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Moving a radiator to a smaller, capped pipe

We are finishing up a gut renovation and have had a bunch of issues with our one pipe steam heating system.

My contractor is recommending moving our large cast iron radiator (43”x20”x8”), which is connected to a 1.25” pipe to a currently unused .75” pipe that is capped. He thinks that pipe goes straight down, but I am not sure what he is basing that on. For what it is worth, we know that pipe is functional because the previous owner had a smaller radiator there.

We are on the top floor and I am concerned that moving to the smaller pipe could lead to trapped water/banging. Any thoughts? I guess we could always move the radiator back if there is a problem.


  • Jamie Hall
    Jamie Hall Member Posts: 23,428
    I don't think so. A 3/4 inch riser would have a maximum EDR capacity of somewhere around 10. The 1.25 has a capacity of around 100...

    You might be able to get away with it by venting the now way oversize radiator with the smallest possible vent you could find, but any effort to get real heat out of it will probably lead to problems.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • KC_Jones
    KC_Jones Member Posts: 5,741
    To be brutally honest if your contractor is suggesting that you need to find a new contractor. He obviously doesn't know steam.

    You already see what the contractor has done with the other problems, why keep taking chances?
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • Fred
    Fred Member Posts: 8,542
    What is it with that guy that he is so enamored by small pipes??? :| You can not support that radiator with a 3/4" supply. Is he trying to make it compensate for whatever smaller pipe he previously used in the wall or ceiling? Won't work, at all, not to mention the hammering you will get because there will be no room for both steam and water (and I might add not much of either).
  • Steamhead
    Steamhead Member Posts: 16,889
    What Jamie, KC and Fred said. Get a Steam Man in there. Where are you located?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • biglebowski99
    biglebowski99 Member Posts: 21
    Thanks! That's why I was double-checking, because it didn't sound like a smart idea (especially with all of the other problems we've had).

    It's one of these very old NYC apartment buildings that is a million degrees in the winter. To compensate, most of the apartments in the building have removed radiators so it is more bearable. I had two HUGE radiators and two LARGE radiators in the living/dining room. We now just have the one HUGE radiator (43x20x8) and it is comfortable. The idea was to move that radiator to where one of the LARGE radiators (35x20x8) was before the renovation. But, it sounds like the existing 3/4" pipe is probably undersized for the larger 43" radiator. Makes sense :-)
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    No way you'd get enough steam. Even if you did, your asking for wet steam, and hammer as the counterflow of the condensate meets in incoming steam. Expect the vent to chirp a lot and have some and faint pinging sounds.