today is 32F, and I have my Design Supply right now set at 106F (again for an outdoor of -5F) and the house is being heated quite adequately with a boiler supply of 99F water.

    Is my logic and calculations wrong?
  • CMadatMeCMadatMe Member Posts: 3,066
    If Your Comfortable

    with your design and the house is comfortable then you are ok. The numbers get you in the ballpark but an individuals comfort is a homerun. I'm sure you will keep tweaking throught the season to get it where you want. After that you should never have to touch it again.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Charlie from wmassCharlie from wmass Member Posts: 4,158
    The edrs are not exact.

    The heat emmitters have allowances and factors built in. So if they are rated say 9 sq ft the may actually produce 11 sq ft. It is funny how when the units need to be rated they are still rated low for output. I say adjust to a temp that suits you as the other poster said and leave it be. Even the heat loss is padded as no one wants to get that call " you said it would heat the house!!!!!" and it to be 1 degrees below design temp and the house won't get above 50 F.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
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