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Yes, I know it should be called a convector :-)

The obvious effect is that a coat of paint, which is generally a thermal insulator, can limit the rate of heat transfer from the metal to the air. But how important is that effect to the overall efficiency of heating my house or room?

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If the convection efficiency is lowered by an insulating layer, the water returns hotter. I chose some ballpark numbers, under the (not really true) assumption that the heating is only convective:

  • Power = 1000W

  • Area = 1 m^2

  • Thermal Conductivity = 0.2 W/mK (median of some plastics on the internet)

  • Thickness = 2E-04 m (200 micron ballpark for paint)

Using dimensional analysis (because I don't remember the equation) I get

delta T (temperature drop across the paint thickness) in K:

[Thickness (m) * Power (W)] / [Area (m^2) Conductivity (W/mK)] = 1 (K) !

So a ballpark number would be a 1 degree K (or C) difference in surface temperature.

If the room is cold (10C) and the radiator is almost too hot to touch (140F = 60C) then it's a few percent effect on the convective efficiency, roughly speaking, and probably no more than 10%.

But a radiator temperature of 60C and room temperature of 10C, emissivity of 0.85, and area of 1 m^2 gives about 600 W of outgoing radiation, and ~360 W of incoming radiation from the room, or a net radiation of 240 W, so really can't ignore either one.

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Assuming your house is well insulated, the First Law of Thermodynamics applies. This means that the total amount of energy in your home should be constant. If you were opening and closing doors and/or had leaky windows those effects would outweigh paint coverings on an insulator.

As you mention, Radiation and Convection are different. Convection is heat generated from contact with the air. Radiation is generated from the radiator to you or other objects, without mediating through the air. This site discusses the various advantages of differing paint styles. It estimates about a 9% improvement if you use black paint. However, as mentioned on the site, metallic paints of any kind are a big no no. So don't use them. In addition, the paints used on the site are specifically designed for radiators, so you'll want to look for black radiator paint rather than just typical black paint.

For a more theoretical treatment looking at the ratio of the Stephan-Boltzman Law equations, look at the radiative power section, shows the same.

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  • $\begingroup$ 1st law does not a apply. A house is not enclosed by walls through which can pass neither matter nor energy. $\endgroup$ – paparazzo Aug 17 '15 at 2:26
  • $\begingroup$ @Frisbee Then I guess your insulator problems will be drowned out by the uninsulated nature of your house...it only need be practically a closed system. $\endgroup$ – Zach466920 Aug 17 '15 at 2:28
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From the surface to room you have radiation and convection.
A dark color will have better radiation so if you chose a good radiant color then should have no loss (don't pick white).
A painted versus non painted should have the same convection.

So the only difference is the conductive thermal insulation of the the paint.
Paint is pretty thin - not much insulation there.

And you you need to consider the boiler does not do more work. The produced heat gets into the house. It just takes the radiator a little longer to release the heat and it is released at a very slightly lower temperature.

Paint em. The difference is very very small.

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  • $\begingroup$ Sorry - do not paint them. The radiator operates at much lower temperature than sun or incandescent bulb; it emits mid- and far-infrared radiation. In this portion of spectrum, it may easily happen that the white paint will be "blacker" than some black paint. Unless you know the infrared reflectance, it does not make any sense to paint them. $\endgroup$ – dominecf Feb 4 '16 at 20:04
  • $\begingroup$ @dominecf Then get the can of white that is whiter than black. $\endgroup$ – paparazzo Feb 4 '16 at 20:17
  • $\begingroup$ As a matter of fact, most organic materials are pretty close to absolute black in the middle infrared. $\endgroup$ – dominecf Feb 4 '16 at 20:39
  • $\begingroup$ @dominecf And I said "The difference is very very small" $\endgroup$ – paparazzo Feb 4 '16 at 20:42
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    $\begingroup$ Just for further reference, a nice table of IR emmissivity of different materials is here: infrared-thermography.com/material-1.htm $\endgroup$ – dominecf Feb 5 '16 at 11:58

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