Why are radiators so often painted white?

December 13, 2004 7:27 AM

Why are radiators (the things connected to your hot water system that heat a room) usually painted white, when white is the worst colour for heat radiation?
posted by Mwongozi to (19 comments total)

Because white is the most common color of interior walls, particulalry in rental units. Most people consider radiators an eyesore and want them to blend into the surroundings.
posted by alball at 7:32 AM on December 13, 2004

Wait. Does the paint color actually affect heat radiation when heat is radiating out? This isn't radiation by light/EM waves, so I wouldn't expect the color to make any difference whatsoever.
posted by stopgap at 7:40 AM on December 13, 2004

So, assuming you don't care about aesthetics, would painting your radiator black make a better radiator?
posted by Mwongozi at 7:40 AM on December 13, 2004

I have seen radiators hidden behind wood boxes with openings at the top and bottom too. Probably for aesthetics too of course, but it is almost as if they are trying to minimize the amount of radiation for some reason...

Wait. Does the paint color actually affect heat radiation when heat is radiating out? This isn't radiation by light/EM waves, so I wouldn't expect the color to make any difference whatsoever.

A significant amount of the heat from a radiator is radiated. Radiated meaning photons of infra red electro magnetic radiation come off the radiator and then hit objects in the room.

Heat transfer mechanisms
Thermal Radiation
posted by Chuckles at 8:08 AM on December 13, 2004

I demand a refund for my college physics classes. But I'm still thinking that a radiator with no paint at all would make a better radiator than one painted black. Am I way off on that too?
posted by stopgap at 8:20 AM on December 13, 2004

I'm probably being dense here, but why would white be worse as radiation emission goes? It'll reflect more radiation, so it'll absorb less, so that would be good. But why would it influence emission?

Also note that all this effects are negligable assuming your radiator isn't abnormally (really abnormally) hot. Also, since (almost) all the radiation coming off your radiator is way in the infrared, the optical colour doesn't matter much.
posted by fvw at 8:20 AM on December 13, 2004

I always assumed that they started out unpainted, and people painted them to match their (usually white) walls, to help them fade into the background.
posted by iconomy at 8:24 AM on December 13, 2004

My understanding was that most of the heat from a modern radiator is convected, so it doesn't really matter what colour it is.
posted by chrispy at 8:25 AM on December 13, 2004

1. Most of the heat is convected.

2. The whole thing is covered in white paint. Some heat is radiated from the metal part of the device directly into the white coating, at which point some of it is absorbed and then convects, and some amount reflects back into the metal, at which point some is absorbed and conducts to the paint where it convects, and some is reflected back to the paint, and...
posted by rxrfrx at 8:32 AM on December 13, 2004

There is a fairly involved article about it here: Heatinghelp.com

Summary of their conclusions:
- Only the last coat of paint matters
- Metallic paint is the worst for heat transmission

That site is excellent for general heating tips you can't find anywhere else - for example How does covering a radiator affect heat output? Another useful site for those with old radiators is here, they have a chart helping you compare heat outputs of different sized radiators if you ever need to replace one.
posted by true at 8:43 AM on December 13, 2004

"I demand a refund for my college physics classes. But I'm still thinking that a radiator with no paint at all would make a better radiator than one painted black. Am I way off on that too?"

Color affecting a radiator's performance? Seems absurd to me. Bare metal would be best. The paint's heat conductivity characteristics would not seem linked to paint color.

Of course, if the heat isn't going through the radiator (due to the paint), it's going somewhere further up the line. And if the location "up the line" also needs to be heated, the whole discussion is academic.

Right?
posted by ParisParamus at 9:01 AM on December 13, 2004

The heat will go somewhere, but depending on how your thermostat works it could be a problem. In a single zone setup if the radiator in the thermostat room isn't producing enough heat, the rest of the rooms will be too hot as the thermostat will keep the system on for longer. Vice versa if it is producing too much heat.

So, it can be a balancing problem. Less so in Europe (where most radiators seem to have individual thermostatic valves) or in a multi-zone setup in the US, where each zone can call for its own heat.
posted by true at 9:07 AM on December 13, 2004

Color affecting a radiator's performance?

Yes, black is the most efficient, white is less so, and silvery and reflective would be even less efficient.
posted by caddis at 9:50 AM on December 13, 2004

Efficiency being defined here purely for the radiated portion of the heat, not the much greater portion which will be transferred by convection.
posted by caddis at 9:53 AM on December 13, 2004

Check out Kirchhoff's law. As I understand it, emission (radiation) and absorption (colour) are independent, but the factors that determine each one are basically the same, so they are usually equal.
posted by cillit bang at 10:06 AM on December 13, 2004

Color affecting a radiator's performance? Seems absurd to me. Bare metal would be best. The paint's heat conductivity characteristics would not seem linked to paint color.



Note: Highly reflective objects have emissivities near 0; ; "dull", black objects have emissivities near 1. Some typical emissivities are shown here. Note that white paint has a high emissivity: it is reflective in the visible but not in the IR!
source
posted by Chuckles at 10:13 AM on December 13, 2004

It's possible that paint provides a small amount of insulation, reducing the efficiency of the radiator slightly. But paint also helps reduce rust and may make it easier to clean off dust, so it's a wash.

Any heat not radiated by the 1st radiator in the loop is available to the next radiator, etc., so how important is a slight loss of emmissivity? (not a rhetorical question - I'm curious about this.)
posted by theora55 at 10:32 AM on December 13, 2004

silvery and reflective would be even less efficient.

Funny, every apartment & house we lived in growing up had radiators and they were always, without exception painted either silver or gold. I just assumed that they had to be painted metallic due to the heat. Learn something new everyday at Askme!
posted by Juicylicious at 12:37 PM on December 13, 2004

Radiators are truly misnamed. They are convectors. That's why they have fins, ribs etc. This is a radiator. Note the black tube (where gas is fired) and the silver reflector. (Okay, it's hard to see the reflector but I have about 200' of this stuff in a building and the reflectors are sheet metal.) Everyone in this thread is right!
posted by Dick Paris at 2:50 PM on December 13, 2004