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In-floor radiator BTU

Norm Harvey
Norm Harvey Member Posts: 684
Hello Guys,

I have a customer in a large residential home that has some under-floor radiators that heat convectively through some ductwork into grilles in the floor.

One of these is leaking and needs to be replaced.

I was thinking or removing it and replacing with fin tube baseboard to match the BTU output of the old radiator (accounting for temperature drop etc,..etc,..

My question is how to figure the BTU of the existing radiator.

Some notes,.. this looks more like a boiler than a radiator,.. 10 sections each about 12-15" high that make a 36" X 30" square. I say they look like boiler sections because the air moves through pins between the sections that resemble a boiler flue.

Any help in finding a way to measure approximately how many BTU's i have would be greatly sappreciated. I'll post some pictures as soon as I can get this new camera phone figured out.


- Norm

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  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    \"Boilers Hanging in the Ductwork\"

    That's an old indirect heating system. It heats fresh, outdoor air that rises via gravity into the rooms. Room air leaves the structure via cracks around windows and such.

    Based on your size estimates, it sounds like they are American Radiator "Perfection Pin" indirect rads in the "extra large" (36 1/4" L; 11 1/2" H; 2 7/8" D.

    Each of these sections is rated at 15 sq.ft. EDR, so the overall size is 150 sq.ft. EDR.

    If low-pressure steam, 150 sq.ft. EDR is good for a whopping 240 * 150 = 36,000 btu/hr.

    If water at typical 180F average rating it's good for 170 * 150 = 25,500 btu/hr.

    Since indirect radiators heat almost exclusively by convection, fin-tube is a suitable replacement, but you do have to pay careful attention to how such is placed inside the "duct". I would HIGHLY suggest Dan's Holohan's (owner of this site) book, "How Come?". It has an entire chapter devoted to this form of heating--while not nearly as in-depth as engineering books of the period, it should help you when you have to replace.

    Slant Fin makes suitable plain fin tube that should be suitable. While you'll certainly need larger diameter tubes/pipes for steam, I'd suggest such for hot water as well since it will keep head loss low--just like the original "boiler" sections.

    Using the "single tier" 1 1/4" IPS steel pipe rated at 925 btu/hr @ 180F AWT, that means at least 27 1/2' of fin-tube. Since you'll be "packing in" the elements I would suggest deducting 15% - 20% from the published ratings as this is not the "normal" configuration.

    Be VERY aware of how this system was designed to work BEFORE you attempt repair. Since these systems were designed for 100% fresh air input, they are both extremely large and extremely energy hungry. Attempts to save energy by "tightening" the house are actually counter-productive. Remember--all that incoming fresh air also has to EXIT the house! If insufficent air is moving the customer will be uncomfortable and his fuel bills will be even higher! Sometimes the fresh air inlet is blocked with the service hatch opened--this lets the heater work off of basement air. Not only can odors be a severe problem, but again, the air has to come from somewhere and there will be no original provision for air from the upstairs rooms to travel into the basement.
  • Norm Harvey
    Norm Harvey Member Posts: 684

    Thank you very much Mike!

    One thing I found odd is that these radiators are enclosed in a sheet metal box with no "inlet" I'm not sure if the boxes are original to the system, but they have been in place long enough for the homeowner to be completely satisfied with the current configuration.

    I imagine if air must be traveling in two directions through the same ductwork, much the samw way steam and condensate can travel in opposite directions.
    From investigating the house I've found only one grate per hung radiator. From the sheetmetal box that forms the enclosure there is an "L" shaped duct to get to the grate.


    Thank you very much for your help Mike, I appreciate it.

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  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Residential indirect heating was preferred by the truly rich around the turn of the 20th century and since I'm in semi-rural southeast Missouri where even the "rich" are "poor" by most standards, I've never seen a real-life indirect residential heating system.

    I have though studied old books carefully so at least I should know what to expect should I ever encounter.

    Sheet metal, wood and even brick or concrete were used for the enclosures--an enclosure of some sort was an original requirement. Originally there would be some form of "duct" (I suppose it could be a joist cavity) leading from the enclosure to the great outdoors. From the outside, I suspect it would look like a foundation vent--probably a decorative cast iron grille with insect screen behind.

    Sometimes these indirect heaters were installed in "cold rooms" in the basement. These rooms were nicely sealed (including the door) from other basement rooms with air inlet(s) in the foundation. In this case I can only assume that you'll see an intentional hole (inlet) in the chamber.

    Some form of damper was most likely included in the chamber. The purpose of such was to bypass outside air directly into the heated warm air during mild weather. Again, these were 100% fresh air systems. The dampers could have been purely mechanical with no thermostatic control, pneumatic with proportional thermostatic control, or electric with on-off control. Likely you'll find that any damper control has been long abandoned.

    When you replace the "boiler sections" with fin-tube, keep these original methods in mind:

    1) The heating elements should fit as tightly as reasonably possible both front-to-back and side-to-side. The object is to prevent air from "short circuiting" around the heating elements.

    2) There should be at least 6"-8" of "cold air space" below the elements.

    3) There should be at least 8" - 10" of "hot air space" above the elements.

    Perhaps however the system was modified at some point to use indoor air instead of outdoor. If so, I would expect to find a "return" cut into the floor of the lowest room being heated. (Note that these heaters often served "stacked" rooms on two or possibly three floors.) The highest floor (occupied by servants) will have standard radiators.

    Assuming that the homeowner has no comfort problems, I believe you'll be fine replacing the "boiler sections" with fin tube. Just keep those old placement ideals in mind... Tight as possible at the ends and sides with 20% or more "hot air space" above than "cold air space" below.

  • Brad White
    Brad White Member Posts: 2,398
    Hard to beat Mike's answer

    Hi Norm!

    Those pin radiators are something. Where is it and shall I take a look with you? Dan's EDR book I thought had some data on those. Mine is on loan. I may have another source but to go with Mike's suggestion is a great start. Keep cast iron with cast iron if you can. Otherwise, separate zone if copper as you know.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    To paraphrase Dan the Man, "you have to keep the old ways in mind when you replace with a nest of fin-tube". No reason it won't work perfectly, especially with the larger diameter fin tubes.
  • Brad White
    Brad White Member Posts: 2,398
    Copper and Iron

    Hi Mike-

    Copper is OK with cast iron in conditions of continuous flow and provided the BTU match-up is close to equal. When the system as a whole (or as a zone) is controlled by a room with one or the other, trouble looms.... :)

    All in all though, those old pin radiators rocked. Some old schools around here used to take residual steam (a form of condenser, especially where they generated their own power) or even prime steam to heat a radiator at the bottom or an exhaust shaft in order to induce convection out of the building. What look like chimneys with copper caps on old schools were either intake or exhaust shafts. Amazing to think of wasting heat just to induce an upward flow, but they did.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • J.C.A._3
    J.C.A._3 Member Posts: 2,981
    Norm, Brad and Mike....

    Norm, these are almost always found still being used up here on the "Gold Coast", and what Brad and Mike said holds true.

    I see them (usually) in the Foyer/Entrance way to old mansions. The reason was that most of the rooms in the homes HAD fireplaces, and that was the only "Welcoming" heat where the guests were being greeted, and some gravity flow from the water heater was available if needed.

    I've seen them in smaller homes also...but only where the systems have been converted from gravity to forced water. In those cases, we loop them together and use a 3 speed pump set to LO speed with a separate thermostat for the area. There is usually some kind of "air damper door" on the duct that can be adjusted...depending on the season.

    I think the BTU/area numbers that Mike T stated are good, unless the units are WAY BIG!(I've seen some that were bigger than a W/M 768 series boiler sections...and I think I might go a BIT less for those!)

    Maybe some nice new cast iron radiators in the space to make up the loss?(ask the customer, give them a price...you might be surprised)

    Let us know how you make out. Chris
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Even in the "poor" part of the country we had the finest school pin indirect emitters with fresh air input under the care of the custodian.
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