We have steam heat in our 1930s-era home and one of the bedrooms is much colder than the other two. Here is a picture of the radiator in the cold room:


The room is 12'x12'x8'. This radiator is 8"x25"x4.5" in size. It has four fins each with four tubes. The tubes look like they are about 0.75" in diameter.

In contrast, our other bedroom of the same size has a larger radiator and the room is well-heated. (If it matters, both rooms are on the north side of the house; the colder one northeast and the warmer one northwest.) The larger radiator is 14"x38"x7.5" in size.

From what I understand the ability of a radiator to heat is proportional to its surface area, but using volume as a ballpark to check the appropriate size, I get that that radiator in the cold room is about 900 cubic inches in volume while the one in the warmer room is almost 4000 cubic inches. That makes me think it's too small.

Otherwise, the radiator seems to function well. It hisses loudly once in a while, but it does warm up—just not enough to heat the room. If the size is not the issue, what else could be the trouble?

  • 1
    You are asking "if size is not the issue..." but the rest of your question pretty much points to the fact that size is the issue. I'm no expert, but I'd say I've never see a radiator that small in anything but a bathroom.
    – DA01
    Jan 22, 2013 at 1:13
  • DA01: My best guess is that it is the size, but not being an expert either I didn't want to ignore the possibility that it was something else. Jan 22, 2013 at 2:14
  • The valve on the right is often adjustable. Have you attempted to increase it to the maximum, and to turn down some of the other rooms, to better balance the system.
    – BMitch
    Jan 23, 2013 at 1:35
  • This appears to be a single pipe steam system, I see no return pipe. This means the valve at the bottom must always be either fully open or fully closed. Something in between will inhibit condensate return, causing the radiator to malfunction. You can somewhat control the output by changing the vent valve size so the heat builds up faster or slower during every heat cycle.
    – bcworkz
    Jan 23, 2013 at 1:58
  • BMItch: The valve on the right isn't adjustable, but I think there are replacement valves which are. Even so, if the radiator is too small no valve is going to make it hot enough to heat the room, right? Jan 23, 2013 at 14:27

1 Answer 1


First of all I assume that the radiator is functioning properly and gets nice and hot(215F for steam) If not than it is not a problem of sizing.

To figure out if this radiator is appropriate for the given room we need to calculate how much heat it emits. It is all based on its surface area. This radiator looks like it could be National Aero Four Tube radiator or American "Corto" Radiator. Those radiators with four sections and in 26 inch size both have 11 sq ft of radiation. 1 sq ft gives off 240 BTUs at 215F. So this radiator emits 2640 BTUs.

Now just calculate how much heat this room of your home looses and you can see whether this radiator is adequate. There are quite a few online tools available to calculate heat loss. None of them will be too precise without blower door test, but you can probably get a ballpark figure. I did a quick google search and this seems to give you necessary steps to do a basic heat loss calc: http://www.pexuniverse.com/content/calculate-heat-loss

There's also software available to help you. For example HeatCAD http://www.avenir-online.com/avenirweb/HeatCAD/HeatCADHome.aspx has a 30 day trial.

It will probably make sense to compare heat loss numbers with other rooms in your house that are known to heat well.

Once you know for sure what is causing the room to be cold, you can remedy the problem. You could either increase the radiator size or spend some time insulating/tightening the room(this would be much cheaper and more efficient)

  • Thanks for doing the math on the BTU output of my radiator. I know the radiator does get hot (too hot to touch; don't know if the full 215F but I can check). I used this calculator and it suggests about 5300 BTU/hr is needed, which would support the supposition that 2640 is not enough! Jan 23, 2013 at 14:24

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