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Sizing New Radiators

I know how to measure my existing radiators, but how do I figure out what size I need for a new one? Two radiators were removed from my house at some point.

Based on the impressions in the floor and the other radiators in the house, I'm guessing one was a Thatcher Gothic five tube with either 17 or 18 sections and 20 inches tall. That would put it at between 45 1/3 to 48 square feet. I'm assuming there was once a window above where it sat, but there's now a doorway to an addition in that spot, so it would have to be moved to another wall. The new location has two windows instead of one. With 17 or 18 sections, the new radiator would overlap each window by a couple inches. Nothing has changed for probably 100 years as far as insulation, floors, or ceiling in that room. I doubt the existing window bays are new, although the windows themselves are new. The only change would be the blown-in insulation in the attic above on the second floor.second floor.

The other missing radiator is from the room right above the first missing radiator. They must have removed this one to eliminate the riser near the new door. This was probably another Gothic with 10 to 12 sections, so 26 2/3 to 32 square feet. It sat under a window on the second floor that is still there. If we reinstall it, it'll go right back in the same spot. Again, the only change is the blown-in insulation in the attic. The second floor is a 1/2 story, so really only the flat part is insulated and the roofline isn't well insulated at all.

Is it safe to assume the old sizes are still appropriate for the rooms? If not, how do I go from room square footage to radiator square footage?


  • ChrisJ
    ChrisJ Member Posts: 15,677

    Just a homeowner but,

    Whatever you do it must stay consistent with the rest of the house.

    Meaning if all of the other rooms have radiators which are 30% too big for the current setup then the ones you install must also be 30% oversized.

    The most important part is that everything matches in the system whether oversized, undersized or just right.
    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
  • Jamie Hall
    Jamie Hall Member Posts: 23,289
    Most llikely

    it is reasonably safe to go with what was there.  It's also simplest...

    To determine the heat load of a space isn't all that hard, though.  You need to determine the total R factor of the various walls and ceilings and windows, their areas, and the temperature on the other side (an outside wall is easy -- that's the design temperature for your area.  It's best to assume that an attic is also at design temperature, unless it is occupied.  Other spaces... are trickier, as you need to factor in whether or not they are heated, and how well, etc.).  Then the heat loss through that surface is simply the area of the surface, times the difference between the design indoor temperature and the temperature on the other side, divided by the total R.  Then add them all up, and there you are.  That's in BTU/hr.  Radiators are measured in EDR, and the EDR of a radiator times 240 is also the BTU/hr.


    Sort of.

    As I say, it's sometimes easier to just go by what was there!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • saikosis
    saikosis Member Posts: 75

    Thanks, guys. I'll take a stab at calculating the heat load (heat load or heat loss?) just to double check that what I got from measuring the old feet marks on the floor isn't too far off. I mostly just want an estimate so I can size my new boiler correctly. I only need 343 square feet now, but I want enough headroom to add those two radiators at some point. I'm also uninsulated at this point, so the extra capacity of the new boiler will cover me until I wrap the pipes.
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