# Why are radiators white? **URL:** \ **Category:** Factual Questions\ **Created:** [November 21, 2011, 10:06am UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626 "2011-11-21T10:06:23Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [November 22, 2011, 3:29am UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/21 "2011-11-22T03:29:50Z")
> [@brainfizz](#): > > So if the colour doesn’t matter but the albedo of the surface does … “Albedo” usually refers to the absorption efficiency in _visible_ light, which does not matter for a radiator. The term you’re looking for is _emissivity_, which measures how efficiently a surface emits/absorbs infrared light. ---
**Author:** ![GameHat](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@GameHat](https://boards.straightdope.com/u/GameHat)\ **Post date:** [November 22, 2011, 4:55am UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/22 "2011-11-22T04:55:13Z")
The radiator in my college apartment was unpainted. Just bare metal. …just saying. ---
**Author:** ![foolsguinea](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/foolsguinea/32/14751_2.png) [@foolsguinea](https://boards.straightdope.com/u/foolsguinea)\ **Post date:** [November 22, 2011, 5:07am UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/23 "2011-11-22T05:07:01Z")
I remember seeing sort of bronze-colored radiators when I was a kid. Maybe the paint had worn off? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [November 22, 2011, 7:37am UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/24 "2011-11-22T07:37:52Z")
> [@gotpasswords](#): > > 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. If the paint stops the heat coming out, where does the heat go? ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [November 22, 2011, 8:59am UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/25 "2011-11-22T08:59:31Z")
> [@Mangetout](#): > > If the paint stops the heat coming out, where does the heat go? Nowhere. Which is a problem if you want to it go from the water and into your apartment. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [November 22, 2011, 10:19am UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/26 "2011-11-22T10:19:29Z")
> [@naita](#): > > Nowhere. Which is a problem if you want to it go from the water and into your apartment. Which means the boiler will stop heating the circulating water sooner - the radiator will be less _effective_, but not less _efficient_. If it’s an electric, oil-filled radiator, it’s literally impossible for it to be inefficient. ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [November 22, 2011, 1:40pm UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/27 "2011-11-22T13:40:32Z")
> [@Mangetout](#): > > Which means the boiler will stop heating the circulating water sooner - the radiator will be less _effective_, but not less _efficient_. > > If it’s an electric, oil-filled radiator, it’s literally impossible for it to be inefficient. Less effective is less efficient, as it means you have to start with higher heat to get to the same desired temperature. Comparing efficiency is either matching input and comparing output, or matching output and comparing input. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [November 22, 2011, 2:26pm UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/28 "2011-11-22T14:26:33Z")
Efficiency has a clear technical meaning. In a home heating situation it is energy into the system versus energy into your room. **Mangetout** ’s example of an electric oil filled radiator is a good boundary case. It is a resistive heater. There is no-where for the energy to go once it heats the element in the radiator. A huge thick layer of paint, or polished metal finish, may reduce the rate of heat transfer for a while - but the energy has nowhere else to go. The end result is that the entire insides of the heater run hotter, and the heat flow increases - eventually reaching a steady state. This steady state is 100% efficient. The only issue is that the radiator is probably not going to last as long, as it may be running outside of its design limits. For a central heating system you do get issues with efficiency. I alluded to this above, but to be more explicit. If the radiators have a higher thermal resistance, they will also get hotter, and eventually too, this increase in temperature will cause a greater heat flow, until the system reaches a steady state. However, the return water will be hotter. This can be a problem. If the boiler was electrically heated, the system just runs hotter - with issues of shorter lifetime, but also greater losses due to leaking heat in the boiler and pipework due to the greater temperature here also creating flow of heat to somewhere you didn’t want it to go. If the boiler is heated with a flame (oil or gas) the increased temperature of the return water means it will pick up less heat from the flame than had it been colder. The heat transfer is proportional to the difference in temperature. This is critical point. The heat not picked up goes up the flue. This **is** a clear reduction in system efficiency. However, how much of a reduction is another matter. But from a technical and definitional point of view, in this case, efficiency is reduced. ---
**Author:** ![The\_Man\_In\_Black](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@The\_Man\_In\_Black](https://boards.straightdope.com/u/The_Man_In_Black)\ **Post date:** [November 22, 2011, 5:11pm UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/29 "2011-11-22T17:11:13Z")
I have almost never seen a radiator painted any color. They are always just the natural metal color. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [November 22, 2011, 5:35pm UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/30 "2011-11-22T17:35:32Z")
If I cover all my radiators in insulation, the water will not get hotter. The central regulator keeps the circulating water within certain parameters by letting differing amounts of hot water from the boiler into circulation. So for heating my apartment the system will be less effective. ---
**Author:** ![JBDivmstr](https://avatars.discourse-cdn.com/v4/letter/j/b4bc9f/32.png) [@JBDivmstr](https://boards.straightdope.com/u/JBDivmstr)\ **Post date:** [November 22, 2011, 5:37pm UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/31 "2011-11-22T17:37:59Z")
> [@The\_Man\_In\_Black](#): > > I have almost never seen a radiator painted any color. They are always just the natural metal color. Seconded! I removed and recycled (read, sold for scrap) at least 200+ cast iron radiators, during the course of remodeling temporary classroom buildings for the local school district. (HISD) None of them were painted. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [November 22, 2011, 8:21pm UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/32 "2011-11-22T20:21:41Z")
> [@boytyperanma](#): > > It seems to me tradition rather then any important function. Kinda like we usually paint ceilings white. It makes sense for ceilings to be white because it makes a room brighter than otherwise. Of course, the same is true with white walls, but if we painted all internal walls white, we’d get bored of it, whereas ceiling colour is not very noticeable. Also if the ceiling is a neutral colour you’re less likely to need to repaint it if you change your room colours. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [November 22, 2011, 10:24pm UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/33 "2011-11-22T22:24:49Z")
> [@BigT](#): > > Less effective is less efficient, as it means you have to start with higher heat to get to the same desired temperature. Comparing efficiency is either matching input and comparing output, or matching output and comparing input. Less effective is not necessarily less efficient. The water temperature is usually regulated, so nothing will get hotter. What will happen is that the duty cycle of the water heater will be altered - but not necessarily in a way that consumes more fuel over all (it may, or may not). The water in the system will stay hotter, so the boiler will turn on to heat it less frequently, but because heat transfer to the room is slower, the heating system may need to be active for longer, but that’s a longer group of shorter individual burns. But that wasn’t really the point. ‘Effective’ here means how well the system conducts its job of warming the room (at a certain level of insulation, it will become incapable of adequately maintaining the desired temperature - ineffective, but not necessarily inefficient in terms of the fuel consumed vs the heat delivered). ‘Efficient’, on the other hand, means the extent to which the available energy in the fuel is delivered as heat into the rooms. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [November 23, 2011, 1:41am UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/34 "2011-11-23T01:41:35Z")
> [@aerodave](#): > > 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](http://www.frigidn.com/Emissivity%20Table.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. Is the emissivity related to the absorption? (I always thought it is) In that case, shouldn’t black cars be the same temperature as white under the sun? ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [November 23, 2011, 9:53am UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/35 "2011-11-23T09:53:42Z")
> [@AaronX](#): > > Is the emissivity related to the absorption? (I always thought it is) In that case, shouldn’t black cars be the same temperature as white under the sun? The energy lost in emission is related to the temperature of the object. The energy absorbed is related to the radiation. So even if emissivity and absorption are proportional you don’t necessarily end up with black cars having the same temperature as white. Also there’s the issue of what wavelengths are absorbed and which are emitted, but for all I know that might be how my earlier point comes to be… My head hurts now. ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [November 23, 2011, 11:29am UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/36 "2011-11-23T11:29:44Z")
Just for the record, while most radiators I’ve seen have been white, I have on occasion seen them painted black, brown, and a sort of dull metallic chrome color, the non-white versions invariably being in private residences. So I think it’s tradition more than anything else. ---
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [November 23, 2011, 11:53am UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/37 "2011-11-23T11:53:21Z")
> [@AaronX](#): > > Is the emissivity related to the absorption? (I always thought it is) In that case, shouldn’t black cars be the same temperature as white under the sun? Emissivity is equal to absorptivity, always. That’s the basic idea of [Kirchoff’s Law of Thermal Radiation](http://en.wikipedia.org/wiki/Kirchhoff's_law_of_thermal_radiation). Radiation must be absorbed or reflected (we’ll ignore transmittance for see-thru objects). The total amount of energy incident on a surface will be divided between what is absorbed and what is reflected. And what is absorbed must be emitted at the same rate once things reach a steady state, hence emissivity is equal to absorptivity. Highly emissive objects (like flat black finishes) are also good absorbers, and they are poor reflectors. Poorly emissive objects (like polished metal) are also poor absorbers, and are therefore great reflectors. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [November 23, 2011, 12:01pm UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/38 "2011-11-23T12:01:25Z")
So is car paint different from radiator paint, making black cars better absorbers than white? ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [November 23, 2011, 12:29pm UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/39 "2011-11-23T12:29:19Z")
> [@AaronX](#): > > So is car paint different from radiator paint, making black cars better absorbers than white? Here we’re back to what part of the spectrum we’re talking about. Black absorbs light in the visible spectrum better than white does, and the bulk of the suns radiation is in the visible. But what’s emitted is infrared, and there black and white don’t necessarily differ. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [November 23, 2011, 12:46pm UTC](https://boards.straightdope.com/t/why-are-radiators-white/603626/40 "2011-11-23T12:46:04Z")
Cars in the sun are an example of things getting a bit more complex. The problem is that the radiation and absorption of heat are occurring at different wavelengths. Emissivity changes depending upon the material at different wavelengths. A car radiates energy in the far IR (as a black body this is dependent upon the car’s surface temperature), but absorbs it in the near IR and visible wavelengths (because this is dependent upon the sun’s surface temperature). You only see the paint’s colour in visible wavelengths. So you don’t see the full story. A white car has almost the same emissivity in the far IR as a black car. This is why white and black painted radiators are so similar in your home. However the white car reflects lot more visible and near IR away than the black car. So whilst both are capable of radiating away similar amounts of heat in the far IR, they don’t absorb the same amount in the visible and near IR - which is where most of the incoming energy is. So the black car will get hotter. You might recognise this as a form of greenhouse effect. [Previous page](https://boards.straightdope.com/t/why-are-radiators-white/603626.md?page=1) [Next page](https://boards.straightdope.com/t/why-are-radiators-white/603626.md?page=3)