If I recall my school physics lessons, matt black radiates more efficiently than gloss white, yet all of the radiators I’ve seen in people’s houses and workplaces are gloss white. I’m talking about the radiators that are pumped with hot water from some sort of central heating system, but storage heaters also appear to be all white.
Is this some sort of aesthetic choice? If so then why are the radiators that are placed behind panelling still white?
Is there something I’m missing? Would the radiators get too hot and burn to the touch? Would the hot water cool down so much that only a couple of radiators would heat up and the rest would be tepid?
Anyone?
The colour of the radiator makes no significant difference to its radiating power. White is a simple default color that fits the most design schemes.
White paint is cheaper.
Hmm… I think Max Planck might disagree with that. Planck's law - Wikipedia
As far as the cost of the paint goes, people are spending thousands making their houses more energy efficient, a little extra for different paint surely wouldn’t matter that much?
If white radiators aren’t radiating efficiently, where is the heat going?
It’s got to be one of the following:
[ul]
[li]They spend more time cycled off (and thus are not actually inefficient)[/li][li]They overheat and melt (I’d have noticed this if it happened even once)[/li][/ul]
Look for the surfaces that the light from your radiator hits. Are they warmer than other surfaces? If not, I don’t think it’s worth the paint to paint them black. I suspect that the radiator needs to be literally red-hot in order to heat by electromagnetic radiation.
You’re presuming the infrared emissivity of black and white paints would be much different. The sources I can find online (as well as my own engineering experience) disagree.
Here’s one: (PDF). The lowest emissivity value for any paint listed (any enamel, lacquer or oil paint) is 0.87. In fact white lacquer and black lacquer have the exact same emissivity value, and the only difference to be had comes from changing the finish on the black lacquer. I’d bet that dull white lacquer would have a higher emissivity, too.
The way things look in the visible spectrum doesn’t indicate how they will radiate in the infrared. And surface finish generally matters more than color.
Radiators are badly named. They should be called convectors. Their dominant heat transfer mechanism is convection, not radiation.
In principle yes, they should be painted black, it would help a little tiny tiny tiny bit to improve heat transfer into the room, The emissivity of ordinary white paint is, rather remarkably, very little different to black paint. A titanium white has emissivity of about 0.94, compared to 1.0 for a perfect black body. Whilst a polished metal surface can drop well below 0.1 (gold goes to 0.02). It is the polished metal that makes the difference, not the colour.
Ha, ninja’d by seconds 
Ah, but Francis Vaughan, you got the most important part right that I missed: Radiators don’t really radiate much, anyway.
Great, thanks!
So if the colour doesn’t matter but the albedo of the surface does then would repainting the radiators from gloss white to matt white make any noticable difference? I mean I know it wouldn’t save as much money on heating as double-glazed windows or roof lagging but would it be worth the cost of the paint?
I have never thought of it, but now that I think of it, every radiator I can recall has been white.
How can it save money? Converting fuel energy to heat is for many devices an unintentional side-effect of operation. In this case, it’s the primary function.
Unless the system is throwing heat away somewhere else (possible - if it’s a gas-fired system, some heat is lost up the flue), or converting energy to some other form that is escaping (making a loud noise that is escaping through windows), then this isn’t really a question about *efficiency *at all - it’s about effectiveness.
In a word: No.
Gloss white might have emissivity down to about 0.85 versus 0.95 for just about any matte colour. That isn’t a big deal. Not when it isn’t even the dominant heat transfer mechanism.
Also, there is a bit of a logical fallacy here. If the heat isn’t radiated into the room, it isn’t lost. The water in the radiator system simply stays a bit hotter. It is as if your radiator was a bit smaller. But in both cases the end result isn’t a less efficient heating system, it is a system that uses less energy because it injects less energy into your room. So your energy bills are a tiny bit lower. Better insulation of the house does make things more efficient - because your are reducing lost energy. So your room stays hotter for less money spent.
(OK, there are second order effects that say you should try to get the most heat out the water you possibly can, and this does raise system efficiency. Colder return water is losing less heat into those parts of the house you don’t need to heat, and a colder return makes for a slightly improved heat transfer in the boiler. But you are going to make an unmeasurable change to the overall answer by painting the radiator.)
In the terms of blackbody radiation, the color of both a white and black radiator is infrared. 
Well put. If you really wanted to save energy this way, you’d want a paint that is highly absorptive in the infrared (over the 1–100 micron range, say.) These types of pigments probably exist—see here, for example—but as noted, it wouldn’t improve the performance of your radiator all that much, and probably won’t change your heating bills at all.
My parents house had a radiator in one room that was painted a dark gold color (the others were white). A guy who came in to fix something in the central heating told my mother that this was a bad idea, and that it would not radiate with maximum efficiency unless it was white. When she mentioned it to me, I said (on the basis of the same high-school physics mentioned in the OP) that this was wrong, and that darker colors are actually better. It remained gold.
The physics of heat radiation is, I would suggest, slightly counter intuitive. I suspect many people do think that white “gives off” more heat than black. After all, when things such as metals get very hot they start to glow with first red and then white heat.
The consensus of this thread seems to be that it does not matter a damn what color a radiator is painted.and I expect that is right. However, my mother’s experience suggests that it may be commonly (but erroneously) believed amongst central heating technicians that white radiators transfer heat better. That may be the answer to the thread title question.
In my experience residential ones tend to be white while commercial tend to be silver. I’ve seen others colors too. It seems to me tradition rather then any important function. Kinda like we usually paint ceilings white.
Mine aren’t. Well, one is, but that’s because I spray-painted it myself. (Believe it or not, it was PINK before!)
*I see my radiator and I want it painted black
No coldness anymore, I want them to turn black
I see the girls walk by dressed in their summer clothes
I have turned my heat up until my frostiness goes…*
How about thickness of paint? After how many coats of paint might you see a significant drop in efficiency?
Just wondering as I’ve lived in apartments where the cast iron monsters had been painted so many times that the raised lettering (manufacturer’s name, etc.) on them was reduced to faint lumps.