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Sizing cast iron radiators

McPhinn Member Posts: 3

We were referred to this site as the best place to get knowledgable advice for our problem.

We need help sizing hot water cast iron radiators for two rooms in a gutted renovation of a 120 year-old house. We have installed underfloor radiant pex tubing (staple-up) in most of the rest of the house. The water temperature from the new boiler will run at 140 degrees for the heating system. We will insulate thoroughly.

The two rooms that need radiators are each approximately 170 square feet. They are bedrooms that will be used as offices/guest rooms. Our plumber calculated the heat loss at 6,000 btu/hr for room #1, and 7,000 btu/hr for room #2. He has said the bigger the radiator the better, but we are concerned about weight and getting huge radiators up the stairs.

The radiator for room #1 will go on a wall that is 4' long. Will a 38" x 38" (15 section) x 8" radiator be sufficient for this room?

The radiator for room #2 will go under a window that is 27" high. How large a radiator will we need?

Much thanks in advance for your help,



  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    do you know the EDR

    of the radiators you are looking at?

    failing that how many "columns" in each section? 2, 3...?
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • McPhinn
    McPhinn Member Posts: 3
    sizing cast iron radiators

    Thanks, I do not know the EDR. We are shopping for old cast iron radiators on Craigslist or from local vendors, so we need to work with what is available. Is there a way to calculate the minimum (and maximum) size radiators that would be appropriate for each of the rooms?

    Much thanks,

  • VictoriaEnergy
    VictoriaEnergy Member Posts: 126

    Room 1: A 15 section (2 1/2") 4 column 38" H would be too small (5737 BTUs), if you could find a 5 col 15 section, it would work better @ 6750 BTUs with a thermostatic rad valve.

    Room 2:  at least 22 sec of 5 col. 26"h again you want a little bigger and add a TRV, especially when  mixing staple up heat with cast rads.
    Home Owners Please Note:

    You are receiving advice from some very skilled pros completely free of charge. One of the reasons I participate is to sharpen my own troubleshooting skills. So; did we get it right? I would be grateful if you extend this courtesy back by posting the final outcome of the issue you are inquiring about. Thanks
  • Ironman
    Ironman Member Posts: 7,352
    EDR Table

    See the attached table. Multiply the square footage x 110, if you're using 140* water, to get the btu output.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • McPhinn
    McPhinn Member Posts: 3
    Sizing cast iron radiators

    Thank you all for sharing your expertise and advice, we can now search for the correct sized radiators!

  • Radiators

    Did you get them yet? I'm coming down today.

    Thanks, Bob Gagnon
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,314
    WHat size are you looking for?

    With the height and edr you need I could check my inventory.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • sandrine182
    sandrine182 Member Posts: 1
    Sizing cast iron radiators


    We also want to calculate the BTU's generated from our old cast iron radiator. We are doing some renos in the kitchen and we need the space used by the cast iron. We are trying to replace the tube type radiator, for one at the wall or something similar that will use less space than the actual one.

    The Tube Radiator contains 5 tubes, 12 sections. It is 8 inches wide, 3 inches long and Height is 20 inches.

    The room size is 14X 13,5 X 9.

    The actual cast iron is underneath a window. The plan is to convert that window into a door or patio door to access our newly built deck.

    Would you have any suggestion?

    We are trying to buy second-hand wall cast iron and we are a bit limited on what is on the market. What would be the smallest and the larger option to heat this room?

    Thanks in advance for you advice!