# Is this radiator too small?

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?

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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 '13 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. – Matthew Leingang Jan 22 '13 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 '13 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 '13 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? – Matthew Leingang Jan 23 '13 at 14:27

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