[JP]

A back-of-the-envelope calculation for a radiator operating at 100 C in a 20 C room shows that it could maximally emit about 680 Watts/m² radiated energy. 

That seems to be about consistent with my back-of-the-hand measurement  []

If your bro was able to burn himself, those radiators must have been using really hot water, or even steam, like the ones in NYC. We were able to sit on top of the ones in our house in the UK.

I think I know the type of finned baseboard radiator you are referring to. Pretty ugly things, and not at all like the superior Scandinavian designed (did I mention they were enclosed in teak) variety that I installed in our bungalow in Renfrew.

Yeah--the ones we had (and have now) are steam.  The radiators we had as kids were the baseboard variety with fins. 

[Bored chemist]

All the places I have lived didn't run radiators anything like as hot as 100C. We didn't want to get burned so 50 or 60C was probably the upper limit.
That makes a difference to the power output and, presumably, our perception of whether they radiate or convect..

[lightarrow]

Why Are Most Radiators White ?

Maybe because most walls are the same colour.

[CZARCAR]

Actually, I'm not convinced radiation isn't important (though the paint color is probably fairly unimportant).  A back-of-the-envelope calculation for a radiator operating at 100 C in a 20 C room shows that it could maximally emit about 680 Watts/m² radiated energy.  That's fairly significant, thought not huge.  It's peak wavelength would be at 7700 nm, which is in the infra red. 

http://en.wikipedia.org/wiki/Zeroth_law_of_thermodynamics

[Airthumbs]

The radiator in room 5 ewe sea is white like the rest.  I am sticking stuff up behind all the radiators that is like about 5mm thick foam covered in a layer of foily substance from Germany.  It states that it reflects 95% of something back into the room instead of into the wall underneath the window.  What is the best colour for a radiator?

[CZARCAR]

foil reflects radiant heat the same as a flashlight. Zeroth law sayeth black absorbs radiant best so it has to radiate best also. What are u trying to do?

[JP]

Actually, I'm not convinced radiation isn't important (though the paint color is probably fairly unimportant).  A back-of-the-envelope calculation for a radiator operating at 100 C in a 20 C room shows that it could maximally emit about 680 Watts/m² radiated energy.  That's fairly significant, thought not huge.  It's peak wavelength would be at 7700 nm, which is in the infra red. 

http://en.wikipedia.org/wiki/Zeroth_law_of_thermodynamics

Ok, but what does the zeroth law have to do with what I was pointing out?

[CZARCAR]

Actually, I'm not convinced radiation isn't important (though the paint color is probably fairly unimportant).  A back-of-the-envelope calculation for a radiator operating at 100 C in a 20 C room shows that it could maximally emit about 680 Watts/m² radiated energy.  That's fairly significant, thought not huge.  It's peak wavelength would be at 7700 nm, which is in the infra red. 

http://en.wikipedia.org/wiki/Zeroth_law_of_thermodynamics

Ok, but what does the zeroth law have to do with what I was pointing out?

"paint color is fairly unimportant"?

[JP]

Ah, but the zeroth law is about thermal equilibrium between two bodies.  The direct statement of emission being equal to absorption is Kirchoff's law: http://en.wikipedia.org/wiki/Kirchhoff's_law_of_thermal_radiation

Regardless, paint color is still going to be fairly unimportant because we're talking about emission primarily in the thermal range of infra red, while the color of paint has to do with the visible spectrum--two completely different ranges of radiation.  Paint which appears white in the visible spectrum still tends to be highly absorptive in the infra red. 

[CZARCAR]

i was thinking solar panel where black is prefered color for absorption & the implied inverse for radiation?

[JP]

Quite true true, but a solar panel is operating to absorb light from the sun, which is strongly peaked in the visible range: http://en.wikipedia.org/wiki/File:Solar_Spectrum.png  Black paint looks black because it absorbs well in the visible.  Much of the sun's energy is in the visible range, which is why black things left out in the sun get very hot, while white things don't. 

For a steam/hot water radiator, most of the radiation will be in the infra red (way off the right-hand side of that chart) which is far from visible radiation, so I doubt paint "color" matters much (since color refers to what we see).  I believe most house paints absorb fairly well in the infra red, though I'm sure there are special paints that are designed to reflect it.  If you polished your radiator so it was highly reflective, it would, however, be very poor at radiating heat.

I could be completely wrong, though--maybe white paints are also reflective in the infra red.  If you do a search, though, you find companies selling specialized paint of various colors that reflects in the infra red.

[Geezer]

To settle this one, I've set up an experiment:

I have suspended a piece of sliced bread one inch from a radiator. We should be able to get a pretty good idea of the amount of IR the radiator emits by seeing how long it takes to toast one side.

[Bored chemist]

Quite a lot of things that look black- like soot for example, are also pretty black in the IR. However things that reflect white light well are often fairly good absorbers (and emitters) of IR.
The colour of the paint is only an indication of how well it absorbs or reflects visible light and since radiators don't do that, the colour isn't important.
Polishing the radiator to a mirror finish would be a bad idea but I don't expect any real effect from any other change in the surface finish.

[CZARCAR]

To account for a body's outgoing radiation (or its emissive power, defined as the heat flux per unit time), one makes a comparison to a perfect body who emits as much thermal radiation as possible. Such an object is known as a blackbody, and the ratio of the actual emissive power E to the emissive power of a blackbody is defined as the surface emissivity e,

By stating that a body's surface emissivity is equal to its absorption fraction, Kirchhoff's Identity binds incoming and outgoing radiation into a useful dependent relationship,
Found this & aint it saying that if black absorbs best it has to radiate best also?

[Bored chemist]

Yes,black is the best absorber and therefore the best radiator. It's just that the difference isn't that big and the things we call radiators actually transfer quite a lot of the heat by convection.(which explains the design of them- they are usually set up to have quite a lot of surface area with ribs + ribs etc. if they were pure radiators this wouldn't help.) The complicated bits of this, fairly typical, design
http://www.wickes.co.uk/double-universal-radiator/invt/201484/
are all facing eachother in the middle of the radiator. Since they face eachother they can't radiate heat out to the room, but they can transfer a lot by convection.

Also, the rated power 1360 W is much higher than the possible radiated power especially as one face of it can only radiate towards a wall, and once the wall warms up there's much less net heat radiated in that direction so the item only has about half a square metre of effective area. It only could radiate a few hundred watts, even if it were run at 100C.

[damocles]

For a genuine radiator -- i.e. maximum radiant heat output -- the best colour is black. I suspect that the reason that the water-based heaters you are describing are mostly white, or occasionally pastel colours, has to do with the aesthetics of their not clashing with wall colours. (as lightarrow suggests).

I think the reason these water-based heating devices are called "radiators" even though they mainly work by conduction/convection is because they perform the same function as genuine radiators: the electric devices that make a ceramic bar glow red hot (or a wire coil in some older models).

Radiative efficiency is exactly the same as absorption efficiency. Something which absorbs all visible light (black) will be a 100% efficient radiator of visible light. But a device operating at 100°C or less will not emit a significant amount of visible light anyway! The bottom line is that in terms of radiation efficiency for a water-heating device, it makes absolutely no difference what colour it is, as far as visible colour is concerned! The important thing is that it should be "black" (that is, totally absorbent) in the infrared.

[Geezer]

I think the reason these water-based heating devices are called "radiators" even though they mainly work by conduction/convection is because they perform the same function as genuine radiators: the electric devices that make a ceramic bar glow red hot (or a wire coil in some older models).

I sort of doubt it. Haven't the water filled variety been around longer?  Also, water cooled IC powered vehicles have radiators too, and it's pretty clear they work because of conduction []

Where are all the etymologists when you need them? (Out catching butterflies as usual?)

[CZARCAR]

For a genuine radiator -- i.e. maximum radiant heat output -- the best colour is black. I suspect that the reason that the water-based heaters you are describing are mostly white, or occasionally pastel colours, has to do with the aesthetics of their not clashing with wall colours. (as lightarrow suggests).

I think the reason these water-based heating devices are called "radiators" even though they mainly work by conduction/convection is because they perform the same function as genuine radiators: the electric devices that make a ceramic bar glow red hot (or a wire coil in some older models).

Radiative efficiency is exactly the same as absorption efficiency. Something which absorbs all visible light (black) will be a 100% efficient radiator of visible light. But a device operating at 100°C or less will not emit a significant amount of visible light anyway! The bottom line is that in terms of radiation efficiency for a water-heating device, it makes absolutely no difference what colour it is, as far as visible colour is concerned! The important thing is that it should be "black" (that is, totally absorbent) in the infrared.

my non-frostfree fridge has a black heat dissipation grid mounted up the outer backwall of the fridge & the compressor + all freon piping is black. The fridge's main job is to remove heat from its interior, not to heat my kitchen

[damocles]

I think the reason these water-based heating devices are called "radiators" even though they mainly work by conduction/convection is because they perform the same function as genuine radiators: the electric devices that make a ceramic bar glow red hot (or a wire coil in some older models).

...(snip)... Also, water cooled IC powered vehicles have radiators too, and it's pretty clear they work because of conduction [] ,,,(snip)...

Telling point, Geezer! I had temporarily forgotten about automotive radiators. I stand corrected.

[damocles]

my non-frostfree fridge has a black heat dissipation grid mounted up the outer backwall of the fridge & the compressor + all freon piping is black. The fridge's main job is to remove heat from its interior, not to heat my kitchen

Although the main (intended) job of your fridge is to remove heat from the cold box, it actually does a slightly better job of heating your kitchen.

From the Law of Conservation of Energy:

Heat released into kitchen = heat removed from cold box + heat released due to the conversion of mechanical energy as the result of friction in the compressor.

In terms of logic, the fact that your heat exchanger or that "all" freon piping is black cannot be taken as evidence that black is a more efficient colour for low temperature radiators, particularly when theory would tell us that colour (in the visible part of the spectrum) is irrelevant.