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radiators: an energy diff. painting 'em black?
Old 09-06-2007, 04:02 AM   #1
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radiators: an energy diff. painting 'em black?

So, I'm remembering having heard somewhere that radiators supposedly work better when painted black. We have big heating bills and the light grey/cream radiators could withstand a touch-up (a matte dark charcoal grey color could be nice)... but "the Google" gives me conflicting stories.

Plenty o' academic types reasoning why it's irrelevant or negligible (because "radiators" heat more by convection than by true radiation) but some practical folks claiming it works.

Anyone have a real-life experience similar to [or different from] this guy?
I remember making a mistake with radiator color in my first home, which had a similar steam system to yours... a former coal furnace... except it burned fuel oil. There was one bathroom in the house... a very small one on the second floor that had you sitting inches from the radiator... enough said! Anyway, I thought it would be aesthetically pleasing (read "look cool") to paint the radiator black. Well, the BTU output from this tiny radiator was so intense that I ended up repainting it white to make the room useable. After that I selectively repainted radiators black or white based on my heating experience in each room. Since most of the radiators were under covers, the color change was not noticeable, but the temperature difference sure was!

OTOH MOST people seem convinced that metallic paints reduce the radiators' efficiency. I am hard-pressed to think why that'd be so; the metal used in metallic paint is aluminum, which is the same material as some of our radiators, anyway. So why in this case would people claim it 'reflects' (the heat back into the base metal) rather than conducts (the heat from the base metal to the air)?

Others say metallic is actually better because it goes on in a thinner coat, or because it is more emissive than non-metallic paint. There are also other home-grown theories, such as the bare cast iron having more surface area than painted (paint fills in tiny surface irregularities).

I failed thermodynamics and don't have a horse in this race.
Lotsa theories; have any of you put it to the test?

Related.. refrigerators more efficient when painted white? (talking about in a home rather than the blazing desert...)

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Old 09-06-2007, 05:58 AM   #2
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I can't see how the color of the radiator makes any difference in the heat it radiates. That heat comes from inside out, not outside in and color is pretty much a factor of reflected light. In fact, the exterior color seems so irrelevent that maybe imagining it sitting in the dark would help clarify this. The radiator doesn't really care if the lights are on or not and, without light, color doesn't actually exist.

Now if your paint had some insulative property, that would make a difference. Imagine that your black paint was styrofoam and it encased the radiator, 6" thick. That would certainly reduce it's ability to radiate the heat.

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Old 09-06-2007, 06:02 AM   #3
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I agree.

The surface temperature of the white might be a tiny bit higher than the black one, but I doubt if you would notice the difference.
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Old 09-06-2007, 08:09 AM   #4
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Originally Posted by ladelfina View Post
Related.. refrigerators more efficient when painted white? (talking about in a home rather than the blazing desert...)
I don't know anything about radiators, but I can tell you that refrigerators are more efficient when they are recent models. I think that matters more than the color.

Besides, people are more likely to paint an old refrigerator (it seems to me), that probably needs replacing pretty soon anyway.

I bought a new, very large black refrigerator from Sears after Katrina, since I left 50 pounds of chicken, shrimp, and fish in the freezer of my previous 15-20 year old white refrigerator when I evacuated (and the wretched smell permeated the insulation). It is amazing how little electricity my big new black refrigerator uses compared with the old, smaller white refrigerator.
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Old 09-06-2007, 08:58 AM   #5
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Old-House Journal Try this site. The following was taken from an article about painting radiators.

Bronze Beauties
If you can live with its flaws, bronzing in shades from silver to gold is a period way to paint your radiator.
By Gordon Bock

"In the 1920s, heating engineers took radiators into the lab to determine as accurately as possible what made the best coating for a radiator. In terms of physics, radiators heat a room though two modes: convection (heating the surrounding air) and radiation (heat energy emitted directly from the metal as waves). While tests revealed that radiator paint had no effect on convection it could dramatically influence radiation. As it turned out, only the last, exposed coat of paint had any impact and, among the oil-based paints tested, color made no appreciable difference. Ironically, metallic paints (and galvanizing) cut radiation by 7.4 to 9.2 percent."
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Old 09-06-2007, 09:27 AM   #6
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Most of our radiators have the original bronze/gold paint.

I think the best thing to do is put that insulating foil on the wall behind the radiators.

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Old 09-06-2007, 07:19 PM   #7
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Our house has the big ol' cast iron radiators, and over the years they've been painted both very light tan, and are now (and for the past many years) painted a chocolate brown, except the one in the kitchen which is a light yellow. As far as I can tell, there is absolutely NO difference in their heating characteristics. Half of the brown ones are painted with gloss paint, and the the others are flat paint.....they heat the same. The brown one in the living room heats just the same as the yellow one in the kitchen.

Paint color makes NO difference!
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Old 09-07-2007, 11:50 AM   #8
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Just to throw another wrench in this: don't confuse 'efficiency' with 'effectiveness'.

In theory, if you do something to reduce the amount of heat coming from that radiator, the steam or water is just retaining that heat, so it does not need as much re-heating on it's way through the boiler/furnace. The energy isn't 'lost' it just is not coming out of the system as effectively as you want it.

That said, there will be secondary effects that will reduce efficiency - hotter water in the lines losing heat where you don't want to. Lower temperature differential in the furnace will reduce heat transfer, you need to run the pump longer to get the heat you need, probably some others.

I think Martha's comment on reflective/insulated foil behind the radiator is probably the most cost effective way to improve both effectiveness and efficiency.

I think... - ERD50 (edit/add) yes - black is for absorbing heat, little/no difference radiating it
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Old 09-08-2007, 02:27 AM   #9
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ok.. all good comments, thanks. I'd just been wondering if anyone had a real-life experience to back up the "Natural Handyman".

People are also right to point out that, even if radiator color did make a difference, there are more important elements of the system. We do have insulating foil behind some (not all) so that's the first step to take with the others. True, too, about the heat circulating in the pipes. Since our house has a stone/brick exterior, an air space, and interior walls made of hollow bricks with a stucco/cement surface, the pipes between radiators are in theory heating up that air space. I tend to doubt they are insulated but have no proof one way or the other since we have never opened up the wall (that would be a big, messy, largely pointless endeavor). Since the radiators on exterior walls are inset (backed up by just the outside stone wall) the reflectors are particularly important in those spots.

ERD50.. yeah, in terms of "effectiveness" I get your point entirely. That's why I'm not sold on the compact fluorescent bulbs.. I actually appreciate the extra heat they throw off into the room 9-10 months of the year. Brrrrrr.

We just spent almost $6000 having all the windows and doors re-finished and double-glazing retrofitted everywhere to replace the single-pane original glass. There are no storm windows or storm doors and no real possibility of such. There are interior wooden shutters which help insulate (but make the rooms dark). It's really a nightmare house from the p.o.v. of energy efficiency and I really wasn't aware how bad it could be switching from the low-ceilinged, small-roomed wooden house in the US, to the high-ceilinged, larger stone/tile place.

It's actually far more airtight than my US house (you can actually feel the air pressure changing as you open and close interior doors) and gets stuffier in winter than my old house. The Italian ladies usually do a morning "airing" of the house, throwing open all the windows, some even going to the extent of airing the bedding out the window on the sill. The first electronic thermostat we bought had a button on it with a pictogram of a lady with a broom: this shut the heat off for 2 hours and then restarted it!! Because "cleaning" = also a complete air exchange. Interesting how the cleaning habits are so ingrained they get reflected in the consumer products.

I'm curious to find out what it takes to heat two rooms of equivalent size: one of brick and one of wood/drywall. We feel the heat from the air, but to what extent is that absorbed also by the surrounding materials, leaving less "effective" heat for us? Should I spend megabucks in wood parquet and paneling? (Rhetorical at this point; what with the window replacement and the sewer hook-up, my budget this year has enough for a couple pairs of fingerless gloves.)

I know the roof is apparently reasonably well-insulated (not because I know it has actual insulation.. again, it's a cement slab with tile over it and who knows what). I just go by the fact that snow doesn't seem to melt appreciably, which was the New England way of judging heat loss. They don't seem to use degree-days so I have no way of comparing the gas bills season to season like I used to be able to do.

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